原文： https://howtodoinjava.com/spring-boot2/springbootapplication-auto-configuration/

Spring Boot 非常易于使用，它在后台执行了许多操作，您可能没有意识到。 将来，一个好的开发人员将确切地了解 spring boot 自动配置背后的工作，如何以自己的喜好使用它，以及如何禁用某些不需要的部分到您的项目中。

为了了解 Spring Boot 背后的大多数基本知识，我们将创建一个具有单个依赖项和单个启动类文件的最小启动应用程序。 然后，我们将分析启动日志以获取见解。

使用启动类创建 Spring Boot 应用程序

  1. 在 Eclipse 中创建一个新的 Maven 项目，原型为maven-archetype-quickstart。

  2. 使用spring-boot-starter-web依赖和插件信息更新pom.xml文件。

    1. <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    2.  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd;
    3.  <modelVersion>4.0.0</modelVersion>
    5.  <groupId>com.howtodoinjava</groupId>
    6.  <artifactId>springbootdemo</artifactId>
    7.  <version>0.0.1-SNAPSHOT</version>
    8.  <packaging>jar</packaging>
    10.  <name>springbootdemo</name>
    11.  <url>http://maven.apache.org</url>
    13.  <parent>
    14.      <groupId>org.springframework.boot</groupId>
    15.      <artifactId>spring-boot-starter-parent</artifactId>
    16.      <version>2.0.0.RELEASE</version>
    17.  </parent>
    19.  <properties>
    20.      <java.version>1.8</java.version>
    21.      <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    22.  </properties>
    24.  <dependencies>
    25.      <dependency>
    26.          <groupId>org.springframework.boot</groupId>
    27.          <artifactId>spring-boot-starter-web</artifactId>
    28.      </dependency>
    29.  </dependencies>
    31.  <build>
    32.      <plugins>
    33.          <plugin>
    34.              <groupId>org.springframework.boot</groupId>
    35.              <artifactId>spring-boot-maven-plugin</artifactId>
    36.          </plugin>
    37.      </plugins>
    38.  </build>
    40.  <repositories>
    41.      <repository>
    42.          <id>repository.spring.release</id>
    43.          <name>Spring GA Repository</name>
    44.          <url>http://repo.spring.io/release</url>
    45.      </repository>
    46.  </repositories>
    47. </project>
  1. 创建启动应用。 ```java package com.howtodoinjava.demo;

import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.context.ApplicationContext;

@SpringBootApplication public class App { public static void main(String[] args) { ApplicationContext ctx = SpringApplication.run(App.class, args); } }

  2. <a name="2ec0a61c"></a>
  3. #### 这个入门班是做什么的？
  5. <br />上面的类称为 Spring Boot 应用程序启动类。 它曾经通过 Java `main()`方法来运行和启动 Spring 应用程序。 它通常会执行以下操作：
  7.    - 创建 Spring 的`ApplicationContext`的实例。
  8.    - 启用该功能以接受命令行参数并将其公开为 Spring 属性。
  9.    - 根据配置加载所有 Spring bean。 您还可以根据项目需要进行其他操作。
  11. <a name="4fdfe769"></a>
  12. ## `@SpringBootApplication`注解
  14. 此注解是在一条语句中应用 3 个注解的快捷方式：
  16. 1. 
  17. <a name="9e5eeb13"></a>
  18. #### `@SpringBootConfiguration`
  20. <br />[@SpringBootConfiguration](https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/SpringBootConfiguration.html) 是 Spring Boot 2 中的新注解。以前，我们一直在使用[`@Configuration`](https://docs.spring.io/spring-framework/docs/5.0.4.RELEASE/javadoc-api/org/springframework/context/annotation/Configuration.html)注解。 您可以代替使用`@Configuration`。 两者都是同一回事。
  21. <br />它指示一个类提供 Spring Boot 应用程序`@Configuration`。 它仅表示带注解的类是配置类，应对其进行扫描以获取进一步的配置和 Bean 定义。
  23. 2. 
  24. <a name="49874534"></a>
  25. #### `@EnableAutoConfiguration`
  27. <br />该注解用于启用 Spring 应用下上文的自动配置，尝试猜测和配置您可能需要的 bean。 通常根据您的类路径和定义的 bean 来应用自动配置类。
  28. <br />自动配置会尝试尽可能智能化，并且在您定义更多自己的配置时会自动退出。 您始终可以使用两种方法手动排除您永远不想应用的任何配置：
  29. <br />i）使用`excludeName()`
  30. <br />ii）使用属性文件中的`spring.autoconfigure.exclude`属性。 例如
  31. ```java
  32. @EnableAutoConfiguration(excludeName = {"multipartResolver","mbeanServer"})


自动配置始终在注册用户定义的 bean 之后应用。

  1. @ComponentScan


该注解提供与 Spring XML 的context:component-scan元素并行的支持。
可以指定basePackageClasses()或basePackages()定义要扫描的特定程序包。 如果未定义特定的程序包，则将从声明此注解的类的程序包中进行扫描。

运行启动应用程序并检查日志

让我们以最简单的选项开始运行它 - 作为 Java 应用程序运行。 在您的 IDE 中，右键单击应用程序类，然后将其作为 Java 应用程序运行。 为了了解已注册的 bean，我添加了如下修改的启动应用程序。

  1. @SpringBootApplication
  2. public class App 
  3. {
  4.     public static void main(String[] args) 
  5.     {
  6.         ApplicationContext ctx = SpringApplication.run(App.class, args);
  8.         String[] beanNames = ctx.getBeanDefinitionNames();
  10.         Arrays.sort(beanNames);
  12.         for (String beanName : beanNames) {
  13.             System.out.println(beanName);
  14.         }
  15.     }
  16. }

现在查看日志：

  1.   .   ____          _            __ _ _
  2.  /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
  3. ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
  4.  \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  5.   '  |____| .__|_| |_|_| |_\__, | / / / /
  6.  =========|_|==============|___/=/_/_/_/
  7.  :: Spring Boot ::        (v2.0.0.RELEASE)
  9. 2018-04-02 13:09:41.100  INFO 11452 --- [           main] com.howtodoinjava.demo.App               : Starting App on FFC15B4E9C5AA with PID 11452 (C:\Users\zkpkhua\IDPPaymentTransfers_Integrated\springbootdemo\target\classes started by zkpkhua in C:\Users\zkpkhua\IDPPaymentTransfers_Integrated\springbootdemo)
  10. 2018-04-02 13:09:41.108  INFO 11452 --- [           main] com.howtodoinjava.demo.App               : No active profile set, falling back to default profiles: default
  11. 2018-04-02 13:09:41.222  INFO 11452 --- [           main] ConfigServletWebServerApplicationContext : Refreshing org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@4450d156: startup date [Mon Apr 02 13:09:41 IST 2018]; root of context hierarchy
  12. 2018-04-02 13:09:43.474  INFO 11452 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
  13. 2018-04-02 13:09:43.526  INFO 11452 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
  14. 2018-04-02 13:09:43.526  INFO 11452 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/8.5.28
  15. 2018-04-02 13:09:43.748  INFO 11452 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
  16. 2018-04-02 13:09:43.748  INFO 11452 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 2531 ms
  17. 2018-04-02 13:09:43.964  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean  : Servlet dispatcherServlet mapped to [/]
  18. 2018-04-02 13:09:43.969  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'characterEncodingFilter' to: [/*]
  19. 2018-04-02 13:09:43.970  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
  20. 2018-04-02 13:09:43.970  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'httpPutFormContentFilter' to: [/*]
  21. 2018-04-02 13:09:43.970  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'requestContextFilter' to: [/*]
  22. 2018-04-02 13:09:44.480  INFO 11452 --- [           main] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@4450d156: startup date [Mon Apr 02 13:09:41 IST 2018]; root of context hierarchy
  23. 2018-04-02 13:09:44.627  INFO 11452 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
  24. 2018-04-02 13:09:44.630  INFO 11452 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
  25. 2018-04-02 13:09:44.681  INFO 11452 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
  26. 2018-04-02 13:09:44.682  INFO 11452 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
  27. 2018-04-02 13:09:44.747  INFO 11452 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**/favicon.ico] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
  28. 2018-04-02 13:09:45.002  INFO 11452 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
  29. 2018-04-02 13:09:45.070  INFO 11452 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
  30. 2018-04-02 13:09:45.076  INFO 11452 --- [           main] com.howtodoinjava.demo.App               : Started App in 4.609 seconds (JVM running for 5.263)
  31. app
  32. basicErrorController
  33. beanNameHandlerMapping
  34. beanNameViewResolver
  35. characterEncodingFilter
  36. conventionErrorViewResolver
  37. defaultServletHandlerMapping
  38. defaultValidator
  39. defaultViewResolver
  40. dispatcherServlet
  41. dispatcherServletRegistration
  42. error
  43. errorAttributes
  44. errorPageCustomizer
  45. errorPageRegistrarBeanPostProcessor
  46. faviconHandlerMapping
  47. faviconRequestHandler
  48. handlerExceptionResolver
  49. hiddenHttpMethodFilter
  50. httpPutFormContentFilter
  51. httpRequestHandlerAdapter
  52. jacksonCodecCustomizer
  53. jacksonObjectMapper
  54. jacksonObjectMapperBuilder
  55. jsonComponentModule
  56. localeCharsetMappingsCustomizer
  57. mappingJackson2HttpMessageConverter
  58. mbeanExporter
  59. mbeanServer
  60. messageConverters
  61. methodValidationPostProcessor
  62. multipartConfigElement
  63. multipartResolver
  64. mvcContentNegotiationManager
  65. mvcConversionService
  66. mvcHandlerMappingIntrospector
  67. mvcPathMatcher
  68. mvcResourceUrlProvider
  69. mvcUriComponentsContributor
  70. mvcUrlPathHelper
  71. mvcValidator
  72. mvcViewResolver
  73. objectNamingStrategy
  74. org.springframework.boot.autoconfigure.AutoConfigurationPackages
  75. org.springframework.boot.autoconfigure.condition.BeanTypeRegistry
  76. org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration
  77. org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration
  78. org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration
  79. org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration$StringHttpMessageConverterConfiguration
  80. org.springframework.boot.autoconfigure.http.JacksonHttpMessageConvertersConfiguration
  81. org.springframework.boot.autoconfigure.http.JacksonHttpMessageConvertersConfiguration$MappingJackson2HttpMessageConverterConfiguration
  82. org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration
  83. org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration$JacksonCodecConfiguration
  84. org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration
  85. org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory
  86. org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration
  87. org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$Jackson2ObjectMapperBuilderCustomizerConfiguration
  88. org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$JacksonObjectMapperBuilderConfiguration
  89. org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$JacksonObjectMapperConfiguration
  90. org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$ParameterNamesModuleConfiguration
  91. org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration
  92. org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration
  93. org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration
  94. org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration
  95. org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration
  96. org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration$TomcatWebServerFactoryCustomizerConfiguration
  97. org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration
  98. org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration$DispatcherServletConfiguration
  99. org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration$DispatcherServletRegistrationConfiguration
  100. org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration
  101. org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration
  102. org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration
  103. org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryConfiguration$EmbeddedTomcat
  104. org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration
  105. org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$EnableWebMvcConfiguration
  106. org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter
  107. org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter$FaviconConfiguration
  108. org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration
  109. org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$DefaultErrorViewResolverConfiguration
  110. org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$WhitelabelErrorViewConfiguration
  111. org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration
  112. org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration$TomcatWebSocketConfiguration
  113. org.springframework.boot.context.properties.ConfigurationBeanFactoryMetadata
  114. org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor
  115. org.springframework.context.annotation.internalAutowiredAnnotationProcessor
  116. org.springframework.context.annotation.internalCommonAnnotationProcessor
  117. org.springframework.context.annotation.internalConfigurationAnnotationProcessor
  118. org.springframework.context.annotation.internalRequiredAnnotationProcessor
  119. org.springframework.context.event.internalEventListenerFactory
  120. org.springframework.context.event.internalEventListenerProcessor
  121. parameterNamesModule
  122. preserveErrorControllerTargetClassPostProcessor
  123. propertySourcesPlaceholderConfigurer
  124. requestContextFilter
  125. requestMappingHandlerAdapter
  126. requestMappingHandlerMapping
  127. resourceHandlerMapping
  128. restTemplateBuilder
  129. server-org.springframework.boot.autoconfigure.web.ServerProperties
  130. servletWebServerFactoryCustomizer
  131. simpleControllerHandlerAdapter
  132. spring.http.encoding-org.springframework.boot.autoconfigure.http.HttpEncodingProperties
  133. spring.info-org.springframework.boot.autoconfigure.info.ProjectInfoProperties
  134. spring.jackson-org.springframework.boot.autoconfigure.jackson.JacksonProperties
  135. spring.mvc-org.springframework.boot.autoconfigure.web.servlet.WebMvcProperties
  136. spring.resources-org.springframework.boot.autoconfigure.web.ResourceProperties
  137. spring.security-org.springframework.boot.autoconfigure.security.SecurityProperties
  138. spring.servlet.multipart-org.springframework.boot.autoconfigure.web.servlet.MultipartProperties
  139. standardJacksonObjectMapperBuilderCustomizer
  140. stringHttpMessageConverter
  141. tomcatServletWebServerFactory
  142. tomcatServletWebServerFactoryCustomizer
  143. tomcatWebServerFactoryCustomizer
  144. viewControllerHandlerMapping
  145. viewResolver
  146. webServerFactoryCustomizerBeanPostProcessor
  147. websocketContainerCustomizer
  148. welcomePageHandlerMapping

您会看到有多少个 bean 自动注册。 那是 SpringBoot 子的美。 如果您想更深入地研究为什么要注册任何特定的 bean？ 通过在应用程序启动时放置调试标志，您可以看到这一点。

只需将-Ddebug=true作为 VM 参数传递。

现在，当您运行该应用程序时，您将获得许多调试日志，它们具有类似的信息：

  1. CodecsAutoConfiguration.JacksonCodecConfiguration matched:
  2.   - @ConditionalOnClass found required class 'com.fasterxml.jackson.databind.ObjectMapper'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
  4. CodecsAutoConfiguration.JacksonCodecConfiguration#jacksonCodecCustomizer matched:
  5.   - @ConditionalOnBean (types: com.fasterxml.jackson.databind.ObjectMapper; SearchStrategy: all) found bean 'jacksonObjectMapper' (OnBeanCondition)
  7. DispatcherServletAutoConfiguration.DispatcherServletConfiguration matched:
  8.   - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
  9.   - Default DispatcherServlet did not find dispatcher servlet beans (DispatcherServletAutoConfiguration.DefaultDispatcherServletCondition)
  11. DispatcherServletAutoConfiguration.DispatcherServletRegistrationConfiguration matched:
  12.   - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
  13.   - DispatcherServlet Registration did not find servlet registration bean (DispatcherServletAutoConfiguration.DispatcherServletRegistrationCondition)
  15.   ...
  16.   ...
  17.   ...

上面的日志说明了为什么将特定的 bean 注册到 spring 上下文中。 当您调试自动配置问题时，此信息非常有用。

同样，每次我们向 Spring Boot 项目添加新的依赖项时，Spring Boot 自动配置都会自动尝试基于该依赖项配置 Bean。

希望以上讨论的信息将来能在调试 Spring Boot 相关问题时为您提供帮助。

学习愉快！