对@SpringBootApplication注解的一些简单理解

@SpringBootApplication

点进去这个注解看它的源码，如下：

接下来先分析这四个元注解：

• @Target：限定注解运用的场景
  • 此处是@Target(ElementType.TYPE)：限定给一个类型进行注解，比如类、接口、枚举
  • 除此以外还有：
    ElementType.ANNOTATION_TYPE 可以给一个注解进行注解
    ElementType.CONSTRUCTOR 可以给构造方法进行注解
    ElementType.FIELD 可以给属性进行注解
    ElementType.LOCAL_VARIABLE 可以给局部变量进行注解
    ElementType.METHOD 可以给方法进行注解
    ElementType.PACKAGE 可以给一个包进行注解
    ElementType.PARAMETER 可以给一个方法内的参数进行注解
• @Retention：该注解定义了这个注解存活的时间
  • 此处是@Retention(RetentionPolicy.RUNTIME)，表明该注解可以保留到程序运行时，会被加载进JVM，运行时可以获得到他们,因此@SpringBootApplication可以在程序运行时被获取到，它的生存周期很长。常见的@Autowired注解就是运行期注解
  • RetentionPolicy.SOURCE 注解只在源码阶段保留，在编译器进行编译时它将被丢弃忽视。
    RetentionPolicy.CLASS 注解只被保留到编译进行的时候，它并不会被加载到 JVM 中。常见的编译期注解：@Override
• @Documented：注解信息会被包含到Javadoc文档中
• @Inherited：如果一个超类被 【【@Inherited 注解过】的注解】 进行注解的话，那么如果它的子类没有被任何注解应用的话，那么这个子类就继承了超类的注解

综上：@SpringBootApplication是一个被限定给类、接口进行注解的运行期注解，并且被@SpringBootApplication注解的类的子类没有被注解的话，子类可以继承该注解

除了上述四个注解，还有一个元注解：@Repeatable，JDK1.8新加入进来的，算是一个新特性，通常是注解的值可以同时取多个（比如一个Person身兼数职，既是学生又是兼职家教老师）

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除去上面四个元注解，还有三个关键性的注解：

1.@SpringBootConfiguration：Spring Boot项目的配置注解

可以看到，@SpringBootConfiguration也是一个复合注解，但是前三个我们一句熟悉了，只有最后一个 @Configuration：它的作用是声明了这是一个配置类

2.@EnableAutoConfiguration：完成自动配置，该注解会使Spring Boot根据项目中依赖的jar包自动配置项目的配置项

 * 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).

public @interface ConditionalOnMissingBean{…}

@Inherited注解会超类来找依赖中的类，完成自动初始化类加载

3.@ComponentScan：用注解配置实现自动扫描，默认会扫描当前包和所有子包，和xml配置自动扫描效果一样，@Filter是排除了两个类

 * Configures component scanning directives for use with @{@link Configuration} classes.
 * Provides support parallel with Spring XML's {@code } element.
 *
 * 
Either {@link #basePackageClasses} or {@link #basePackages} (or its alias
 * {@link #value}) may be specified to define specific packages to scan. If specific
 * packages are not defined, scanning will occur from the package of the
 * class that declares this annotation.
 *
 * 
Note that the {@code } element has an
 * {@code annotation-config} attribute; however, this annotation does not. This is because
 * in almost all cases when using {@code @ComponentScan}, default annotation config
 * processing (e.g. processing {@code @Autowired} and friends) is assumed. Furthermore,
 * when using {@link AnnotationConfigApplicationContext}, annotation config processors are
 * always registered, meaning that any attempt to disable them at the
 * {@code @ComponentScan} level would be ignored.
 *
 * 
See {@link Configuration @Configuration}'s Javadoc for usage examples.

@ComponentScan 注解会自动扫描指定包下的全部标有 @Component注解 的类包括 @Component 下的子注解@Service、@Repository、@Controller等，并注册成bean