@SpringBootApplication理解

@SpringBootApplication是Spring Boot 的核心注解，它是一个组合注解，源码如下：

@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 {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "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
     */
    @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName")
    String[] excludeName() default {};

    /**
     * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
     * for a type-safe alternative to String-based package names.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};

    /**
     * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
     * scan for annotated components. The package of each class specified will be scanned.
     * 

     * Consider creating a special no-op marker class or interface in each package that
     * serves no purpose other than being referenced by this attribute.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    Class[] scanBasePackageClasses() default {};

}

这里有两个作用

1. 自动依赖，就像图中所示的，
   
   可以看到只使引入了一个web，但是它会自动为我们引入starter和tomcat等内容，这就是@EnableAutoConfiguration所起的作用
2. 自动扫描@SpringBootApplication所在类的同级包，如图所示：
   
   Ch523Application类使Spring Boot的一个入口类，里面有@SpringBootApplication注解，有一个main方法，这个方法就是普通的java main方法，和这个类同级别的包都可以自动的扫描，但是不在这个包里的类就不会扫描到，就像这个图片中的com.wisely.test这个包，这里的类就不会被扫描到spring的容器中。