21. Web MVC framework

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21. Web MVC framework

21.1 Introduction to Spring Web MVC framework

The Spring Web model-view-controller (MVC) framework is designed around a DispatcherServlet that dispatches requests to handlers, with configurable handler mappings, view resolution, locale, time zone and theme resolution as well as support for uploading files. The default handler is based on the @Controller and @RequestMapping annotations, offering a wide range of flexible handling methods. With the introduction of Spring 3.0, the @Controller mechanism also allows you to create RESTful Web sites and applications, through the @PathVariable annotation and other features.

“Open for extension…” A key design principle in Spring Web MVC and in Spring in general is the “Open for extension, closed for modification“ principle.

Some methods in the core classes of Spring Web MVC are marked final. As a developer you cannot override these methods to supply your own behavior. This has not been done arbitrarily, but specifically with this principle in mind.

For an explanation of this principle, refer to Expert Spring Web MVC and Web Flow by Seth Ladd and others; specifically see the section “A Look At Design,” on page 117 of the first edition. Alternatively, see

• Bob Martin, The Open-Closed Principle (PDF)

You cannot add advice to final methods when you use Spring MVC. For example, you cannot add advice to the AbstractController.setSynchronizeOnSession() method. Refer to Section 10.6.1, “Understanding AOP proxies” for more information on AOP proxies and why you cannot add advice to final methods.

In Spring Web MVC you can use any object as a command or form-backing object; you do not need to implement a framework-specific interface or base class. Spring’s data binding is highly flexible: for example, it treats type mismatches as validation errors that can be evaluated by the application, not as system errors. Thus you need not duplicate your business objects’ properties as simple, untyped strings in your form objects simply to handle invalid submissions, or to convert the Strings properly. Instead, it is often preferable to bind directly to your business objects.

Spring’s view resolution is extremely flexible. A Controller is typically responsible for preparing a model Map with data and selecting a view name but it can also write directly to the response stream and complete the request. View name resolution is highly configurable through file extension or Accept header content type negotiation, through bean names, a properties file, or even a custom ViewResolver implementation. The model (the M in MVC) is a Map interface, which allows for the complete abstraction of the view technology. You can integrate directly with template based rendering technologies such as JSP, Velocity and Freemarker, or directly generate XML, JSON, Atom, and many other types of content. The model Map is simply transformed into an appropriate format, such as JSP request attributes, a Velocity template model.

21.1.1 Features of Spring Web MVC

Spring Web Flow

Spring Web Flow (SWF) aims to be the best solution for the management of web application page flow.

SWF integrates with existing frameworks like Spring MVC and JSF, in both Servlet and Portlet environments. If you have a business process (or processes) that would benefit from a conversational model as opposed to a purely request model, then SWF may be the solution.

SWF allows you to capture logical page flows as self-contained modules that are reusable in different situations, and as such is ideal for building web application modules that guide the user through controlled navigations that drive business processes.

For more information about SWF, consult the Spring Web Flow website.

Spring’s web module includes many unique web support features:

• Clear separation of roles. Each role — controller, validator, command object, form object, model object, DispatcherServlet, handler mapping, view resolver, and so on — can be fulfilled by a specialized object.
• Powerful and straightforward configuration of both framework and application classes as JavaBeans. This configuration capability includes easy referencing across contexts, such as from web controllers to business objects and validators.
• Adaptability, non-intrusiveness, and flexibility. Define any controller method signature you need, possibly using one of the parameter annotations (such as @RequestParam, @RequestHeader, @PathVariable, and more) for a given scenario.
• Reusable business code, no need for duplication. Use existing business objects as command or form objects instead of mirroring them to extend a particular framework base class.
• Customizable binding and validation. Type mismatches as application-level validation errors that keep the offending value, localized date and number binding, and so on instead of String-only form objects with manual parsing and conversion to business objects.
• Customizable handler mapping and view resolution. Handler mapping and view resolution strategies range from simple URL-based configuration, to sophisticated, purpose-built resolution strategies. Spring is more flexible than web MVC frameworks that mandate a particular technique.
• Flexible model transfer. Model transfer with a name/value Map supports easy integration with any view technology.
• Customizable locale, time zone and theme resolution, support for JSPs with or without Spring tag library, support for JSTL, support for Velocity without the need for extra bridges, and so on.
• A simple yet powerful JSP tag library known as the Spring tag library that provides support for features such as data binding and themes. The custom tags allow for maximum flexibility in terms of markup code. For information on the tag library descriptor, see the appendix entitled Chapter 42, spring JSP Tag Library
• A JSP form tag library, introduced in Spring 2.0, that makes writing forms in JSP pages much easier. For information on the tag library descriptor, see the appendix entitled Chapter 43, spring-form JSP Tag Library
• Beans whose lifecycle is scoped to the current HTTP request or HTTP Session. This is not a specific feature of Spring MVC itself, but rather of the WebApplicationContext container(s) that Spring MVC uses. These bean scopes are described in Section 6.5.4, “Request, session, and global session scopes”

21.1.2 Pluggability of other MVC implementations

Non-Spring MVC implementations are preferable for some projects. Many teams expect to leverage their existing investment in skills and tools, for example with JSF.

If you do not want to use Spring’s Web MVC, but intend to leverage other solutions that Spring offers, you can integrate the web MVC framework of your choice with Spring easily. Simply start up a Spring root application context through its ContextLoaderListener, and access it through its ServletContext attribute (or Spring’s respective helper method) from within any action object. No “plug-ins” are involved, so no dedicated integration is necessary. From the web layer’s point of view, you simply use Spring as a library, with the root application context instance as the entry point.

Your registered beans and Spring’s services can be at your fingertips even without Spring’s Web MVC. Spring does not compete with other web frameworks in this scenario. It simply addresses the many areas that the pure web MVC frameworks do not, from bean configuration to data access and transaction handling. So you can enrich your application with a Spring middle tier and/or data access tier, even if you just want to use, for example, the transaction abstraction with JDBC or Hibernate.

21.2 The DispatcherServlet

Spring’s web MVC framework is, like many other web MVC frameworks, request- driven, designed around a central Servlet that dispatches requests to controllers and offers other functionality that facilitates the development of web applications. Spring’s DispatcherServlet however, does more than just that. It is completely integrated with the Spring IoC container and as such allows you to use every other feature that Spring has.

The request processing workflow of the Spring Web MVC DispatcherServlet is illustrated in the following diagram. The pattern-savvy reader will recognize that the DispatcherServlet is an expression of the “Front Controller” design pattern (this is a pattern that Spring Web MVC shares with many other leading web frameworks).

Figure 21.1. The request processing workflow in Spring Web MVC (high level)

The DispatcherServlet is an actual Servlet (it inherits from the HttpServlet base class), and as such is declared in the web.xml of your web application. You need to map requests that you want the DispatcherServlet to handle, by using a URL mapping in the same web.xml file. This is standard Java EE Servlet configuration; the following example shows such a DispatcherServlet declaration and mapping:

<web-app>
    <servlet>
        <servlet-name>example</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>example</servlet-name>
        <url-pattern>/example/*</url-pattern>
    </servlet-mapping>
</web-app>

In the preceding example, all requests starting with /example will be handled by the DispatcherServlet instance named example. In a Servlet 3.0+ environment, you also have the option of configuring the Servlet container programmatically. Below is the code based equivalent of the above web.xml example:

public class MyWebApplicationInitializer implements WebApplicationInitializer {
    _@Override_
    public void onStartup(ServletContext container) {
        ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet());
        registration.setLoadOnStartup(1);
        registration.addMapping("/example/*");
    }
}

WebApplicationInitializer is an interface provided by Spring MVC that ensures your code-based configuration is detected and automatically used to initialize any Servlet 3 container. An abstract base class implementation of this interface named AbstractDispatcherServletInitializer makes it even easier to register the DispatcherServlet by simply specifying its servlet mapping. See Code-based Servlet container initialization for more details.

The above is only the first step in setting up Spring Web MVC. You now need to configure the various beans used by the Spring Web MVC framework (over and above the DispatcherServlet itself).

As detailed in Section 6.15, “Additional Capabilities of the ApplicationContext”, ApplicationContext instances in Spring can be scoped. In the Web MVC framework, each DispatcherServlet has its own WebApplicationContext, which inherits all the beans already defined in the root WebApplicationContext. These inherited beans can be overridden in the servlet-specific scope, and you can define new scope-specific beans local to a given Servlet instance.

Figure 21.2. Typical context hierarchy in Spring Web MVC

Upon initialization of a DispatcherServlet, Spring MVC looks for a file named [servlet-name]-servlet.xml in the WEB-INF directory of your web application and creates the beans defined there, overriding the definitions of any beans defined with the same name in the global scope.

Consider the following DispatcherServlet Servlet configuration (in the web.xml file):

<web-app>
    <servlet>
        <servlet-name>**golfing**</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>**golfing**</servlet-name>
        <url-pattern>/golfing/*</url-pattern>
    </servlet-mapping>
</web-app>

With the above Servlet configuration in place, you will need to have a file called /WEB-INF/golfing-servlet.xml in your application; this file will contain all of your Spring Web MVC-specific components (beans). You can change the exact location of this configuration file through a Servlet initialization parameter (see below for details).

It is also possible to have just one root context for single DispatcherServlet scenarios.

Figure 21.3. Single root context in Spring Web MVC

This can be configured by setting an empty contextConfigLocation servlet init parameter, as shown below:

<web-app>
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/root-context.xml</param-value>
    </context-param>
    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value></param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
</web-app>

The WebApplicationContext is an extension of the plain ApplicationContext that has some extra features necessary for web applications. It differs from a normal ApplicationContext in that it is capable of resolving themes (see Section 21.9, “Using themes”), and that it knows which Servlet it is associated with (by having a link to the ServletContext). The WebApplicationContext is bound in the ServletContext, and by using static methods on the RequestContextUtils class you can always look up the WebApplicationContext if you need access to it.

21.2.1 Special Bean Types In the WebApplicationContext

The Spring DispatcherServlet uses special beans to process requests and render the appropriate views. These beans are part of Spring MVC. You can choose which special beans to use by simply configuring one or more of them in the WebApplicationContext. However, you don’t need to do that initially since Spring MVC maintains a list of default beans to use if you don’t configure any. More on that in the next section. First see the table below listing the special bean types the DispatcherServlet relies on.

Table 21.1. Special bean types in the WebApplicationContext

HandlerMapping

|

Maps incoming requests to handlers and a list of pre- and post-processors (handler interceptors) based on some criteria the details of which vary by HandlerMapping implementation. The most popular implementation supports annotated controllers but other implementations exists as well.

HandlerAdapter

|

Helps the DispatcherServlet to invoke a handler mapped to a request regardless of the handler is actually invoked. For example, invoking an annotated controller requires resolving various annotations. Thus the main purpose of a HandlerAdapter is to shield the DispatcherServlet from such details.

HandlerExceptionResolver

|

Maps exceptions to views also allowing for more complex exception handling code.

ViewResolver

|

Resolves logical String-based view names to actual View types.

LocaleResolver & LocaleContextResolver

|

Resolves the locale a client is using and possibly their time zone, in order to be able to offer internationalized views

ThemeResolver

|

Resolves themes your web application can use, for example, to offer personalized layouts

MultipartResolver

|

Parses multi-part requests for example to support processing file uploads from HTML forms.

FlashMapManager

|

Stores and retrieves the “input” and the “output” FlashMap that can be used to pass attributes from one request to another, usually across a redirect.

21.2.2 Default DispatcherServlet Configuration

As mentioned in the previous section for each special bean the DispatcherServlet maintains a list of implementations to use by default. This information is kept in the file DispatcherServlet.properties in the package org.springframework.web.servlet.

All special beans have some reasonable defaults of their own. Sooner or later though you’ll need to customize one or more of the properties these beans provide. For example it’s quite common to configure an InternalResourceViewResolver settings its prefix property to the parent location of view files.

Regardless of the details, the important concept to understand here is that once you configure a special bean such as an InternalResourceViewResolver in your WebApplicationContext, you effectively override the list of default implementations that would have been used otherwise for that special bean type. For example if you configure an InternalResourceViewResolver, the default list of ViewResolver implementations is ignored.

In Section 21.16, “Configuring Spring MVC” you’ll learn about other options for configuring Spring MVC including MVC Java config and the MVC XML namespace both of which provide a simple starting point and assume little knowledge of how Spring MVC works. Regardless of how you choose to configure your application, the concepts explained in this section are fundamental should be of help to you.

21.2.3 DispatcherServlet Processing Sequence

After you set up a DispatcherServlet, and a request comes in for that specific DispatcherServlet, the DispatcherServlet starts processing the request as follows:

• The WebApplicationContext is searched for and bound in the request as an attribute that the controller and other elements in the process can use. It is bound by default under the key DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE.
• The locale resolver is bound to the request to enable elements in the process to resolve the locale to use when processing the request (rendering the view, preparing data, and so on). If you do not need locale resolving, you do not need it.
• The theme resolver is bound to the request to let elements such as views determine which theme to use. If you do not use themes, you can ignore it.
• If you specify a multipart file resolver, the request is inspected for multiparts; if multiparts are found, the request is wrapped in a MultipartHttpServletRequest for further processing by other elements in the process. See Section 21.10, “Spring’s multipart (file upload) support” for further information about multipart handling.
• An appropriate handler is searched for. If a handler is found, the execution chain associated with the handler (preprocessors, postprocessors, and controllers) is executed in order to prepare a model or rendering.
• If a model is returned, the view is rendered. If no model is returned, (may be due to a preprocessor or postprocessor intercepting the request, perhaps for security reasons), no view is rendered, because the request could already have been fulfilled.

Handler exception resolvers that are declared in the WebApplicationContext pick up exceptions that are thrown during processing of the request. Using these exception resolvers allows you to define custom behaviors to address exceptions.

The Spring DispatcherServlet also supports the return of the last- modification-date, as specified by the Servlet API. The process of determining the last modification date for a specific request is straightforward: the DispatcherServlet looks up an appropriate handler mapping and tests whether the handler that is found implements the LastModified interface. If so, the value of the long getLastModified(request) method of the LastModified interface is returned to the client.

You can customize individual DispatcherServlet instances by adding Servlet initialization parameters ( init-param elements) to the Servlet declaration in the web.xml file. See the following table for the list of supported parameters.

Table 21.2. DispatcherServlet initialization parameters

contextClass

|

Class that implements WebApplicationContext, which instantiates the context used by this Servlet. By default, the XmlWebApplicationContext is used.

contextConfigLocation

|

String that is passed to the context instance (specified by contextClass) to indicate where context(s) can be found. The string consists potentially of multiple strings (using a comma as a delimiter) to support multiple contexts. In case of multiple context locations with beans that are defined twice, the latest location takes precedence.

namespace

|

Namespace of the WebApplicationContext. Defaults to [servlet- name]-servlet.

21.3 Implementing Controllers

Controllers provide access to the application behavior that you typically define through a service interface. Controllers interpret user input and transform it into a model that is represented to the user by the view. Spring implements a controller in a very abstract way, which enables you to create a wide variety of controllers.

Spring 2.5 introduced an annotation-based programming model for MVC controllers that uses annotations such as @RequestMapping, @RequestParam, @ModelAttribute, and so on. This annotation support is available for both Servlet MVC and Portlet MVC. Controllers implemented in this style do not have to extend specific base classes or implement specific interfaces. Furthermore, they do not usually have direct dependencies on Servlet or Portlet APIs, although you can easily configure access to Servlet or Portlet facilities.

	Tip

Available in the spring-projects Org on Github, a number of web applications leverage the annotation support described in this section including MvcShowcase, MvcAjax, MvcBasic, PetClinic, PetCare, and others.

_@Controller_
public class HelloWorldController {
    _@RequestMapping("/helloWorld")_
    public String helloWorld(Model model) {
        model.addAttribute("message", "Hello World!");
        return "helloWorld";
    }
}

As you can see, the @Controller and @RequestMapping annotations allow flexible method names and signatures. In this particular example the method accepts a Model and returns a view name as a String, but various other method parameters and return values can be used as explained later in this section. @Controller and @RequestMapping and a number of other annotations form the basis for the Spring MVC implementation. This section documents these annotations and how they are most commonly used in a Servlet environment.

21.3.1 Defining a controller with @Controller

The @Controller annotation indicates that a particular class serves the role of a controller. Spring does not require you to extend any controller base class or reference the Servlet API. However, you can still reference Servlet- specific features if you need to.

The @Controller annotation acts as a stereotype for the annotated class, indicating its role. The dispatcher scans such annotated classes for mapped methods and detects @RequestMapping annotations (see the next section).

You can define annotated controller beans explicitly, using a standard Spring bean definition in the dispatcher’s context. However, the @Controller stereotype also allows for autodetection, aligned with Spring general support for detecting component classes in the classpath and auto-registering bean definitions for them.

To enable autodetection of such annotated controllers, you add component scanning to your configuration. Use the spring-context schema as shown in the following XML snippet:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
    <context:component-scan base-package="org.springframework.samples.petclinic.web"/>
    <!-- ... -->
</beans>

21.3.2 Mapping Requests With @RequestMapping

You use the @RequestMapping annotation to map URLs such as /appointments onto an entire class or a particular handler method. Typically the class-level annotation maps a specific request path (or path pattern) onto a form controller, with additional method-level annotations narrowing the primary mapping for a specific HTTP method request method (“GET”, “POST”, etc.) or an HTTP request parameter condition.

The following example from the Petcare sample shows a controller in a Spring MVC application that uses this annotation:

_@Controller_
**@RequestMapping("/appointments")**
public class AppointmentsController {
    private final AppointmentBook appointmentBook;
    _@Autowired_
    public AppointmentsController(AppointmentBook appointmentBook) {
        this.appointmentBook = appointmentBook;
    }
    **@RequestMapping(method = RequestMethod.GET)**
    public Map<String, Appointment> get() {
        return appointmentBook.getAppointmentsForToday();
    }
    **@RequestMapping(path = "/{day}", method = RequestMethod.GET)**
    public Map<String, Appointment> getForDay(_@PathVariable_ _@DateTimeFormat(iso=ISO.DATE)_ Date day, Model model) {
        return appointmentBook.getAppointmentsForDay(day);
    }
    **@RequestMapping(path = "/new", method = RequestMethod.GET)**
    public AppointmentForm getNewForm() {
        return new AppointmentForm();
    }
    **@RequestMapping(method = RequestMethod.POST)**
    public String add(_@Valid_ AppointmentForm appointment, BindingResult result) {
        if (result.hasErrors()) {
            return "appointments/new";
        }
        appointmentBook.addAppointment(appointment);
        return "redirect:/appointments";
    }
}

In the example, the @RequestMapping is used in a number of places. The first usage is on the type (class) level, which indicates that all handling methods on this controller are relative to the /appointments path. The get() method has a further @RequestMapping refinement: it only accepts GET requests, meaning that an HTTP GET for /appointments invokes this method. The add() has a similar refinement, and the getNewForm() combines the definition of HTTP method and path into one, so that GET requests for appointments/new are handled by that method.

The getForDay() method shows another usage of @RequestMapping: URI templates. (See the next section).

A @RequestMapping on the class level is not required. Without it, all paths are simply absolute, and not relative. The following example from the PetClinic sample application shows a multi-action controller using @RequestMapping:

_@Controller_
public class ClinicController {
    private final Clinic clinic;
    _@Autowired_
    public ClinicController(Clinic clinic) {
        this.clinic = clinic;
    }
    **@RequestMapping("/")**
    public void welcomeHandler() {
    }
    **@RequestMapping("/vets")**
    public ModelMap vetsHandler() {
        return new ModelMap(this.clinic.getVets());
    }
}

The above example does not specify GET vs. PUT, POST, and so forth, because @RequestMapping maps all HTTP methods by default. Use @RequestMapping(method=GET) to narrow the mapping.

@Controller and AOP Proxying

In some cases a controller may need to be decorated with an AOP proxy at runtime. One example is if you choose to have @Transactional annotations directly on the controller. When this is the case, for controllers specifically, we recommend using class-based proxying. This is typically the default choice with controllers. However if a controller must implement an interface that is not a Spring Context callback (e.g. InitializingBean, *Aware, etc), you may need to explicitly configure class-based proxying. For example with <tx:annotation-driven/>, change to <tx:annotation-driven proxy-target-class="true"/>.

New Support Classes for @RequestMapping methods in Spring MVC 3.1

Spring 3.1 introduced a new set of support classes for @RequestMapping methods called RequestMappingHandlerMapping and RequestMappingHandlerAdapter respectively. They are recommended for use and even required to take advantage of new features in Spring MVC 3.1 and going forward. The new support classes are enabled by default by the MVC namespace and the MVC Java config but must be configured explicitly if using neither. This section describes a few important differences between the old and the new support classes.

Prior to Spring 3.1, type and method-level request mappings were examined in two separate stages — a controller was selected first by the DefaultAnnotationHandlerMapping and the actual method to invoke was narrowed down second by the AnnotationMethodHandlerAdapter.

With the new support classes in Spring 3.1, the RequestMappingHandlerMapping is the only place where a decision is made about which method should process the request. Think of controller methods as a collection of unique endpoints with mappings for each method derived from type and method-level @RequestMapping information.

This enables some new possibilities. For once a HandlerInterceptor or a HandlerExceptionResolver can now expect the Object-based handler to be a HandlerMethod, which allows them to examine the exact method, its parameters and associated annotations. The processing for a URL no longer needs to be split across different controllers.

There are also several things no longer possible:

• Select a controller first with a SimpleUrlHandlerMapping or BeanNameUrlHandlerMapping and then narrow the method based on @RequestMapping annotations.
• Rely on method names as a fall-back mechanism to disambiguate between two @RequestMapping methods that don’t have an explicit path mapping URL path but otherwise match equally, e.g. by HTTP method. In the new support classes @RequestMapping methods have to be mapped uniquely.
• Have a single default method (without an explicit path mapping) with which requests are processed if no other controller method matches more concretely. In the new support classes if a matching method is not found a 404 error is raised.

The above features are still supported with the existing support classes. However to take advantage of new Spring MVC 3.1 features you’ll need to use the new support classes.

URI Template Patterns

URI templates can be used for convenient access to selected parts of a URL in a @RequestMapping method.

A URI Template is a URI-like string, containing one or more variable names. When you substitute values for these variables, the template becomes a URI. The proposed RFC for URI Templates defines how a URI is parameterized. For example, the URI Template <http://www.example.com/users/{userId}> contains the variable userId. Assigning the value fred to the variable yields <http://www.example.com/users/fred>.

In Spring MVC you can use the @PathVariable annotation on a method argument to bind it to the value of a URI template variable:

_@RequestMapping(path="/owners/{ownerId}", method=RequestMethod.GET)_
public String findOwner(**@PathVariable** String ownerId, Model model) {
    Owner owner = ownerService.findOwner(ownerId);
    model.addAttribute("owner", owner);
    return "displayOwner";
}

The URI Template “ /owners/{ownerId}“ specifies the variable name ownerId. When the controller handles this request, the value of ownerId is set to the value found in the appropriate part of the URI. For example, when a request comes in for /owners/fred, the value of ownerId is fred.

	Tip

To process the @PathVariable annotation, Spring MVC needs to find the matching URI template variable by name. You can specify it in the annotation:

_@RequestMapping(path="/owners/{ownerId}", method=RequestMethod.GET)_
public String findOwner(**@PathVariable("ownerId")** String theOwner, Model model) {
    // implementation omitted
}

Or if the URI template variable name matches the method argument name you can omit that detail. As long as your code is not compiled without debugging information, Spring MVC will match the method argument name to the URI template variable name:

_@RequestMapping(path="/owners/{ownerId}", method=RequestMethod.GET)_
public String findOwner(**@PathVariable** String ownerId, Model model) {
    // implementation omitted
}

A method can have any number of @PathVariable annotations:

_@RequestMapping(path="/owners/{ownerId}/pets/{petId}", method=RequestMethod.GET)_
public String findPet(**@PathVariable** String ownerId, **@PathVariable** String petId, Model model) {
    Owner owner = ownerService.findOwner(ownerId);
    Pet pet = owner.getPet(petId);
    model.addAttribute("pet", pet);
    return "displayPet";
}

When a @PathVariable annotation is used on a Map<String, String> argument, the map is populated with all URI template variables.

A URI template can be assembled from type and path level @RequestMapping annotations. As a result the findPet() method can be invoked with a URL such as /owners/42/pets/21.

_@Controller_
@RequestMapping(**"/owners/{ownerId}"**)
public class RelativePathUriTemplateController {
    @RequestMapping(**"/pets/{petId}"**)
    public void findPet(_@PathVariable_ String ownerId, _@PathVariable_ String petId, Model model) {
        // implementation omitted
    }
}

A @PathVariable argument can be of any simple type such as int, long, Date, etc. Spring automatically converts to the appropriate type or throws a TypeMismatchException if it fails to do so. You can also register support for parsing additional data types. See the section called “Method Parameters And Type Conversion” and the section called “Customizing WebDataBinder initialization”.

URI Template Patterns with Regular Expressions

Sometimes you need more precision in defining URI template variables. Consider the URL "/spring-web/spring-web-3.0.5.jar". How do you break it down into multiple parts?

The @RequestMapping annotation supports the use of regular expressions in URI template variables. The syntax is {varName:regex} where the first part defines the variable name and the second - the regular expression.For example:

_@RequestMapping("/spring-web/{symbolicName:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{extension:\\.[a-z]+}")_
    public void handle(_@PathVariable_ String version, _@PathVariable_ String extension) {
        // ...
    }
}

Path Patterns

In addition to URI templates, the @RequestMapping annotation also supports Ant-style path patterns (for example, /myPath/*.do). A combination of URI template variables and Ant-style globs is also supported (e.g. /owners/*/pets/{petId}).

Path Pattern Comparison

When a URL matches multiple patterns, a sort is used to find the most specific match.

A pattern with a lower count of URI variables and wild cards is considered more specific. For example /hotels/{hotel}/* has 1 URI variable and 1 wild card and is considered more specific than /hotels/{hotel}/** which as 1 URI variable and 2 wild cards.

If two patterns have the same count, the one that is longer is considered more specific. For example /foo/bar* is longer and considered more specific than /foo/*.

When two patterns have the same count and length, the pattern with fewer wild cards is considered more specific. For example /hotels/{hotel} is more specific than /hotels/*.

There are also some additional special rules:

• The default mapping pattern /** is less specific than any other pattern. For example /api/{a}/{b}/{c} is more specific.
• A prefix pattern such as /public/** is less specific than any other pattern that doesn’t contain double wildcards. For example /public/path3/{a}/{b}/{c} is more specific.

For the full details see AntPatternComparator in AntPathMatcher. Note that the PathMatcher can be customized (see Section 21.16.11, “Path Matching” in the section on configuring Spring MVC).

Path Patterns with Placeholders

Patterns in @RequestMapping annotations support ${…} placeholders against local properties and/or system properties and environment variables. This may be useful in cases where the path a controller is mapped to may need to be customized through configuration. For more information on placeholders, see the javadocs of the PropertyPlaceholderConfigurer class.

Suffix Pattern Matching

By default Spring MVC performs ".*" suffix pattern matching so that a controller mapped to /person is also implicitly mapped to /person.*. This makes it easy to request different representations of a resource through the URL path (e.g. /person.pdf, /person.xml).

Suffix pattern matching can be turned off or restricted to a set of path extensions explicitly registered for content negotiation purposes. This is generally recommended to minimize ambiguity with common request mappings such as /person/{id} where a dot might not represent a file extension, e.g. /person/[[email protected]](/cdn-cgi/l/email-protection) vs /person/[[email protected]](/cdn-cgi/l/email-protection). Furthermore as explained in the note below suffix pattern matching as well as content negotiation may be used in some circumstances to attempt malicious attacks and there are good reasons to restrict them meaningfully.

See Section 21.16.11, “Path Matching” for suffix pattern matching configuration and also Section 21.16.6, “Content Negotiation” for content negotiation configuration.

Suffix Pattern Matching and RFD

Reflected file download (RFD) attack was first described in a paper by Trustwave in 2014. The attack is similar to XSS in that it relies on input (e.g. query parameter, URI variable) being reflected in the response. However instead of inserting JavaScript into HTML, an RFD attack relies on the browser switching to perform a download and treating the response as an executable script if double-clicked based on the file extension (e.g. .bat, .cmd).

In Spring MVC @ResponseBody and ResponseEntity methods are at risk because they can render different content types which clients can request including via URL path extensions. Note however that neither disabling suffix pattern matching nor disabling the use of path extensions for content negotiation purposes alone are effective at preventing RFD attacks.

For comprehensive protection against RFD, prior to rendering the response body Spring MVC adds a Content-Disposition:inline;filename=f.txt header to suggest a fixed and safe download file filename. This is done only if the URL path contains a file extension that is neither whitelisted nor explicitly registered for content negotiation purposes. However it may potentially have side effects when URLs are typed directly into a browser.

Many common path extensions are whitelisted by default. Furthermore REST API calls are typically not meant to be used as URLs directly in browsers. Nevertheless applications that use custom HttpMessageConverter implementations can explicitly register file extensions for content negotiation and the Content-Disposition header will not be added for such extensions. See Section 21.16.6, “Content Negotiation”.

	Note

This was originally introduced as part of work for CVE-2015-5211. Below are additional recommendations from the report:

• Encode rather than escape JSON responses. This is also an OWASP XSS recommendation. For an example of how to do that with Spring see spring-jackson-owasp.
• Configure suffix pattern matching to be turned off or restricted to explicitly registered suffixes only.
• Configure content negotiation with the properties “useJaf” and “ignoreUnknownPathExtensions” set to false which would result in a 406 response for URLs with unknown extensions. Note however that this may not be an option if URLs are naturally expected to have a dot towards the end.
• Add X-Content-Type-Options: nosniff header to responses. Spring Security 4 does this by default.

Matrix Variables

The URI specification RFC 3986 defines the possibility of including name-value pairs within path segments. There is no specific term used in the spec. The general “URI path parameters” could be applied although the more unique “Matrix URIs”, originating from an old post by Tim Berners-Lee, is also frequently used and fairly well known. Within Spring MVC these are referred to as matrix variables.

Matrix variables can appear in any path segment, each matrix variable separated with a “;” (semicolon). For example: "/cars;color=red;year=2012". Multiple values may be either “,” (comma) separated "color=red,green,blue" or the variable name may be repeated "color=red;color=green;color=blue".

If a URL is expected to contain matrix variables, the request mapping pattern must represent them with a URI template. This ensures the request can be matched correctly regardless of whether matrix variables are present or not and in what order they are provided.

Below is an example of extracting the matrix variable “q”:

// GET /pets/42;q=11;r=22
_@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET)_
public void findPet(_@PathVariable_ String petId, _@MatrixVariable_ int q) {
    // petId == 42
    // q == 11
}

Since all path segments may contain matrix variables, in some cases you need to be more specific to identify where the variable is expected to be:

// GET /owners/42;q=11/pets/21;q=22
_@RequestMapping(path = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)_
public void findPet(
        _@MatrixVariable(name="q", pathVar="ownerId")_ int q1,
        _@MatrixVariable(name="q", pathVar="petId")_ int q2) {
    // q1 == 11
    // q2 == 22
}

A matrix variable may be defined as optional and a default value specified:

// GET /pets/42
_@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET)_
public void findPet(_@MatrixVariable(required=false, defaultValue="1")_ int q) {
    // q == 1
}

All matrix variables may be obtained in a Map:

// GET /owners/42;q=11;r=12/pets/21;q=22;s=23
_@RequestMapping(path = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)_
public void findPet(
        _@MatrixVariable_ Map<String, String> matrixVars,
        _@MatrixVariable(pathVar="petId"")_ Map<String, String> petMatrixVars) {
    // matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
    // petMatrixVars: ["q" : 11, "s" : 23]
}

Note that to enable the use of matrix variables, you must set the removeSemicolonContent property of RequestMappingHandlerMapping to false. By default it is set to true.

	Tip

The MVC Java config and the MVC namespace both provide options for enabling the use of matrix variables.

If you are using Java config, The Advanced Customizations with MVC Java Config section describes how the RequestMappingHandlerMapping can be customized.

In the MVC namespace, the <mvc:annotation-driven> element has an enable- matrix-variables attribute that should be set to true. By default it is set to false.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc.xsd">
    <mvc:annotation-driven enable-matrix-variables="true"/>
</beans>

Consumable Media Types

You can narrow the primary mapping by specifying a list of consumable media types. The request will be matched only if the Content-Type request header matches the specified media type. For example:

_@Controller_
@RequestMapping(path = "/pets", method = RequestMethod.POST, **consumes="application/json"**)
public void addPet(_@RequestBody_ Pet pet, Model model) {
    // implementation omitted
}

Consumable media type expressions can also be negated as in !text/plain to match to all requests other than those with Content-Type of text/plain. Also consider using constants provided in MediaType such as APPLICATION_JSON_VALUE and APPLICATION_JSON_UTF8_VALUE.

	Tip

The consumes condition is supported on the type and on the method level. Unlike most other conditions, when used at the type level, method-level consumable types override rather than extend type-level consumable types.

Producible Media Types

You can narrow the primary mapping by specifying a list of producible media types. The request will be matched only if the Accept request header matches one of these values. Furthermore, use of the produces condition ensures the actual content type used to generate the response respects the media types specified in the produces condition. For example:

_@Controller_
@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET, **produces = MediaType.APPLICATION_JSON_UTF8_VALUE**)
_@ResponseBody_
public Pet getPet(_@PathVariable_ String petId, Model model) {
    // implementation omitted
}

	Note

Be aware that the media type specified in the produces condition can also optionally specify a character set. For example, in the code snippet above we specify the same media type than the default one configured in MappingJackson2HttpMessageConverter, including the UTF-8 charset.

Just like with consumes, producible media type expressions can be negated as in !text/plain to match to all requests other than those with an Accept header value of text/plain. Also consider using constants provided in MediaType such as APPLICATION_JSON_VALUE and APPLICATION_JSON_UTF8_VALUE.

	Tip

The produces condition is supported on the type and on the method level. Unlike most other conditions, when used at the type level, method-level producible types override rather than extend type-level producible types.

Request Parameters and Header Values

You can narrow request matching through request parameter conditions such as "myParam", "!myParam", or "myParam=myValue". The first two test for request parameter presence/absence and the third for a specific parameter value. Here is an example with a request parameter value condition:

_@Controller_
_@RequestMapping("/owners/{ownerId}")_
public class RelativePathUriTemplateController {
    @RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET, **params="myParam=myValue"**)
    public void findPet(_@PathVariable_ String ownerId, _@PathVariable_ String petId, Model model) {
        // implementation omitted
    }
}

The same can be done to test for request header presence/absence or to match based on a specific request header value:

_@Controller_
_@RequestMapping("/owners/{ownerId}")_
public class RelativePathUriTemplateController {
    @RequestMapping(path = "/pets", method = RequestMethod.GET, **headers="myHeader=myValue"**)
    public void findPet(_@PathVariable_ String ownerId, _@PathVariable_ String petId, Model model) {
        // implementation omitted
    }
}

	Tip

Although you can match to Content-Type and Accept header values using media type wild cards (for example “content-type=text/*” will match to “text/plain” and “text/html”), it is recommended to use the consumes and produces conditions respectively instead. They are intended specifically for that purpose.

21.3.3 Defining @RequestMapping handler methods

An @RequestMapping handler method can have a very flexible signatures. The supported method arguments and return values are described in the following section. Most arguments can be used in arbitrary order with the only exception of BindingResult arguments. This is described in the next section.

	Note

Spring 3.1 introduced a new set of support classes for @RequestMapping methods called RequestMappingHandlerMapping and RequestMappingHandlerAdapter respectively. They are recommended for use and even required to take advantage of new features in Spring MVC 3.1 and going forward. The new support classes are enabled by default from the MVC namespace and with use of the MVC Java config but must be configured explicitly if using neither.

Supported method argument types

The following are the supported method arguments:

• Request or response objects (Servlet API). Choose any specific request or response type, for example ServletRequest or HttpServletRequest.
• Session object (Servlet API): of type HttpSession. An argument of this type enforces the presence of a corresponding session. As a consequence, such an argument is never null.

	Note

Session access may not be thread-safe, in particular in a Servlet environment. Consider setting the RequestMappingHandlerAdapter’s “synchronizeOnSession” flag to “true” if multiple requests are allowed to access a session concurrently.

• org.springframework.web.context.request.WebRequest or org.springframework.web.context.request.NativeWebRequest. Allows for generic request parameter access as well as request/session attribute access, without ties to the native Servlet/Portlet API.
• java.util.Locale for the current request locale, determined by the most specific locale resolver available, in effect, the configured LocaleResolver / LocaleContextResolver in an MVC environment.
• java.util.TimeZone (Java 6+) / java.time.ZoneId (on Java 8) for the time zone associated with the current request, as determined by a LocaleContextResolver.
• java.io.InputStream / java.io.Reader for access to the request’s content. This value is the raw InputStream/Reader as exposed by the Servlet API.
• java.io.OutputStream / java.io.Writer for generating the response’s content. This value is the raw OutputStream/Writer as exposed by the Servlet API.
• org.springframework.http.HttpMethod for the HTTP request method.
• java.security.Principal containing the currently authenticated user.
• @PathVariable annotated parameters for access to URI template variables. See the section called “URI Template Patterns”.
• @MatrixVariable annotated parameters for access to name-value pairs located in URI path segments. See the section called “Matrix Variables”.
• @RequestParam annotated parameters for access to specific Servlet request parameters. Parameter values are converted to the declared method argument type. See the section called “Binding request parameters to method parameters with @RequestParam”.
• @RequestHeader annotated parameters for access to specific Servlet request HTTP headers. Parameter values are converted to the declared method argument type. See the section called “Mapping request header attributes with the @RequestHeader annotation”.
• @RequestBody annotated parameters for access to the HTTP request body. Parameter values are converted to the declared method argument type using HttpMessageConverters. See the section called “Mapping the request body with the @RequestBody annotation”.
• @RequestPart annotated parameters for access to the content of a “multipart/form-data” request part. See Section 21.10.5, “Handling a file upload request from programmatic clients” and Section 21.10, “Spring’s multipart (file upload) support”.
• HttpEntity<?> parameters for access to the Servlet request HTTP headers and contents. The request stream will be converted to the entity body using HttpMessageConverters. See the section called “Using HttpEntity”.
• java.util.Map / org.springframework.ui.Model / org.springframework.ui.ModelMap for enriching the implicit model that is exposed to the web view.
• org.springframework.web.servlet.mvc.support.RedirectAttributes to specify the exact set of attributes to use in case of a redirect and also to add flash attributes (attributes stored temporarily on the server-side to make them available to the request after the redirect). See the section called “Passing Data To the Redirect Target” and Section 21.6, “Using flash attributes”.
• Command or form objects to bind request parameters to bean properties (via setters) or directly to fields, with customizable type conversion, depending on @InitBinder methods and/or the HandlerAdapter configuration. See the webBindingInitializer property on RequestMappingHandlerAdapter. Such command objects along with their validation results will be exposed as model attributes by default, using the command class class name - e.g. model attribute “orderAddress” for a command object of type “some.package.OrderAddress”. The ModelAttribute annotation can be used on a method argument to customize the model attribute name used.
• org.springframework.validation.Errors / org.springframework.validation.BindingResult validation results for a preceding command or form object (the immediately preceding method argument).
• org.springframework.web.bind.support.SessionStatus status handle for marking form processing as complete, which triggers the cleanup of session attributes that have been indicated by the @SessionAttributes annotation at the handler type level.
• org.springframework.web.util.UriComponentsBuilder a builder for preparing a URL relative to the current request’s host, port, scheme, context path, and the literal part of the servlet mapping.

The Errors or BindingResult parameters have to follow the model object that is being bound immediately as the method signature might have more than one model object and Spring will create a separate BindingResult instance for each of them so the following sample won’t work:

Invalid ordering of BindingResult and @ModelAttribute.

_@RequestMapping(method = RequestMethod.POST)_
public String processSubmit(**@ModelAttribute("pet") Pet pet**, Model model, **BindingResult result**) { ... }

Note, that there is a Model parameter in between Pet and BindingResult. To get this working you have to reorder the parameters as follows:

_@RequestMapping(method = RequestMethod.POST)_
public String processSubmit(**@ModelAttribute("pet") Pet pet**, **BindingResult result**, Model model) { ... }

	Note

JDK 1.8’s java.util.Optional is supported as a method parameter type with annotations that have a required attribute (e.g. @RequestParam, @RequestHeader, etc. The use of java.util.Optional in those cases is equivalent to having required=false.

Supported method return types

The following are the supported return types:

• A ModelAndView object, with the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.
• A Model object, with the view name implicitly determined through a RequestToViewNameTranslator and the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.
• A Map object for exposing a model, with the view name implicitly determined through a RequestToViewNameTranslator and the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.
• A View object, with the model implicitly determined through command objects and @ModelAttribute annotated reference data accessor methods. The handler method may also programmatically enrich the model by declaring a Model argument (see above).
• A String value that is interpreted as the logical view name, with the model implicitly determined through command objects and @ModelAttribute annotated reference data accessor methods. The handler method may also programmatically enrich the model by declaring a Model argument (see above).
• void if the method handles the response itself (by writing the response content directly, declaring an argument of type ServletResponse / HttpServletResponse for that purpose) or if the view name is supposed to be implicitly determined through a RequestToViewNameTranslator (not declaring a response argument in the handler method signature).
• If the method is annotated with @ResponseBody, the return type is written to the response HTTP body. The return value will be converted to the declared method argument type using HttpMessageConverters. See the section called “Mapping the response body with the @ResponseBody annotation”.
• An HttpEntity<?> or ResponseEntity<?> object to provide access to the Servlet response HTTP headers and contents. The entity body will be converted to the response stream using HttpMessageConverters. See the section called “Using HttpEntity”.
• An HttpHeaders object to return a response with no body.
• A Callable<?> can be returned when the application wants to produce the return value asynchronously in a thread managed by Spring MVC.
• A DeferredResult<?> can be returned when the application wants to produce the return value from a thread of its own choosing.
• A ListenableFuture<?> can be returned when the application wants to produce the return value from a thread of its own choosing.
• A ResponseBodyEmitter can be returned to write multiple objects to the response asynchronously; also supported as the body within a ResponseEntity.
• An SseEmitter can be returned to write Server-Sent Events to the response asynchronously; also supported as the body within a ResponseEntity.
• A StreamingResponseBody can be returned to write to the response OutputStream asynchronously; also supported as the body within a ResponseEntity.
• Any other return type is considered to be a single model attribute to be exposed to the view, using the attribute name specified through @ModelAttribute at the method level (or the default attribute name based on the return type class name). The model is implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.

Binding request parameters to method parameters with @RequestParam

Use the @RequestParam annotation to bind request parameters to a method parameter in your controller.

The following code snippet shows the usage:

_@Controller_
_@RequestMapping("/pets")_
_@SessionAttributes("pet")_
public class EditPetForm {
    // ...
    _@RequestMapping(method = RequestMethod.GET)_
    public String setupForm(**@RequestParam("petId") int petId**, ModelMap model) {
        Pet pet = this.clinic.loadPet(petId);
        model.addAttribute("pet", pet);
        return "petForm";
    }
    // ...
}

Parameters using this annotation are required by default, but you can specify that a parameter is optional by setting @RequestParam’s required attribute to false (e.g., @RequestParam(path="id", required=false)).

Type conversion is applied automatically if the target method parameter type is not String. See the section called “Method Parameters And Type Conversion”.

When an @RequestParam annotation is used on a Map<String, String> or MultiValueMap<String, String> argument, the map is populated with all request parameters.

Mapping the request body with the @RequestBody annotation

The @RequestBody method parameter annotation indicates that a method parameter should be bound to the value of the HTTP request body. For example:

_@RequestMapping(path = "/something", method = RequestMethod.PUT)_
public void handle(_@RequestBody_ String body, Writer writer) throws IOException {
    writer.write(body);
}

You convert the request body to the method argument by using an HttpMessageConverter. HttpMessageConverter is responsible for converting from the HTTP request message to an object and converting from an object to the HTTP response body. The RequestMappingHandlerAdapter supports the @RequestBody annotation with the following default HttpMessageConverters:

• ByteArrayHttpMessageConverter converts byte arrays.
• StringHttpMessageConverter converts strings.
• FormHttpMessageConverter converts form data to/from a MultiValueMap<String, String>.
• SourceHttpMessageConverter converts to/from a javax.xml.transform.Source.

For more information on these converters, see Message Converters. Also note that if using the MVC namespace or the MVC Java config, a wider range of message converters are registered by default. See Section 21.16.1, “Enabling the MVC Java Config or the MVC XML Namespace” for more information.

If you intend to read and write XML, you will need to configure the MarshallingHttpMessageConverter with a specific Marshaller and an Unmarshaller implementation from the org.springframework.oxm package. The example below shows how to do that directly in your configuration but if your application is configured through the MVC namespace or the MVC Java config see Section 21.16.1, “Enabling the MVC Java Config or the MVC XML Namespace” instead.

<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
    <property name="messageConverters">
        <util:list id="beanList">
            <ref bean="stringHttpMessageConverter"/>
            <ref bean="marshallingHttpMessageConverter"/>
        </util:list>
    </property
</bean>
<bean id="stringHttpMessageConverter"
        class="org.springframework.http.converter.StringHttpMessageConverter"/>
<bean id="marshallingHttpMessageConverter"
        class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter">
    <property name="marshaller" ref="castorMarshaller"/>
    <property name="unmarshaller" ref="castorMarshaller"/>
</bean>
<bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller"/>

An @RequestBody method parameter can be annotated with @Valid, in which case it will be validated using the configured Validator instance. When using the MVC namespace or the MVC Java config, a JSR-303 validator is configured automatically assuming a JSR-303 implementation is available on the classpath.

Just like with @ModelAttribute parameters, an Errors argument can be used to examine the errors. If such an argument is not declared, a MethodArgumentNotValidException will be raised. The exception is handled in the DefaultHandlerExceptionResolver, which sends a 400 error back to the client.

	Note

Also see Section 21.16.1, “Enabling the MVC Java Config or the MVC XML Namespace” for information on configuring message converters and a validator through the MVC namespace or the MVC Java config.

Mapping the response body with the @ResponseBody annotation

The @ResponseBody annotation is similar to @RequestBody. This annotation can be put on a method and indicates that the return type should be written straight to the HTTP response body (and not placed in a Model, or interpreted as a view name). For example:

_@RequestMapping(path = "/something", method = RequestMethod.PUT)_
_@ResponseBody_
public String helloWorld() {
    return "Hello World";
}

The above example will result in the text Hello World being written to the HTTP response stream.

As with @RequestBody, Spring converts the returned object to a response body by using an HttpMessageConverter. For more information on these converters, see the previous section and Message Converters.

Creating REST Controllers with the @RestController annotation

It’s a very common use case to have Controllers implement a REST API, thus serving only JSON, XML or custom MediaType content. For convenience, instead of annotating all your @RequestMapping methods with @ResponseBody, you can annotate your Controller Class with @RestController.

@RestController is a stereotype annotation that combines @ResponseBody and @Controller. More than that, it gives more meaning to your Controller and also may carry additional semantics in future releases of the framework.

As with regular @Controllers, a @RestController may be assisted by a @ControllerAdvice Bean. See the the section called “Advising controllers with @ControllerAdvice” section for more details.

Using HttpEntity

The HttpEntity is similar to @RequestBody and @ResponseBody. Besides getting access to the request and response body, HttpEntity (and the response-specific subclass ResponseEntity) also allows access to the request and response headers, like so:

_@RequestMapping("/something")_
public ResponseEntity<String> handle(HttpEntity<byte[]> requestEntity) throws UnsupportedEncodingException {
    String requestHeader = requestEntity.getHeaders().getFirst("MyRequestHeader"));
    byte[] requestBody = requestEntity.getBody();
    // do something with request header and body
    HttpHeaders responseHeaders = new HttpHeaders();
    responseHeaders.set("MyResponseHeader", "MyValue");
    return new ResponseEntity<String>("Hello World", responseHeaders, HttpStatus.CREATED);
}

The above example gets the value of the MyRequestHeader request header, and reads the body as a byte array. It adds the MyResponseHeader to the response, writes Hello World to the response stream, and sets the response status code to 201 (Created).

As with @RequestBody and @ResponseBody, Spring uses HttpMessageConverter to convert from and to the request and response streams. For more information on these converters, see the previous section and Message Converters.

Using @ModelAttribute on a method

The @ModelAttribute annotation can be used on methods or on method arguments. This section explains its usage on methods while the next section explains its usage on method arguments.

An @ModelAttribute on a method indicates the purpose of that method is to add one or more model attributes. Such methods support the same argument types as @RequestMapping methods but cannot be mapped directly to requests. Instead @ModelAttribute methods in a controller are invoked before @RequestMapping methods, within the same controller. A couple of examples:

// Add one attribute
// The return value of the method is added to the model under the name "account"
// You can customize the name via @ModelAttribute("myAccount")
_@ModelAttribute_
public Account addAccount(_@RequestParam_ String number) {
    return accountManager.findAccount(number);
}
// Add multiple attributes
_@ModelAttribute_
public void populateModel(_@RequestParam_ String number, Model model) {
    model.addAttribute(accountManager.findAccount(number));
    // add more ...
}

@ModelAttribute methods are used to populate the model with commonly needed attributes for example to fill a drop-down with states or with pet types, or to retrieve a command object like Account in order to use it to represent the data on an HTML form. The latter case is further discussed in the next section.

Note the two styles of @ModelAttribute methods. In the first, the method adds an attribute implicitly by returning it. In the second, the method accepts a Model and adds any number of model attributes to it. You can choose between the two styles depending on your needs.

A controller can have any number of @ModelAttribute methods. All such methods are invoked before @RequestMapping methods of the same controller.

@ModelAttribute methods can also be defined in an @ControllerAdvice- annotated class and such methods apply to many controllers. See the the section called “Advising controllers with @ControllerAdvice” section for more details.

	Tip

What happens when a model attribute name is not explicitly specified? In such cases a default name is assigned to the model attribute based on its type. For example if the method returns an object of type Account, the default name used is “account”. You can change that through the value of the @ModelAttribute annotation. If adding attributes directly to the Model, use the appropriate overloaded addAttribute(..) method - i.e., with or without an attribute name.

The @ModelAttribute annotation can be used on @RequestMapping methods as well. In that case the return value of the @RequestMapping method is interpreted as a model attribute rather than as a view name. The view name is derived from view name conventions instead much like for methods returning void — see Section 21.13.3, “The View - RequestToViewNameTranslator”.

Using @ModelAttribute on a method argument

As explained in the previous section @ModelAttribute can be used on methods or on method arguments. This section explains its usage on method arguments.

An @ModelAttribute on a method argument indicates the argument should be retrieved from the model. If not present in the model, the argument should be instantiated first and then added to the model. Once present in the model, the argument’s fields should be populated from all request parameters that have matching names. This is known as data binding in Spring MVC, a very useful mechanism that saves you from having to parse each form field individually.

_@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)_
public String processSubmit(**@ModelAttribute Pet pet**) { }

Given the above example where can the Pet instance come from? There are several options:

• It may already be in the model due to use of @SessionAttributes — see the section called “Using @SessionAttributes to store model attributes in the HTTP session between requests”.
• It may already be in the model due to an @ModelAttribute method in the same controller — as explained in the previous section.
• It may be retrieved based on a URI template variable and type converter (explained in more detail below).
• It may be instantiated using its default constructor.

An @ModelAttribute method is a common way to to retrieve an attribute from the database, which may optionally be stored between requests through the use of @SessionAttributes. In some cases it may be convenient to retrieve the attribute by using an URI template variable and a type converter. Here is an example:

_@RequestMapping(path = "/accounts/{account}", method = RequestMethod.PUT)_
public String save(_@ModelAttribute("account")_ Account account) {
}

In this example the name of the model attribute (i.e. “account”) matches the name of a URI template variable. If you register Converter<String, Account> that can turn the String account value into an Account instance, then the above example will work without the need for an @ModelAttribute method.

The next step is data binding. The WebDataBinder class matches request parameter names — including query string parameters and form fields — to model attribute fields by name. Matching fields are populated after type conversion (from String to the target field type) has been applied where necessary. Data binding and validation are covered in Chapter 8, Validation, Data Binding, and Type Conversion. Customizing the data binding process for a controller level is covered in the section called “Customizing WebDataBinder initialization”.

As a result of data binding there may be errors such as missing required fields or type conversion errors. To check for such errors add a BindingResult argument immediately following the @ModelAttribute argument:

_@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)_
public String processSubmit(**@ModelAttribute("pet") Pet pet**, BindingResult result) {
    if (result.hasErrors()) {
        return "petForm";
    }
    // ...
}

With a BindingResult you can check if errors were found in which case it’s common to render the same form where the errors can be shown with the help of Spring’s <errors> form tag.

In addition to data binding you can also invoke validation using your own custom validator passing the same BindingResult that was used to record data binding errors. That allows for data binding and validation errors to be accumulated in one place and subsequently reported back to the user:

_@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)_
public String processSubmit(**@ModelAttribute("pet") Pet pet**, BindingResult result) {
    new PetValidator().validate(pet, result);
    if (result.hasErrors()) {
        return "petForm";
    }
    // ...
}

Or you can have validation invoked automatically by adding the JSR-303 @Valid annotation:

_@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)_
public String processSubmit(**@Valid @ModelAttribute("pet") Pet pet**, BindingResult result) {
    if (result.hasErrors()) {
        return "petForm";
    }
    // ...
}

See Section 8.8, “Spring Validation” and Chapter 8, Validation, Data Binding, and Type Conversion for details on how to configure and use validation.

Using @SessionAttributes to store model attributes in the HTTP session

between requests

The type-level @SessionAttributes annotation declares session attributes used by a specific handler. This will typically list the names of model attributes or types of model attributes which should be transparently stored in the session or some conversational storage, serving as form-backing beans between subsequent requests.

The following code snippet shows the usage of this annotation, specifying the model attribute name:

_@Controller_
_@RequestMapping("/editPet.do")_
**@SessionAttributes("pet")**
public class EditPetForm {
    // ...
}

Working with “application/x-www-form-urlencoded” data

The previous sections covered use of @ModelAttribute to support form submission requests from browser clients. The same annotation is recommended for use with requests from non-browser clients as well. However there is one notable difference when it comes to working with HTTP PUT requests. Browsers can submit form data via HTTP GET or HTTP POST. Non-browser clients can also submit forms via HTTP PUT. This presents a challenge because the Servlet specification requires the ServletRequest.getParameter*() family of methods to support form field access only for HTTP POST, not for HTTP PUT.

To support HTTP PUT and PATCH requests, the spring-web module provides the filter HttpPutFormContentFilter, which can be configured in web.xml:

<filter>
    <filter-name>httpPutFormFilter</filter-name>
    <filter-class>org.springframework.web.filter.HttpPutFormContentFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>httpPutFormFilter</filter-name>
    <servlet-name>dispatcherServlet</servlet-name>
</filter-mapping>
<servlet>
    <servlet-name>dispatcherServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
</servlet>

The above filter intercepts HTTP PUT and PATCH requests with content type application/x-www-form-urlencoded, reads the form data from the body of the request, and wraps the ServletRequest in order to make the form data available through the ServletRequest.getParameter*() family of methods.

	Note

As HttpPutFormContentFilter consumes the body of the request, it should not be configured for PUT or PATCH URLs that rely on other converters for application/x-www-form-urlencoded. This includes @RequestBody MultiValueMap<String, String> and HttpEntity<MultiValueMap<String, String>>.

Mapping cookie values with the @CookieValue annotation

The @CookieValue annotation allows a method parameter to be bound to the value of an HTTP cookie.

Let us consider that the following cookie has been received with an http request:

JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

The following code sample demonstrates how to get the value of the JSESSIONID cookie:

_@RequestMapping("/displayHeaderInfo.do")_
public void displayHeaderInfo(**@CookieValue("JSESSIONID")** String cookie) {
    //...
}

Type conversion is applied automatically if the target method parameter type is not String. See the section called “Method Parameters And Type Conversion”.

This annotation is supported for annotated handler methods in Servlet and Portlet environments.

Mapping request header attributes with the @RequestHeader annotation

The @RequestHeader annotation allows a method parameter to be bound to a request header.

Here is a sample request header:

Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300

The following code sample demonstrates how to get the value of the Accept- Encoding and Keep-Alive headers:

_@RequestMapping("/displayHeaderInfo.do")_
public void displayHeaderInfo(**@RequestHeader("Accept-Encoding")** String encoding,
        **@RequestHeader("Keep-Alive")** long keepAlive) {
    //...
}

Type conversion is applied automatically if the method parameter is not String. See the section called “Method Parameters And Type Conversion”.

When an @RequestHeader annotation is used on a Map<String, String>, MultiValueMap<String, String>, or HttpHeaders argument, the map is populated with all header values.

	Tip

Built-in support is available for converting a comma-separated string into an array/collection of strings or other types known to the type conversion system. For example a method parameter annotated with @RequestHeader("Accept") may be of type String but also String[] or List<String>.

This annotation is supported for annotated handler methods in Servlet and Portlet environments.

Method Parameters And Type Conversion

String-based values extracted from the request including request parameters, path variables, request headers, and cookie values may need to be converted to the target type of the method parameter or field (e.g., binding a request parameter to a field in an @ModelAttribute parameter) they’re bound to. If the target type is not String, Spring automatically converts to the appropriate type. All simple types such as int, long, Date, etc. are supported. You can further customize the conversion process through a WebDataBinder (see the section called “Customizing WebDataBinder initialization”) or by registering Formatters with the FormattingConversionService (see Section 8.6, “Spring Field Formatting”).

Customizing WebDataBinder initialization

To customize request parameter binding with PropertyEditors through Spring’s WebDataBinder, you can use @InitBinder-annotated methods within your controller, @InitBinder methods within an @ControllerAdvice class, or provide a custom WebBindingInitializer. See the the section called “Advising controllers with @ControllerAdvice” section for more details.

Customizing data binding with @InitBinder

Annotating controller methods with @InitBinder allows you to configure web data binding directly within your controller class. @InitBinder identifies methods that initialize the WebDataBinder that will be used to populate command and form object arguments of annotated handler methods.

Such init-binder methods support all arguments that @RequestMapping supports, except for command/form objects and corresponding validation result objects. Init-binder methods must not have a return value. Thus, they are usually declared as void. Typical arguments include WebDataBinder in combination with WebRequest or java.util.Locale, allowing code to register context-specific editors.

The following example demonstrates the use of @InitBinder to configure a CustomDateEditor for all java.util.Date form properties.

_@Controller_
public class MyFormController {
    **@InitBinder**
    public void initBinder(WebDataBinder binder) {
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
        dateFormat.setLenient(false);
        binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
    }
    // ...
}

Alternatively, as of Spring 4.2, consider using addCustomFormatter to specify Formatter implementations instead of PropertyEditor instances. This is particularly useful if you happen to have a Formatter-based setup in a shared FormattingConversionService as well, with the same approach to be reused for controller-specific tweaking of the binding rules.

_@Controller_
public class MyFormController {
    **@InitBinder**
    public void initBinder(WebDataBinder binder) {
        binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
    }
    // ...
}

Configuring a custom WebBindingInitializer

To externalize data binding initialization, you can provide a custom implementation of the WebBindingInitializer interface, which you then enable by supplying a custom bean configuration for an AnnotationMethodHandlerAdapter, thus overriding the default configuration.

The following example from the PetClinic application shows a configuration using a custom implementation of the WebBindingInitializer interface, org.springframework.samples.petclinic.web.ClinicBindingInitializer, which configures PropertyEditors required by several of the PetClinic controllers.

<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
    <property name="cacheSeconds" value="0"/>
    <property name="webBindingInitializer">
        <bean class="org.springframework.samples.petclinic.web.ClinicBindingInitializer"/>
    </property>
</bean>

@InitBinder methods can also be defined in an @ControllerAdvice-annotated class in which case they apply to matching controllers. This provides an alternative to using a WebBindingInitializer. See the the section called “Advising controllers with @ControllerAdvice” section for more details.

Advising controllers with @ControllerAdvice

The @ControllerAdvice annotation is a component annotation allowing implementation classes to be auto-detected through classpath scanning. It is automatically enabled when using the MVC namespace or the MVC Java config.

Classes annotated with @ControllerAdvice can contain @ExceptionHandler, @InitBinder, and @ModelAttribute annotated methods, and these methods will apply to @RequestMapping methods across all controller hierarchies as opposed to the controller hierarchy within which they are declared.

The @ControllerAdvice annotation can also target a subset of controllers with its attributes:

// Target all Controllers annotated with @RestController
_@ControllerAdvice(annotations = RestController.class)_
public class AnnotationAdvice {}
// Target all Controllers within specific packages
_@ControllerAdvice("org.example.controllers")_
public class BasePackageAdvice {}
// Target all Controllers assignable to specific classes
_@ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class})_
public class AssignableTypesAdvice {}

Check out the @ControllerAdvice documentation for more details.

Jackson Serialization View Support

It can sometimes be useful to filter contextually the object that will be serialized to the HTTP response body. In order to provide such capability, Spring MVC has built-in support for rendering with Jackson’s Serialization Views.

To use it with an @ResponseBody controller method or controller methods that return ResponseEntity, simply add the @JsonView annotation with a class argument specifying the view class or interface to be used:

_@RestController_
public class UserController {
    _@RequestMapping(path = "/user", method = RequestMethod.GET)_
    _@JsonView(User.WithoutPasswordView.class)_
    public User getUser() {
        return new User("eric", "7!jd#h23");
    }
}
public class User {
    public interface WithoutPasswordView {};
    public interface WithPasswordView extends WithoutPasswordView {};
    private String username;
    private String password;
    public User() {
    }
    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }
    _@JsonView(WithoutPasswordView.class)_
    public String getUsername() {
        return this.username;
    }
    _@JsonView(WithPasswordView.class)_
    public String getPassword() {
        return this.password;
    }
}

	Note

Note that despite @JsonView allowing for more than one class to be specified, the use on a controller method is only supported with exactly one class argument. Consider the use of a composite interface if you need to enable multiple views.

For controllers relying on view resolution, simply add the serialization view class to the model:

_@Controller_
public class UserController extends AbstractController {
    _@RequestMapping(path = "/user", method = RequestMethod.GET)_
    public String getUser(Model model) {
        model.addAttribute("user", new User("eric", "7!jd#h23"));
        model.addAttribute(JsonView.class.getName(), User.WithoutPasswordView.class);
        return "userView";
    }
}

Jackson JSONP Support

In order to enable JSONP support for @ResponseBody and ResponseEntity methods, declare an @ControllerAdvice bean that extends AbstractJsonpResponseBodyAdvice as shown below where the constructor argument indicates the JSONP query parameter name(s):

_@ControllerAdvice_
public class JsonpAdvice extends AbstractJsonpResponseBodyAdvice {
    public JsonpAdvice() {
        super("callback");
    }
}

For controllers relying on view resolution, JSONP is automatically enabled when the request has a query parameter named jsonp or callback. Those names can be customized through jsonpParameterNames property.

21.3.4 Asynchronous Request Processing

Spring MVC 3.2 introduced Servlet 3 based asynchronous request processing. Instead of returning a value, as usual, a controller method can now return a java.util.concurrent.Callable and produce the return value from a Spring MVC managed thread. Meanwhile the main Servlet container thread is exited and released and allowed to process other requests. Spring MVC invokes the Callable in a separate thread with the help of a TaskExecutor and when the Callable returns, the request is dispatched back to the Servlet container to resume processing using the value returned by the Callable. Here is an example of such a controller method:

_@RequestMapping(method=RequestMethod.POST)_
public Callable<String> processUpload(final MultipartFile file) {
    return new Callable<String>() {
        public String call() throws Exception {
            // ...
            return "someView";
        }
    };
}

Another option is for the controller method to return an instance of DeferredResult. In this case the return value will also be produced from any thread, i.e. one that is not managed by Spring MVC. For example the result may be produced in response to some external event such as a JMS message, a scheduled task, and so on. Here is an example of such a controller method:

_@RequestMapping("/quotes")_
_@ResponseBody_
public DeferredResult<String> quotes() {
    DeferredResult<String> deferredResult = new DeferredResult<String>();
    // Save the deferredResult somewhere..
    return deferredResult;
}
// In some other thread...
deferredResult.setResult(data);

This may be difficult to understand without any knowledge of the Servlet 3.0 asynchronous request processing features. It would certainly help to read up on that. Here are a few basic facts about the underlying mechanism:

• A ServletRequest can be put in asynchronous mode by calling request.startAsync(). The main effect of doing so is that the Servlet, as well as any Filters, can exit but the response will remain open to allow processing to complete later.
• The call to request.startAsync() returns AsyncContext which can be used for further control over async processing. For example it provides the method dispatch, that is similar to a forward from the Servlet API except it allows an application to resume request processing on a Servlet container thread.
• The ServletRequest provides access to the current DispatcherType that can be used to distinguish between processing the initial request, an async dispatch, a forward, and other dispatcher types.

With the above in mind, the following is the sequence of events for async request processing with a Callable:

• Controller returns a Callable.
• Spring MVC starts asynchronous processing and submits the Callable to a TaskExecutor for processing in a separate thread.
• The DispatcherServlet and all Filter’s exit the Servlet container thread but the response remains open.
• The Callable produces a result and Spring MVC dispatches the request back to the Servlet container to resume processing.
• The DispatcherServlet is invoked again and processing resumes with the asynchronously produced result from the Callable.

The sequence for DeferredResult is very similar except it’s up to the application to produce the asynchronous result from any thread:

• Controller returns a DeferredResult and saves it in some in-memory queue or list where it can be accessed.
• Spring MVC starts async processing.
• The DispatcherServlet and all configured Filter’s exit the request processing thread but the response remains open.
• The application sets the DeferredResult from some thread and Spring MVC dispatches the request back to the Servlet container.
• The DispatcherServlet is invoked again and processing resumes with the asynchronously produced result.

For further background on the motivation for async request processing and when or why to use it please read this blog post series.

Exception Handling for Async Requests

What happens if a Callable returned from a controller method raises an Exception while being executed? The short answer is the same as what happens when a controller method raises an exception. It goes through the regular exception handling mechanism. The longer explanation is that when a Callable raises an Exception Spring MVC dispatches to the Servlet container with the Exception as the result and that leads to resume request processing with the Exception instead of a controller method return value. When using a DeferredResult you have a choice whether to call setResult or setErrorResult with an Exception instance.

Intercepting Async Requests

A HandlerInterceptor can also implement AsyncHandlerInterceptor in order to implement the afterConcurrentHandlingStarted callback, which is called instead of postHandle and afterCompletion when asynchronous processing starts.

A HandlerInterceptor can also register a CallableProcessingInterceptor or a DeferredResultProcessingInterceptor in order to integrate more deeply with the lifecycle of an asynchronous request and for example handle a timeout event. See the Javadoc of AsyncHandlerInterceptor for more details.

The DeferredResult type also provides methods such as onTimeout(Runnable) and onCompletion(Runnable). See the Javadoc of DeferredResult for more details.

When using a Callable you can wrap it with an instance of WebAsyncTask which also provides registration methods for timeout and completion.

HTTP Streaming

A controller method can use DeferredResult and Callable to produce its return value asynchronously and that can be used to implement techniques such as long polling where the server can push an event to the client as soon as possible.

What if you wanted to push multiple events on a single HTTP response? This is a technique related to “Long Polling” that is known as “HTTP Streaming”. Spring MVC makes this possible through the ResponseBodyEmitter return value type which can be used to send multiple Objects, instead of one as is normally the case with @ResponseBody, where each Object sent is written to the response with an HttpMessageConverter.

Here is an example of that:

_@RequestMapping("/events")_
public ResponseBodyEmitter handle() {
    ResponseBodyEmitter emitter = new ResponseBodyEmitter();
    // Save the emitter somewhere..
    return emitter;
}
// In some other thread
emitter.send("Hello once");
// and again later on
emitter.send("Hello again");
// and done at some point
emitter.complete();

Note that ResponseBodyEmitter can also be used as the body in a ResponseEntity in order to customize the status and headers of the response.

HTTP Streaming With Server-Sent Events

SseEmitter is a sub-class of ResponseBodyEmitter providing support for Server-Sent Events. Server-sent events is a just another variation on the same “HTTP Streaming” technique except events pushed from the server are formatted according to the W3C Server-Sent Events specification.

Server-Sent Events can be used for their intended purpose, that is to push events from the server to clients. It is quite easy to do in Spring MVC and requires simply returning a value of type SseEmitter.

Note however that Internet Explorer does not support Server-Sent Events and that for more advanced web application messaging scenarios such as online games, collaboration, financial applicatinos, and others it’s better to consider Spring’s WebSocket support that includes SockJS-style WebSocket emulation falling back to a very wide range of browsers (including Internet Explorer) and also higher-level messaging patterns for interacting with clients through a publish-subscribe model within a more messaging-centric architecture. For further background on this see the following blog post.

HTTP Streaming Directly To The OutputStream

ResponseBodyEmitter allows sending events by writing Objects to the response through an HttpMessageConverter. This is probably the most common case, for example when writing JSON data. However sometimes it is useful to bypass message conversion and write directly to the response OutputStream for example for a file download. This can be done with the help of the StreamingResponseBody return value type.

Here is an example of that:

_@RequestMapping("/download")_
public StreamingResponseBody handle() {
    return new StreamingResponseBody() {
        _@Override_
        public void writeTo(OutputStream outputStream) throws IOException {
            // write...
        }
    };
}

Note that StreamingResponseBody can also be used as the body in a ResponseEntity in order to customize the status and headers of the response.

Configuring Asynchronous Request Processing

Servlet Container Configuration

For applications configured with a web.xml be sure to update to version 3.0:

<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            http://java.sun.com/xml/ns/javaee
            http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">
    ...
</web-app>

Asynchronous support must be enabled on the DispatcherServlet through the <async-supported>true</async-supported> web.xml sub-element. Additionally any Filter that participates in asyncrequest processing must be configured to support the ASYNC dispatcher type. It should be safe to enable the ASYNC dispatcher type for all filters provided with the Spring Framework since they usually extend OncePerRequestFilter and that has runtime checks for whether the filter needs to be involved in async dispatches or not.

Below is some example web.xml configuration:

    <web-app xmlns="http://java.sun.com/xml/ns/javaee"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="
                http://java.sun.com/xml/ns/javaee
                http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
        version="3.0">
        <filter>
            <filter-name>Spring OpenEntityManagerInViewFilter</filter-name>
            <filter-class>org.springframework.~.OpenEntityManagerInViewFilter</filter-class>
            <async-supported>true</async-supported>
        </filter>
        <filter-mapping>
            <filter-name>Spring OpenEntityManagerInViewFilter</filter-name>
            <url-pattern>/*</url-pattern>
            <dispatcher>REQUEST</dispatcher>
            <dispatcher>ASYNC</dispatcher>
        </filter-mapping>
    </web-app>

If using Servlet 3, Java based configuration for example via WebApplicationInitializer, you’ll also need to set the “asyncSupported” flag as well as the ASYNC dispatcher type just like with web.xml. To simplify all this configuration, consider extending AbstractDispatcherServletInitializer or AbstractAnnotationConfigDispatcherServletInitializer which automatically set those options and make it very easy to register Filter instances.

Spring MVC Configuration

The MVC Java config and the MVC namespace provide options for configuring asynchronous request processing. WebMvcConfigurer has the method configureAsyncSupport while <mvc:annotation-driven> has an <async- support> sub-element.

Those allow you to configure the default timeout value to use for async requests, which if not set depends on the underlying Servlet container (e.g. 10 seconds on Tomcat). You can also configure an AsyncTaskExecutor to use for executing Callable instances returned from controller methods. It is highly recommended to configure this property since by default Spring MVC uses SimpleAsyncTaskExecutor. The MVC Java config and the MVC namespace also allow you to register CallableProcessingInterceptor and DeferredResultProcessingInterceptor instances.

If you need to override the default timeout value for a specific DeferredResult, you can do so by using the appropriate class constructor. Similarly, for a Callable, you can wrap it in a WebAsyncTask and use the appropriate class constructor to customize the timeout value. The class constructor of WebAsyncTask also allows providing an AsyncTaskExecutor.

21.3.5 Testing Controllers

The spring-test module offers first class support for testing annotated controllers. See Section 14.6, “Spring MVC Test Framework”.

21.4 Handler mappings

In previous versions of Spring, users were required to define one or more HandlerMapping beans in the web application context to map incoming web requests to appropriate handlers. With the introduction of annotated controllers, you generally don’t need to do that because the RequestMappingHandlerMapping automatically looks for @RequestMapping annotations on all @Controller beans. However, do keep in mind that all HandlerMapping classes extending from AbstractHandlerMapping have the following properties that you can use to customize their behavior:

• interceptors List of interceptors to use. HandlerInterceptors are discussed in Section 21.4.1, “Intercepting requests with a HandlerInterceptor”.
• defaultHandler Default handler to use, when this handler mapping does not result in a matching handler.
• order Based on the value of the order property (see the org.springframework.core.Ordered interface), Spring sorts all handler mappings available in the context and applies the first matching handler.
• alwaysUseFullPath If true , Spring uses the full path within the current Servlet context to find an appropriate handler. If false (the default), the path within the current Servlet mapping is used. For example, if a Servlet is mapped using /testing/* and the alwaysUseFullPath property is set to true, /testing/viewPage.html is used, whereas if the property is set to false, /viewPage.html is used.
• urlDecode Defaults to true, as of Spring 2.5. If you prefer to compare encoded paths, set this flag to false. However, the HttpServletRequest always exposes the Servlet path in decoded form. Be aware that the Servlet path will not match when compared with encoded paths.

The following example shows how to configure an interceptor:

<beans>
    <bean id="handlerMapping" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping">
        <property name="interceptors">
            <bean class="example.MyInterceptor"/>
        </property>
    </bean>
<beans>

21.4.1 Intercepting requests with a HandlerInterceptor

Spring’s handler mapping mechanism includes handler interceptors, which are useful when you want to apply specific functionality to certain requests, for example, checking for a principal.

Interceptors located in the handler mapping must implement HandlerInterceptor from the org.springframework.web.servlet package. This interface defines three methods: preHandle(..) is called before the actual handler is executed; postHandle(..) is called after the handler is executed; and afterCompletion(..) is called after the complete request has finished. These three methods should provide enough flexibility to do all kinds of preprocessing and postprocessing.

The preHandle(..) method returns a boolean value. You can use this method to break or continue the processing of the execution chain. When this method returns true, the handler execution chain will continue; when it returns false, the DispatcherServlet assumes the interceptor itself has taken care of requests (and, for example, rendered an appropriate view) and does not continue executing the other interceptors and the actual handler in the execution chain.

Interceptors can be configured using the interceptors property, which is present on all HandlerMapping classes extending from AbstractHandlerMapping. This is shown in the example below:

<beans>
    <bean id="handlerMapping"
            class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping">
        <property name="interceptors">
            <list>
                <ref bean="officeHoursInterceptor"/>
            </list>
        </property>
    </bean>
    <bean id="officeHoursInterceptor"
            class="samples.TimeBasedAccessInterceptor">
        <property name="openingTime" value="9"/>
        <property name="closingTime" value="18"/>
    </bean>
<beans>
package samples;
public class TimeBasedAccessInterceptor extends HandlerInterceptorAdapter {
    private int openingTime;
    private int closingTime;
    public void setOpeningTime(int openingTime) {
        this.openingTime = openingTime;
    }
    public void setClosingTime(int closingTime) {
        this.closingTime = closingTime;
    }
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response,
            Object handler) throws Exception {
        Calendar cal = Calendar.getInstance();
        int hour = cal.get(HOUR_OF_DAY);
        if (openingTime <= hour && hour < closingTime) {
            return true;
        }
        response.sendRedirect("http://host.com/outsideOfficeHours.html");
        return false;
    }
}

Any request handled by this mapping is intercepted by the TimeBasedAccessInterceptor. If the current time is outside office hours, the user is redirected to a static HTML file that says, for example, you can only access the website during office hours.

	Note

When using the RequestMappingHandlerMapping the actual handler is an instance of HandlerMethod which identifies the specific controller method that will be invoked.

As you can see, the Spring adapter class HandlerInterceptorAdapter makes it easier to extend the HandlerInterceptor interface.

	Tip

In the example above, the configured interceptor will apply to all requests handled with annotated controller methods. If you want to narrow down the URL paths to which an interceptor applies, you can use the MVC namespace or the MVC Java config, or declare bean instances of type MappedInterceptor to do that. See Section 21.16.1, “Enabling the MVC Java Config or the MVC XML Namespace”.

Note that the postHandle method of HandlerInterceptor is not always ideally suited for use with @ResponseBody and ResponseEntity methods. In such cases an HttpMessageConverter writes to and commits the response before postHandle is called which makes it impossible to change the response, for example to add a header. Instead an application can implement ResponseBodyAdvice and either declare it as an @ControllerAdvice bean or configure it directly on RequestMappingHandlerAdapter.

21.5 Resolving views

All MVC frameworks for web applications provide a way to address views. Spring provides view resolvers, which enable you to render models in a browser without tying you to a specific view technology. Out of the box, Spring enables you to use JSPs, Velocity templates and XSLT views, for example. See Chapter 22, View technologies for a discussion of how to integrate and use a number of disparate view technologies.

The two interfaces that are important to the way Spring handles views are ViewResolver and View. The ViewResolver provides a mapping between view names and actual views. The View interface addresses the preparation of the request and hands the request over to one of the view technologies.

21.5.1 Resolving views with the ViewResolver interface

As discussed in Section 21.3, “Implementing Controllers”, all handler methods in the Spring Web MVC controllers must resolve to a logical view name, either explicitly (e.g., by returning a String, View, or ModelAndView) or implicitly (i.e., based on conventions). Views in Spring are addressed by a logical view name and are resolved by a view resolver. Spring comes with quite a few view resolvers. This table lists most of them; a couple of examples follow.

Table 21.3. View resolvers

AbstractCachingViewResolver

|

Abstract view resolver that caches views. Often views need preparation before they can be used; extending this view resolver provides caching.

XmlViewResolver

|

Implementation of ViewResolver that accepts a configuration file written in XML with the same DTD as Spring’s XML bean factories. The default configuration file is /WEB-INF/views.xml.

ResourceBundleViewResolver

|

Implementation of ViewResolver that uses bean definitions in a ResourceBundle, specified by the bundle base name. Typically you define the bundle in a properties file, located in the classpath. The default file name is views.properties.

UrlBasedViewResolver

|

Simple implementation of the ViewResolver interface that effects the direct resolution of logical view names to URLs, without an explicit mapping definition. This is appropriate if your logical names match the names of your view resources in a straightforward manner, without the need for arbitrary mappings.

InternalResourceViewResolver

|

Convenient subclass of UrlBasedViewResolver that supports InternalResourceView (in effect, Servlets and JSPs) and subclasses such as JstlView and TilesView. You can specify the view class for all views generated by this resolver by using setViewClass(..). See the UrlBasedViewResolver javadocs for details.

VelocityViewResolver / FreeMarkerViewResolver

|

Convenient subclass of UrlBasedViewResolver that supports VelocityView (in effect, Velocity templates) or FreeMarkerView ,respectively, and custom subclasses of them.

ContentNegotiatingViewResolver

|

Implementation of the ViewResolver interface that resolves a view based on the request file name or Accept header. See Section 21.5.4, “ContentNegotiatingViewResolver”.

As an example, with JSP as a view technology, you can use the UrlBasedViewResolver. This view resolver translates a view name to a URL and hands the request over to the RequestDispatcher to render the view.

<bean id="viewResolver"
        class="org.springframework.web.servlet.view.UrlBasedViewResolver">
    <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
    <property name="prefix" value="/WEB-INF/jsp/"/>
    <property name="suffix" value=".jsp"/>
</bean>

When returning test as a logical view name, this view resolver forwards the request to the RequestDispatcher that will send the request to /WEB- INF/jsp/test.jsp.

When you combine different view technologies in a web application, you can use the ResourceBundleViewResolver:

<bean id="viewResolver"
        class="org.springframework.web.servlet.view.ResourceBundleViewResolver">
    <property name="basename" value="views"/>
    <property name="defaultParentView" value="parentView"/>
</bean>

The ResourceBundleViewResolver inspects the ResourceBundle identified by the basename, and for each view it is supposed to resolve, it uses the value of the property [viewname].(class) as the view class and the value of the property [viewname].url as the view url. Examples can be found in the next chapter which covers view technologies. As you can see, you can identify a parent view, from which all views in the properties file “extend”. This way you can specify a default view class, for example.

	Note

Subclasses of AbstractCachingViewResolver cache view instances that they resolve. Caching improves performance of certain view technologies. It’s possible to turn off the cache by setting the cache property to false. Furthermore, if you must refresh a certain view at runtime (for example when a Velocity template is modified), you can use the removeFromCache(String viewName, Locale loc) method.

21.5.2 Chaining ViewResolvers

Spring supports multiple view resolvers. Thus you can chain resolvers and, for example, override specific views in certain circumstances. You chain view resolvers by adding more than one resolver to your application context and, if necessary, by setting the order property to specify ordering. Remember, the higher the order property, the later the view resolver is positioned in the chain.

In the following example, the chain of view resolvers consists of two resolvers, an InternalResourceViewResolver, which is always automatically positioned as the last resolver in the chain, and an XmlViewResolver for specifying Excel views. Excel views are not supported by the InternalResourceViewResolver.

<bean id="jspViewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
    <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
    <property name="prefix" value="/WEB-INF/jsp/"/>
    <property name="suffix" value=".jsp"/>
</bean>
<bean id="excelViewResolver" class="org.springframework.web.servlet.view.XmlViewResolver">
    <property name="order" value="1"/>
    <property name="location" value="/WEB-INF/views.xml"/>
</bean>
<!-- in views.xml -->
<beans>
    <bean name="report" class="org.springframework.example.ReportExcelView"/>
</beans>

If a specific view resolver does not result in a view, Spring examines the context for other view resolvers. If additional view resolvers exist, Spring continues to inspect them until a view is resolved. If no view resolver returns a view, Spring throws a ServletException.

The contract of a view resolver specifies that a view resolver can return null to indicate the view could not be found. Not all view resolvers do this, however, because in some cases, the resolver simply cannot detect whether or not the view exists. For example, the InternalResourceViewResolver uses the RequestDispatcher internally, and dispatching is the only way to figure out if a JSP exists, but this action can only execute once. The same holds for the VelocityViewResolver and some others. Check the javadocs of the specific view resolver to see whether it reports non-existing views. Thus, putting an InternalResourceViewResolver in the chain in a place other than the last results in the chain not being fully inspected, because the InternalResourceViewResolver will always return a view!

21.5.3 Redirecting to Views

As mentioned previously, a controller typically returns a logical view name, which a view resolver resolves to a particular view technology. For view technologies such as JSPs that are processed through the Servlet or JSP engine, this resolution is usually handled through the combination of InternalResourceViewResolver and InternalResourceView, which issues an internal forward or include via the Servlet API’s RequestDispatcher.forward(..) method or RequestDispatcher.include() method. For other view technologies, such as Velocity, XSLT, and so on, the view itself writes the content directly to the response stream.

It is sometimes desirable to issue an HTTP redirect back to the client, before the view is rendered. This is desirable, for example, when one controller has been called with POST data, and the response is actually a delegation to another controller (for example on a successful form submission). In this case, a normal internal forward will mean that the other controller will also see the same POST data, which is potentially problematic if it can confuse it with other expected data. Another reason to perform a redirect before displaying the result is to eliminate the possibility of the user submitting the form data multiple times. In this scenario, the browser will first send an initial POST; it will then receive a response to redirect to a different URL; and finally the browser will perform a subsequent GET for the URL named in the redirect response. Thus, from the perspective of the browser, the current page does not reflect the result of a POST but rather of a GET. The end effect is that there is no way the user can accidentally re- POST the same data by performing a refresh. The refresh forces a GET of the result page, not a resend of the initial POST data.

RedirectView

One way to force a redirect as the result of a controller response is for the controller to create and return an instance of Spring’s RedirectView. In this case, DispatcherServlet does not use the normal view resolution mechanism. Rather because it has been given the (redirect) view already, the DispatcherServlet simply instructs the view to do its work. The RedirectView in turn calls HttpServletResponse.sendRedirect() to send an HTTP redirect to the client browser.

If you use RedirectView and the view is created by the controller itself, it is recommended that you configure the redirect URL to be injected into the controller so that it is not baked into the controller but configured in the context along with the view names. The the section called “The redirect: prefix” facilitates this decoupling.

Passing Data To the Redirect Target

By default all model attributes are considered to be exposed as URI template variables in the redirect URL. Of the remaining attributes those that are primitive types or collections/arrays of primitive types are automatically appended as query parameters.

Appending primitive type attributes as query parameters may be the desired result if a model instance was prepared specifically for the redirect. However, in annotated controllers the model may contain additional attributes added for rendering purposes (e.g. drop-down field values). To avoid the possibility of having such attributes appear in the URL, an @RequestMapping method can declare an argument of type RedirectAttributes and use it to specify the exact attributes to make available to RedirectView. If the method does redirect, the content of RedirectAttributes is used. Otherwise the content of the model is used.

The RequestMappingHandlerAdapter provides a flag called "ignoreDefaultModelOnRedirect" that can be used to indicate the content of the default Model should never be used if a controller method redirects. Instead the controller method should declare an attribute of type RedirectAttributes or if it doesn’t do so no attributes should be passed on to RedirectView. Both the MVC namespace and the MVC Java config keep this flag set to false in order to maintain backwards compatibility. However, for new applications we recommend setting it to true

Note that URI template variables from the present request are automatically made available when expanding a redirect URL and do not need to be added explicitly neither through Model nor RedirectAttributes. For example:

_@RequestMapping(path = "/files/{path}", method = RequestMethod.POST)_
public String upload(...) {
    // ...
    return "redirect:files/{path}";
}

Another way of passing data to the redirect target is via Flash Attributes. Unlike other redirect attributes, flash attributes are saved in the HTTP session (and hence do not appear in the URL). See Section 21.6, “Using flash attributes” for more information.

The redirect: prefix

While the use of RedirectView works fine, if the controller itself creates the RedirectView, there is no avoiding the fact that the controller is aware that a redirection is happening. This is really suboptimal and couples things too tightly. The controller should not really care about how the response gets handled. In general it should operate only in terms of view names that have been injected into it.

The special redirect: prefix allows you to accomplish this. If a view name is returned that has the prefix redirect:, the UrlBasedViewResolver (and all subclasses) will recognize this as a special indication that a redirect is needed. The rest of the view name will be treated as the redirect URL.

The net effect is the same as if the controller had returned a RedirectView, but now the controller itself can simply operate in terms of logical view names. A logical view name such as redirect:/myapp/some/resource will redirect relative to the current Servlet context, while a name such as redirect:http://myhost.com/some/arbitrary/path will redirect to an absolute URL.

Note that the controller handler is annotated with the @ResponseStatus, the annotation value takes precedence over the response status set by RedirectView.

The forward: prefix

It is also possible to use a special forward: prefix for view names that are ultimately resolved by UrlBasedViewResolver and subclasses. This creates an InternalResourceView (which ultimately does a RequestDispatcher.forward()) around the rest of the view name, which is considered a URL. Therefore, this prefix is not useful with InternalResourceViewResolver and InternalResourceView (for JSPs for example). But the prefix can be helpful when you are primarily using another view technology, but still want to force a forward of a resource to be handled by the Servlet/JSP engine. (Note that you may also chain multiple view resolvers, instead.)

As with the redirect: prefix, if the view name with the forward: prefix is injected into the controller, the controller does not detect that anything special is happening in terms of handling the response.

21.5.4 ContentNegotiatingViewResolver

The ContentNegotiatingViewResolver does not resolve views itself but rather delegates to other view resolvers, selecting the view that resembles the representation requested by the client. Two strategies exist for a client to request a representation from the server:

• Use a distinct URI for each resource, typically by using a different file extension in the URI. For example, the URI <http://www.example.com/users/fred.pdf> requests a PDF representation of the user fred, and <http://www.example.com/users/fred.xml> requests an XML representation.
• Use the same URI for the client to locate the resource, but set the Accept HTTP request header to list the media types that it understands. For example, an HTTP request for <http://www.example.com/users/fred> with an Accept header set to application/pdf requests a PDF representation of the user fred, while <http://www.example.com/users/fred> with an Accept header set to text/xml requests an XML representation. This strategy is known as content negotiation.

	Note

One issue with the Accept header is that it is impossible to set it in a web browser within HTML. For example, in Firefox, it is fixed to:

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

For this reason it is common to see the use of a distinct URI for each representation when developing browser based web applications.

To support multiple representations of a resource, Spring provides the ContentNegotiatingViewResolver to resolve a view based on the file extension or Accept header of the HTTP request. ContentNegotiatingViewResolver does not perform the view resolution itself but instead delegates to a list of view resolvers that you specify through the bean property ViewResolvers.

The ContentNegotiatingViewResolver selects an appropriate View to handle the request by comparing the request media type(s) with the media type (also known as Content-Type) supported by the View associated with each of its ViewResolvers. The first View in the list that has a compatible Content- Type returns the representation to the client. If a compatible view cannot be supplied by the ViewResolver chain, then the list of views specified through the DefaultViews property will be consulted. This latter option is appropriate for singleton Views that can render an appropriate representation of the current resource regardless of the logical view name. The Accept header may include wild cards, for example text/*, in which case a View whose Content-Type was text/xml is a compatible match.

To support custom resolution of a view based on a file extension, use a ContentNegotiationManager: see Section 21.16.6, “Content Negotiation”.

Here is an example configuration of a ContentNegotiatingViewResolver:

<bean class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver">
    <property name="viewResolvers">
        <list>
            <bean class="org.springframework.web.servlet.view.BeanNameViewResolver"/>
            <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
                <property name="prefix" value="/WEB-INF/jsp/"/>
                <property name="suffix" value=".jsp"/>
            </bean>
        </list>
    </property>
    <property name="defaultViews">
        <list>
            <bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
        </list>
    </property>
</bean>
<bean id="content" class="com.foo.samples.rest.SampleContentAtomView"/>

The InternalResourceViewResolver handles the translation of view names and JSP pages, while the BeanNameViewResolver returns a view based on the name of a bean. (See “[Resolving views with the ViewResolver interface]($mvc.html

mvc-viewresolver-resolver “21.5.1 Resolving views with the ViewResolver

interface” )” for more details on how Spring looks up and instantiates a view.) In this example, the content bean is a class that inherits from AbstractAtomFeedView, which returns an Atom RSS feed. For more information on creating an Atom Feed representation, see the section Atom Views.

In the above configuration, if a request is made with an .html extension, the view resolver looks for a view that matches the text/html media type. The InternalResourceViewResolver provides the matching view for text/html. If the request is made with the file extension .atom, the view resolver looks for a view that matches the application/atom+xml media type. This view is provided by the BeanNameViewResolver that maps to the SampleContentAtomView if the view name returned is content. If the request is made with the file extension .json, the MappingJackson2JsonView instance from the DefaultViews list will be selected regardless of the view name. Alternatively, client requests can be made without a file extension but with the Accept header set to the preferred media-type, and the same resolution of request to views would occur.

	Note

If `ContentNegotiatingViewResolver’s list of ViewResolvers is not configured explicitly, it automatically uses any ViewResolvers defined in the application context.

The corresponding controller code that returns an Atom RSS feed for a URI of the form <http://localhost/content.atom> or <http://localhost/content> with an Accept header of application/atom+xml is shown below.

_@Controller_
public class ContentController {
    private List<SampleContent> contentList = new ArrayList<SampleContent>();
    _@RequestMapping(path="/content", method=RequestMethod.GET)_
    public ModelAndView getContent() {
        ModelAndView mav = new ModelAndView();
        mav.setViewName("content");
        mav.addObject("sampleContentList", contentList);
        return mav;
    }
}

21.6 Using flash attributes

Flash attributes provide a way for one request to store attributes intended for use in another. This is most commonly needed when redirecting — for example, the Post/Redirect/Get pattern. Flash attributes are saved temporarily before the redirect (typically in the session) to be made available to the request after the redirect and removed immediately.

Spring MVC has two main abstractions in support of flash attributes. FlashMap is used to hold flash attributes while FlashMapManager is used to store, retrieve, and manage FlashMap instances.

Flash attribute support is always “on” and does not need to enabled explicitly although if not used, it never causes HTTP session creation. On each request there is an “input” FlashMap with attributes passed from a previous request (if any) and an “output” FlashMap with attributes to save for a subsequent request. Both FlashMap instances are accessible from anywhere in Spring MVC through static methods in RequestContextUtils.

Annotated controllers typically do not need to work with FlashMap directly. Instead an @RequestMapping method can accept an argument of type RedirectAttributes and use it to add flash attributes for a redirect scenario. Flash attributes added via RedirectAttributes are automatically propagated to the “output” FlashMap. Similarly, after the redirect, attributes from the “input” FlashMap are automatically added to the Model of the controller serving the target URL.

Matching requests to flash attributes

The concept of flash attributes exists in many other Web frameworks and has proven to be exposed sometimes to concurrency issues. This is because by definition flash attributes are to be stored until the next request. However the very “next” request may not be the intended recipient but another asynchronous request (e.g. polling or resource requests) in which case the flash attributes are removed too early.

To reduce the possibility of such issues, RedirectView automatically “stamps” FlashMap instances with the path and query parameters of the target redirect URL. In turn the default FlashMapManager matches that information to incoming requests when looking up the “input” FlashMap.

This does not eliminate the possibility of a concurrency issue entirely but nevertheless reduces it greatly with information that is already available in the redirect URL. Therefore the use of flash attributes is recommended mainly for redirect scenarios .

21.7 Building URIs

Spring MVC provides a mechanism for building and encoding a URI using UriComponentsBuilder and UriComponents.

For example you can expand and encode a URI template string:

UriComponents uriComponents = UriComponentsBuilder.fromUriString(
        "http://example.com/hotels/{hotel}/bookings/{booking}").build();
URI uri = uriComponents.expand("42", "21").encode().toUri();

Note that UriComponents is immutable and the expand() and encode() operations return new instances if necessary.

You can also expand and encode using individual URI components:

UriComponents uriComponents = UriComponentsBuilder.newInstance()
        .scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build()
        .expand("42", "21")
        .encode();

In a Servlet environment the ServletUriComponentsBuilder sub-class provides static factory methods to copy available URL information from a Servlet requests:

HttpServletRequest request = ...
// Re-use host, scheme, port, path and query string
// Replace the "accountId" query param
ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromRequest(request)
        .replaceQueryParam("accountId", "{id}").build()
        .expand("123")
        .encode();

Alternatively, you may choose to copy a subset of the available information up to and including the context path:

// Re-use host, port and context path
// Append "/accounts" to the path
ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromContextPath(request)
        .path("/accounts").build()

Or in cases where the DispatcherServlet is mapped by name (e.g. /main/*), you can also have the literal part of the servlet mapping included:

// Re-use host, port, context path
// Append the literal part of the servlet mapping to the path
// Append "/accounts" to the path
ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromServletMapping(request)
        .path("/accounts").build()

21.7.1 Building URIs to Controllers and methods

Spring MVC also provides a mechanism for building links to controller methods. For example, given:

_@Controller_
_@RequestMapping("/hotels/{hotel}")_
public class BookingController {
    _@RequestMapping("/bookings/{booking}")_
    public String getBooking(_@PathVariable_ Long booking) {
    // ...
}

You can prepare a link by referring to the method by name:

UriComponents uriComponents = MvcUriComponentsBuilder
    .fromMethodName(BookingController.class, "getBooking", 21).buildAndExpand(42);
URI uri = uriComponents.encode().toUri();

In the above example we provided actual method argument values, in this case the long value 21, to be used as a path variable and inserted into the URL. Furthermore, we provided the value 42 in order to fill in any remaining URI variables such as the “hotel” variable inherited from the type-level request mapping. If the method had more arguments you can supply null for arguments not needed for the URL. In general only @PathVariable and @RequestParam arguments are relevant for constructing the URL.

There are additional ways to use MvcUriComponentsBuilder. For example you can use a technique akin to mock testing through proxies to avoid referring to the controller method by name (the example assumes static import of MvcUriComponentsBuilder.on):

UriComponents uriComponents = MvcUriComponentsBuilder
    .fromMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);
URI uri = uriComponents.encode().toUri();

The above examples use static methods in MvcUriComponentsBuilder. Internally they rely on ServletUriComponentsBuilder to prepare a base URL from the scheme, host, port, context path and servlet path of the current request. This works well in most cases, however sometimes it may be insufficient. For example you may be outside the context of a request (e.g. a batch process that prepares links) or perhaps you need to insert a path prefix (e.g. a locale prefix that was removed from the request path and needs to be re-inserted into links).

For such cases you can use the static “fromXxx” overloaded methods that accept a UriComponentsBuilder to use base URL. Or you can create an instance of MvcUriComponentsBuilder with a base URL and then use the instance-based “withXxx” methods. For example:

UriComponentsBuilder base = ServletUriComponentsBuilder.fromCurrentContextPath().path("/en");
MvcUriComponentsBuilder builder = MvcUriComponentsBuilder.relativeTo(base);
builder.withMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);
URI uri = uriComponents.encode().toUri();

21.7.2 Building URIs to Controllers and methods from views

You can also build links to annotated controllers from views such as JSP, Thymeleaf, FreeMarker. This can be done using the fromMappingName method in MvcUriComponentsBuilder which refers to mappings by name.

Every @RequestMapping is assigned a default name based on the capital letters of the class and the full method name. For example, the method getFoo in class FooController is assigned the name “FC#getFoo”. This strategy can be replaced or customized by creating an instance of HandlerMethodMappingNamingStrategy and plugging it into your RequestMappingHandlerMapping. The default strategy implementation also looks at the name attribute on @RequestMapping and uses that if present. That means if the default mapping name assigned conflicts with another (e.g. overloaded methods) you can assign a name explicitly on the @RequestMapping.

	Note

The assigned request mapping names are logged at TRACE level on startup.

The Spring JSP tag library provides a function called mvcUrl that can be used to prepare links to controller methods based on this mechanism.

For example given:

_@RequestMapping("/people/{id}/addresses")_
public class PersonAddressController {
    _@RequestMapping("/{country}")_
    public HttpEntity getAddress(_@PathVariable_ String country) { ... }
}

You can prepare a link from a JSP as follows:

<%@ taglib uri="http://www.springframework.org/tags" prefix="s" %>
...
<a href="${s:mvcUrl(''PAC#getAddress'').arg(0,''US'').buildAndExpand(''123'')}">Get Address</a>

The above example relies on the mvcUrl JSP function declared in the Spring tag library (i.e. META-INF/spring.tld). For more advanced cases (e.g. a custom base URL as explained in the previous section), it is easy to define your own function, or use a custom tag file, in order to use a specific instance of MvcUriComponentsBuilder with a custom base URL.

21.8 Using locales

Most parts of Spring’s architecture support internationalization, just as the Spring web MVC framework does. DispatcherServlet enables you to automatically resolve messages using the client’s locale. This is done with LocaleResolver objects.

When a request comes in, the DispatcherServlet looks for a locale resolver, and if it finds one it tries to use it to set the locale. Using the RequestContext.getLocale() method, you can always retrieve the locale that was resolved by the locale resolver.

In addition to automatic locale resolution, you can also attach an interceptor to the handler mapping (see Section 21.4.1, “Intercepting requests with a HandlerInterceptor” for more information on handler mapping interceptors) to change the locale under specific circumstances, for example, based on a parameter in the request.

Locale resolvers and interceptors are defined in the org.springframework.web.servlet.i18n package and are configured in your application context in the normal way. Here is a selection of the locale resolvers included in Spring.

21.8.1 Obtaining Time Zone Information

In addition to obtaining the client’s locale, it is often useful to know their time zone. The LocaleContextResolver interface offers an extension to LocaleResolver that allows resolvers to provide a richer LocaleContext, which may include time zone information.

When available, the user’s TimeZone can be obtained using the RequestContext.getTimeZone() method. Time zone information will automatically be used by Date/Time Converter and Formatter objects registered with Spring’s ConversionService.

21.8.2 AcceptHeaderLocaleResolver

This locale resolver inspects the accept-language header in the request that was sent by the client (e.g., a web browser). Usually this header field contains the locale of the client’s operating system. Note that this resolver does not support time zone information.

21.8.3 CookieLocaleResolver

This locale resolver inspects a Cookie that might exist on the client to see if a Locale or TimeZone is specified. If so, it uses the specified details. Using the properties of this locale resolver, you can specify the name of the cookie as well as the maximum age. Find below an example of defining a CookieLocaleResolver.

<bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver">
    <property name="cookieName" value="clientlanguage"/>
    <!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) -->
    <property name="cookieMaxAge" value="100000">
</bean>

Table 21.4. CookieLocaleResolver properties

cookieName

|

classname + LOCALE

|

The name of the cookie

cookieMaxAge

|

Integer.MAX_INT

|

The maximum time a cookie will stay persistent on the client. If -1 is specified, the cookie will not be persisted; it will only be available until the client shuts down their browser.

cookiePath

|

/

|

Limits the visibility of the cookie to a certain part of your site. When cookiePath is specified, the cookie will only be visible to that path and the paths below it.

21.8.4 SessionLocaleResolver

The SessionLocaleResolver allows you to retrieve Locale and TimeZone from the session that might be associated with the user’s request. In contrast to CookieLocaleResolver, this strategy stores locally chosen locale settings in the Servlet container’s HttpSession. As a consequence, those settings are just temporary for each session and therefore lost when each session terminates.

Note that there is no direct relationship with external session management mechanisms such as the Spring Session project. This SessionLocaleResolver will simply evaluate and modify corresponding HttpSession attributes against the current HttpServletRequest.

21.8.5 LocaleChangeInterceptor

You can enable changing of locales by adding the LocaleChangeInterceptor to one of the handler mappings (see [Section 21.4, “Handler mappings”]($mvc.html

mvc-handlermapping “21.4 Handler mappings” )). It will detect a parameter in

the request and change the locale. It calls setLocale() on the LocaleResolver that also exists in the context. The following example shows that calls to all *.view resources containing a parameter named siteLanguage will now change the locale. So, for example, a request for the following URL, <http://www.sf.net/home.view?siteLanguage=nl> will change the site language to Dutch.

<bean id="localeChangeInterceptor"
        class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">
    <property name="paramName" value="siteLanguage"/>
</bean>
<bean id="localeResolver"
        class="org.springframework.web.servlet.i18n.CookieLocaleResolver"/>
<bean id="urlMapping"
        class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
    <property name="interceptors">
        <list>
            <ref bean="localeChangeInterceptor"/>
        </list>
    </property>
    <property name="mappings">
        <value>/**/*.view=someController</value>
    </property>
</bean>

21.9 Using themes

21.9.1 Overview of themes

You can apply Spring Web MVC framework themes to set the overall look-and-feel of your application, thereby enhancing user experience. A theme is a collection of static resources, typically style sheets and images, that affect the visual style of the application.

21.9.2 Defining themes

To use themes in your web application, you must set up an implementation of the org.springframework.ui.context.ThemeSource interface. The WebApplicationContext interface extends ThemeSource but delegates its responsibilities to a dedicated implementation. By default the delegate will be an org.springframework.ui.context.support.ResourceBundleThemeSource implementation that loads properties files from the root of the classpath. To use a custom ThemeSource implementation or to configure the base name prefix of the ResourceBundleThemeSource, you can register a bean in the application context with the reserved name themeSource. The web application context automatically detects a bean with that name and uses it.

When using the ResourceBundleThemeSource, a theme is defined in a simple properties file. The properties file lists the resources that make up the theme. Here is an example:

styleSheet=/themes/cool/style.css
background=/themes/cool/img/coolBg.jpg

The keys of the properties are the names that refer to the themed elements from view code. For a JSP, you typically do this using the spring:theme custom tag, which is very similar to the spring:message tag. The following JSP fragment uses the theme defined in the previous example to customize the look and feel:

<%@ taglib prefix="spring" uri="http://www.springframework.org/tags"%>
<html>
    <head>
        <link rel="stylesheet" href="<spring:theme code=''styleSheet''/>" type="text/css"/>
    </head>
    <body style="background=<spring:theme code=''background''/>">
        ...
    </body>
</html>

By default, the ResourceBundleThemeSource uses an empty base name prefix. As a result, the properties files are loaded from the root of the classpath. Thus you would put the cool.properties theme definition in a directory at the root of the classpath, for example, in /WEB-INF/classes. The ResourceBundleThemeSource uses the standard Java resource bundle loading mechanism, allowing for full internationalization of themes. For example, we could have a /WEB-INF/classes/cool_nl.properties that references a special background image with Dutch text on it.

21.9.3 Theme resolvers

After you define themes, as in the preceding section, you decide which theme to use. The DispatcherServlet will look for a bean named themeResolver to find out which ThemeResolver implementation to use. A theme resolver works in much the same way as a LocaleResolver. It detects the theme to use for a particular request and can also alter the request’s theme. The following theme resolvers are provided by Spring:

Table 21.5. ThemeResolver implementations

FixedThemeResolver

|

Selects a fixed theme, set using the defaultThemeName property.

SessionThemeResolver

|

The theme is maintained in the user’s HTTP session. It only needs to be set once for each session, but is not persisted between sessions.

CookieThemeResolver

|

The selected theme is stored in a cookie on the client.

Spring also provides a ThemeChangeInterceptor that allows theme changes on every request with a simple request parameter.

21.10 Spring’s multipart (file upload) support

21.10.1 Introduction

Spring’s built-in multipart support handles file uploads in web applications. You enable this multipart support with pluggable MultipartResolver objects, defined in the org.springframework.web.multipart package. Spring provides one MultipartResolver implementation for use with Commons FileUpload and another for use with Servlet 3.0 multipart request parsing.

By default, Spring does no multipart handling, because some developers want to handle multiparts themselves. You enable Spring multipart handling by adding a multipart resolver to the web application’s context. Each request is inspected to see if it contains a multipart. If no multipart is found, the request continues as expected. If a multipart is found in the request, the MultipartResolver that has been declared in your context is used. After that, the multipart attribute in your request is treated like any other attribute.

21.10.2 Using a MultipartResolver with Commons FileUpload

The following example shows how to use the CommonsMultipartResolver:

<bean id="multipartResolver"
        class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
    <!-- one of the properties available; the maximum file size in bytes -->
    <property name="maxUploadSize" value="100000"/>
</bean>