[作者简介] 魏民,信息部售后组研发工程师

基本概念

Wiki 中是这样描述内省的:

在计算机科学中,内省是指计算机程序在运行时(Run time)检查对象(Object)类型的一种能力,通常也可以称作运行时类型检查。

这个描述非常宽泛,但有三个关键词:

  • 运行时
  • 对象
  • 类型

Java 官方对 Java Beans 内省的定义:

At runtime and in the builder environment we need to be able to figure out which properties, events, and methods a Java Bean supports. We call this process introspection.

从 Java Bean 的角度来看,这里的对象就是 Bean 对象,主要关注点是属性、方法和事件等,也就是说在运行时可以获取相应的信息进行一些处理,这就是 Java Beans 的内省机制。

与反射的区别

Java Beans 内省其实就是对反射的一种封装,这个从源码中或者官方文档中都能看到:

By default we will use a low level reflection mechanism to study the methods supported by a target bean and then apply simple design patterns to deduce from those methods what properties, events, and public methods are supported.

Java Beans 内省机制

核心类库

Java Beans 内省机制的核心类是 Introspector

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* The Introspector class provides a standard way for tools to learn about* the properties, events, and methods supported by a target Java Bean.

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操作范围主要包括但不局限于 Java Beans 的属性,事件和方法,具体是基于以下几个类实现:

  • BeanInfo

    • Java Bean 信息类
  • PropertyDescriptor

    • 属性描述类
  • MethodDescriptor

    • 方法描述类
  • EventSetDescriptor

    • 事件描述集合

先看一个示例:

定义一个 Java Bean:

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public class User {    private String username;    private Integer age;

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测试代码:

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@Testpublic void test1() throws IntrospectionException {        BeanInfo userBeanInfo = Introspector.getBeanInfo(User.class);        PropertyDescriptor[] propertyDescriptors = userBeanInfo.getPropertyDescriptors();    System.out.println("属性描述:");    Stream.of(propertyDescriptors).forEach(System.out::println);        System.out.println("方法描述:");    MethodDescriptor[] methodDescriptors = userBeanInfo.getMethodDescriptors();    Stream.of(methodDescriptors).forEach(System.out::println);        System.out.println("事件描述:");    EventSetDescriptor[] eventSetDescriptors = userBeanInfo.getEventSetDescriptors();    Stream.of(eventSetDescriptors).forEach(System.out::println);}

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输出结果:

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属性描述:java.beans.PropertyDescriptor[name=age; propertyType=class java.lang.Integer; readMethod=public java.lang.Integer introspector.bean.User.getAge(); writeMethod=public void introspector.bean.User.setAge(java.lang.Integer)]java.beans.PropertyDescriptor[name=class; propertyType=class java.lang.Class; readMethod=public final native java.lang.Class java.lang.Object.getClass()]java.beans.PropertyDescriptor[name=username; propertyType=class java.lang.String; readMethod=public java.lang.String introspector.bean.User.getUsername(); writeMethod=public void introspector.bean.User.setUsername(java.lang.String)]方法描述:java.beans.MethodDescriptor[name=getClass; method=public final native java.lang.Class java.lang.Object.getClass()]java.beans.MethodDescriptor[name=setAge; method=public void introspector.bean.User.setAge(java.lang.Integer)]java.beans.MethodDescriptor[name=getAge; method=public java.lang.Integer introspector.bean.User.getAge()]java.beans.MethodDescriptor[name=wait; method=public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException]java.beans.MethodDescriptor[name=notifyAll; method=public final native void java.lang.Object.notifyAll()]java.beans.MethodDescriptor[name=notify; method=public final native void java.lang.Object.notify()]java.beans.MethodDescriptor[name=getUsername; method=public java.lang.String introspector.bean.User.getUsername()]java.beans.MethodDescriptor[name=wait; method=public final void java.lang.Object.wait() throws java.lang.InterruptedException]java.beans.MethodDescriptor[name=hashCode; method=public native int java.lang.Object.hashCode()]java.beans.MethodDescriptor[name=setUsername; method=public void introspector.bean.User.setUsername(java.lang.String)]java.beans.MethodDescriptor[name=wait; method=public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException]java.beans.MethodDescriptor[name=equals; method=public boolean java.lang.Object.equals(java.lang.Object)]java.beans.MethodDescriptor[name=toString; method=public java.lang.String introspector.bean.User.toString()]事件描述:

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可以看出通过内省机制可以获取 Java Bean 的属性、方法描述,这里事件描述是空的(关于事件相关会在后面介绍)。由于 Java 类都会继承 Object 类,可以看到这里将 Object 类相关的属性和方法描述也输出了,如果想将某个类的描述信息排除可以使用 java.beans.Introspector#getBeanInfo(java.lang.Class<?>, java.lang.Class<?>) 这个方法。

属性处理

配置绑定

通过 PropertyDescriptor 可以基于字段名为可写属性设置值。

比如我们经常会使用这样的配置文件:

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user:  username: zhangsan  age: 1

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配置文件会与对象进行数据绑定。测试代码:

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@Testpublic void test2() throws IntrospectionException {    YamlPropertiesFactoryBean yaml = new YamlPropertiesFactoryBean();    yaml.setResources(new ClassPathResource("application.yml"));    String path = "user.";    Properties properties = yaml.getObject();    System.out.println(properties);    User user = new User();

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输出结果:

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User{username='zhangsan', age=1}

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在 Spring 中的使用

在传统的 Spring 开发中我们需要在 web.xml 中指定一些配置参数,比如:

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<servlet>    <servlet-name>app</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>

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这里有一个 contextConfigLocation 参数,这个参数最终是与 FrameworkServlet 类中的一个属性进行绑定:

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public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {  private String contextConfigLocation;}

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那么 Spring 是如何将 web.xml 中的配置项与属性进行绑定的呢,可以参数看org.springframework.web.servlet.HttpServletBean#init() 方法:

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@Overridepublic final void init() throws ServletException {    PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);  if (!pvs.isEmpty()) {    try {      BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);      ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());      bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));      initBeanWrapper(bw);      bw.setPropertyValues(pvs, true);    }    catch (BeansException ex) {      if (logger.isErrorEnabled()) {        logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);      }      throw ex;    }  }    initServletBean();}

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可以看到 Spring 是通过 BeanWrapper 完成对属性的绑定:

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public interface BeanWrapper extends ConfigurablePropertyAccessor {

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BeanWrapper 又继承了 PropertyAccessor 接口:

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public interface PropertyAccessor {

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也就是说 Spring 中 BeanWrapper 基于 Java 的内省机制实现了对属性的赋值工作,但是 Spring 并未局限于 Java 提供的 API,而是也进行了扩展和进一步的封装,如 TypeDescriptor

可以参考 org.springframework.web.servlet.HttpServletBean#init()BeanWrapper 的使用来实现对 User 对象的属性赋值:

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@Testpublic void test5(){    User user = new User();    BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(user);    MutablePropertyValues pvs = new MutablePropertyValues();    pvs.add("username","zhangsan");    pvs.add("age",1);    bw.setPropertyValues(pvs);    System.out.println(user);}

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输出结果:

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User{username='zhangsan', age=1}

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类型转换

有属性赋值,必然就会有类型转换。说白了我们从配置文件读取的数据是字符串,与属性进行参数绑定的过程中势必会有类型转换,java.beans 中提供了相应的 API:

  • PropertyEditor

    • 属性编辑器顶层接口
  • PropertyEditorSupport

    • 属性编辑器实现类
  • PropertyEditorManager

    • 属性编辑器管理器
    • 在 Spring 中提供了一个 PropertyEditorRegistrar

先看一个例子:

User 类增加 Date 属性:

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public class User {    private String username;    private Integer age;      private Date createTime;

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日期转换器:

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public class DatPropertyEditor extends PropertyEditorSupport {    @Override    public void setAsText(String text) {        try {            setValue((text == null) ? null : new SimpleDateFormat("yyyy-MM-dd").parse(text));        } catch (ParseException e) {            e.printStackTrace();        }    }}

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在之前的例子中内省设置属性值都是直接通过 PropertyDescriptor 获取属性的写方法通过反射去赋值,而如果需要对值进行类型转换,则需要通过 PropertyEditorSupport#setAsText 调用 setValue 方法,然后 setValue 方法触发属性属性修改事件:

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public class PropertyEditorSupport implements PropertyEditor {    public void setValue(Object value) {        this.value = value;        firePropertyChange();    }}

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要注意这里的 value 实际上是临时存储在 PropertyEditorSupport 中,PropertyEditorSupport 则作为事件源,从而得到类型转换后的 value,再通过 PropertyDescriptor 获取属性的写方法通过反射去赋值。

测试代码:

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@Testpublic void test6() throws IntrospectionException, FileNotFoundException {   Map<String,Object> properties = ImmutableMap.of("age",1,"username","zhangsan","createTime","2020-01-01");    User user = new User();        BeanInfo userBeanInfo = Introspector.getBeanInfo(User.class, Object.class);        PropertyDescriptor[] propertyDescriptors = userBeanInfo.getPropertyDescriptors();    Stream.of(propertyDescriptors).forEach(propertyDescriptor -> {                String property = propertyDescriptor.getName();                Object value = properties.get(property);        if (Objects.equals("createTime", property)) {                        propertyDescriptor.setPropertyEditorClass(DatPropertyEditor.class);                        PropertyEditor propertyEditor = propertyDescriptor.createPropertyEditor(user);                        propertyEditor.addPropertyChangeListener(evt -> {                                Object value1 = propertyEditor.getValue();                setPropertyValue(user, propertyDescriptor, value1);            });            propertyEditor.setAsText(String.valueOf(value));            return;        }        setPropertyValue(user, propertyDescriptor, value);    });    System.out.println(user);}private void setPropertyValue(User user, PropertyDescriptor propertyDescriptor, Object value1) {    try {        propertyDescriptor.getWriteMethod().invoke(user, value1);    } catch (IllegalAccessException | InvocationTargetException ignored) {    }}

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输出结果:

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User{username='zhangsan', age=1, createTime=2020-1-1 0:00:00}

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事件监听

先举一个的例子,在 Mac 中设置飞书通知方式的时候,当界面右侧选择 “提示” 的时候,那么左侧也会相应的显示为“提示”:

小议 Java 内省机制 - 图1

如果将右侧看成一个 Java Bean,那么这中间势必存在一个属性变化监听。java.beans 包中也提供了相应实现:

  • PropertyChangeEvent

    • 属性变化事件
  • PropertyChangeListener

    • 属性(生效)变化监听器
  • PropertyChangeSupport

    • 属性(生效)变化监听器管理器’
  • VetoableChangeListener

    • 属性(否决)变化监听器
  • VetoableChangeSupport

    • 属性(否决)变化监听器管理器

PropertyChangeEvent 的构造方法:

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public PropertyChangeEvent(Object source, String propertyName,    Object oldValue, Object newValue) {    super(source);    this.propertyName = propertyName;    this.newValue = newValue;    this.oldValue = oldValue;}

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通过这个构造方法可以看出属性变化监听的关注点:

  • source

    • 事件源
  • propertyName

    • 发生变化的属性名称
  • oldValue

    • 旧值
  • newValue

    • 新值

示例代码:

User 中增加属性(生效)变化监听:

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public class User {    private String username;    private Integer age;        private PropertyChangeSupport propertyChangeSupport = new PropertyChangeSupport(this);        private void firePropertyChange(String propertyName, String oldValue, String newValue) {        PropertyChangeEvent event = new PropertyChangeEvent(this, propertyName, oldValue, newValue);        propertyChangeSupport.firePropertyChange(event);    }        public void addPropertyChangeListener(PropertyChangeListener listener){        propertyChangeSupport.addPropertyChangeListener(listener);    }        public void removePropertyChangeListener(PropertyChangeListener listener){        propertyChangeSupport.removePropertyChangeListener(listener);    }        public PropertyChangeListener[] getPropertyChangeListeners() {        return propertyChangeSupport.getPropertyChangeListeners();    }    public void setUsername(String username) {        String oldValue = this.username;        this.username = username;        firePropertyChange("username", oldValue, username);    }

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测试代码:

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@Testpublic void test3(){    User user = new User();    user.setAge(1);    user.setUsername("zhangsan");    user.addPropertyChangeListener(System.out::println);    user.setUsername("lisi");    user.setUsername("wangwu");}

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输出结果:

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java.beans.PropertyChangeEvent[propertyName=name; oldValue=zhangsan; newValue=lisi; propagationId=null; source=User{username='lisi', age=1}]java.beans.PropertyChangeEvent[propertyName=name; oldValue=lisi; newValue=wangwu; propagationId=null; source=User{username='wangwu', age=1}]

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可以看到在添加了监听器后,当 username 属性发生变化的时候会出发监听事件。

再看看另外一种监听器 VetoableChangeListener。在 User 中添加监听器:

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private VetoableChangeSupport vetoableChangeSupport = new VetoableChangeSupport(this);private void fireVetoableChange(String propertyName, String oldValue, String newValue) throws PropertyVetoException {    PropertyChangeEvent event = new PropertyChangeEvent(this, propertyName, oldValue, newValue);    vetoableChangeSupport.fireVetoableChange(event);}public void addVetoableChangeListener(VetoableChangeListener listener){    vetoableChangeSupport.addVetoableChangeListener(listener);}public void removeVetoableChangeListener(VetoableChangeListener listener){    vetoableChangeSupport.removeVetoableChangeListener(listener);}public void setUsername(String username) throws PropertyVetoException {    String oldValue = this.username;    fireVetoableChange("username",oldValue,username);    this.username = username;    firePropertyChange("username", oldValue, username);}

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测试代码:

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@Testpublic void test3() throws PropertyVetoException {    User user = new User();    user.setAge(1);    user.addVetoableChangeListener(evt -> {        System.out.println(evt.getNewValue()+",,"+evt.getOldValue());        if (Objects.equals(evt.getNewValue(), evt.getOldValue())) {            throw new PropertyVetoException("当前属性值未发生任何变化", evt);        }    });    user.addPropertyChangeListener(System.out::println);    user.setUsername("lisi");    user.setUsername("zhangsan");    user.setUsername("zhangsan");}

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运行时发现一直无法抛出异常。查看源码发现 PropertyChangeSupportVetoableChangeSupport 当新旧值相等时不会触发监听,于是修改测试代码:

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@Testpublic void test3() throws PropertyVetoException {    User user = new User();    user.setAge(1);    user.addVetoableChangeListener(evt -> {        System.out.println(evt.getNewValue()+",,"+evt.getOldValue());        if (Objects.isNull(evt.getNewValue())) {            throw new PropertyVetoException("username 不能为null", evt);        }    });    user.addPropertyChangeListener(System.out::println);    user.setUsername("lisi");    user.setUsername(null);}

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运行结果:

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lisi,,nulljava.beans.PropertyChangeEvent[propertyName=username; oldValue=null; newValue=lisi; propagationId=null; source=User{username='lisi', age=1}]null,,lisijava.beans.PropertyVetoException: username 不能为null  at introspector.test.IntrospectorTest.lambda$test3$1(IntrospectorTest.java:78)  at java.beans.VetoableChangeSupport.fireVetoableChange(VetoableChangeSupport.java:375)

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可以发现当符合 “否决” 属性变化的条件时,会抛出 PropertyVetoException 异常阻断属性的变化。

在之前的示例中 userBeanInfo 输出的 EventSetDescriptor 为空,这是因为并未到 User 类中增加事件。现在再测试一下获取 EventSetDescriptor

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@Testpublic void test1() throws IntrospectionException {    BeanInfo userBeanInfo = Introspector.getBeanInfo(User.class, Object.class);    EventSetDescriptor[] eventSetDescriptors = userBeanInfo.getEventSetDescriptors();    Stream.of(eventSetDescriptors).forEach(System.out::println);}

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输出结果:

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java.beans.EventSetDescriptor[name=propertyChange; inDefaultEventSet; listenerType=interface java.beans.PropertyChangeListener; getListenerMethod=public java.beans.PropertyChangeListener[] introspector.bean.User.getPropertyChangeListeners(); addListenerMethod=public void introspector.bean.User.addPropertyChangeListener(java.beans.PropertyChangeListener); removeListenerMethod=public void introspector.bean.User.removePropertyChangeListener(java.beans.PropertyChangeListener)]java.beans.EventSetDescriptor[name=vetoableChange; inDefaultEventSet; listenerType=interface java.beans.VetoableChangeListener; addListenerMethod=public void introspector.bean.User.addVetoableChangeListener(java.beans.VetoableChangeListener); removeListenerMethod=public void introspector.bean.User.removeVetoableChangeListener(java.beans.VetoableChangeListener)]

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其他

在 Java 生态飞速发展的今天,很多底层技术细节都被高级框架所屏蔽,而 Java Beans 就是其中一种。也许平时根本就用不到,但是其代码设计和思想理念不应该被忽视。Dubbo 2.7 之后提出了 “服务自省” 的概念,其灵感就来源于 Java Beans 内省机制。

References