springboot源码分析系列（一）--核心注解@SpringBootApplication

柔光的暖阳◎ 2021-11-29 10:14 330阅读 0赞

SpringBoot项目一般都会有Application的入口类，入口类中会有main方法，这是一个标准的java应用程序的入口方法。@SpringBootApplication用于Spring主类上最最最核心的注解，表示这是一个SpringBoot项目，用于开启SpringBoot的各项能力。  
  下面我们来分析一下@SpringBootApplication这个注解的组成以及作用

/** * Indicates a {@link Configuration configuration} class that declares one or more * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience * annotation that is equivalent to declaring {@code @Configuration}, * {@code @EnableAutoConfiguration} and {@code @ComponentScan}. * * @author Phillip Webb * @author Stephane Nicoll * @since 1.2.0 */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(excludeFilters = {  @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
    		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
    public @interface SpringBootApplication { 
    	
    	……
    	……
    	……
    }

通过上面的代码我们可以看出来@SpringBootApplication是一个组合注解，主要由@SpringBootConfiguration、@EnableAutoConfiguration和@ComponentScan这三个注解组成的。

# @SpringBootConfiguration #

/** * Indicates that a class provides Spring Boot application * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's * standard {@code @Configuration} annotation so that configuration can be found * automatically (for example in tests). * * Application should only ever include one {@code @SpringBootConfiguration} and * most idiomatic Spring Boot applications will inherit it from * {@code @SpringBootApplication}. * * @author Phillip Webb * @since 1.4.0 */
 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 @Configuration
 public @interface SpringBootConfiguration { 
 
 }

通过源码可以看出@SpringBootConfiguration也是一个组合注解。@SpringBootConfiguration是SpringBoot项目的配置注解，标识这是一个被装载的Bean，在SpringBoot项目中推荐使用@SpringBootConfiguration替代@Configuration

# @EnableAutoConfiguration #

@EnableAutoConfiguration的作用是开启自动配置功能。以前我们需要配置的东西，SpringBoot帮我们自动配置；@EnableAutoConfiguration告诉SpringBoot开启自动配置功能；这样自动配置才能生效

/** * Enable auto-configuration of the Spring Application Context, attempting to guess and * configure beans that you are likely to need. Auto-configuration classes are usually * applied based on your classpath and what beans you have defined. For example, if you * have {@code tomcat-embedded.jar} on your classpath you are likely to want a * {@link TomcatServletWebServerFactory} (unless you have defined your own * {@link ServletWebServerFactory} bean). * * When using {@link SpringBootApplication}, the auto-configuration of the context is * automatically enabled and adding this annotation has therefore no additional effect. * * Auto-configuration tries to be as intelligent as possible and will back-away as you * define more of your own configuration. You can always manually {@link #exclude()} any * configuration that you never want to apply (use {@link #excludeName()} if you don't * have access to them). You can also exclude them via the * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied * after user-defined beans have been registered. * * The package of the class that is annotated with {@code @EnableAutoConfiguration}, * usually via {@code @SpringBootApplication}, has specific significance and is often used * as a 'default'. For example, it will be used when scanning for {@code @Entity} classes. * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're * not using {@code @SpringBootApplication}) in a root package so that all sub-packages * and classes can be searched. * * Auto-configuration classes are regular Spring {@link Configuration} beans. They are * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class). * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most * often using {@link ConditionalOnClass @ConditionalOnClass} and * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations). * * @author Phillip Webb * @author Stephane Nicoll * @see ConditionalOnBean * @see ConditionalOnMissingBean * @see ConditionalOnClass * @see AutoConfigureAfter * @see SpringBootApplication */
 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 @Inherited
 @AutoConfigurationPackage
 @Import(AutoConfigurationImportSelector.class)
 public @interface EnableAutoConfiguration { 
 
 String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
 
 /** * Exclude specific auto-configuration classes such that they will never be applied. * @return the classes to exclude */
 Class<?>[] exclude() default { };
 
 /** * Exclude specific auto-configuration class names such that they will never be * applied. * @return the class names to exclude * @since 1.3.0 */
 String[] excludeName() default { };
 
 }

当我们启用了@EnableAutoConfiguration注解，启动自动配置后，该注解会使SpringBoot根据项目中依赖的jar包自动配置项目的配置项。例如：引入了Spring-boot-starter-web的依赖，项目中也就会引入SpringMVC的依赖，SpringBoot就会自动配置tomcat和SpringMVC。  
  关于自动配置的具体实现内容，会在后面的文章中介绍。

# @ComponentScan #

@ComponentScan注解会自动扫描包路径下的所有@Controller、@Service、@Repository、@Component的类，不配置包路径的话，SpringBoot中默认扫描@SpringBootApplication注解所在类的同级目录以及子目录下的相关注解。