在Spring Boot中大量使用了@Inherited注解。我们来了解一下这个注解的用法，注解的源码：
package java.lang.annotation;
/**
* Indicates that an annotation type is automatically inherited. If
* an Inherited meta-annotation is present on an annotation type
* declaration, and the user queries the annotation type on a class
* declaration, and the class declaration has no annotation for this type,
* then the class's superclass will automatically be queried for the
* annotation type. This process will be repeated until an annotation for this
* type is found, or the top of the class hierarchy (Object)
* is reached. If no superclass has an annotation for this type, then
* the query will indicate that the class in question has no such annotation.
*
* <p>Note that this meta-annotation type has no effect if the annotated
* type is used to annotate anything other than a class. Note also
* that this meta-annotation only causes annotations to be inherited
* from superclasses; annotations on implemented interfaces have no
* effect.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.3.3 @Inherited
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}
注解的作用：
当某个注解类在它的类上定义了@Inherited注解，例如SpringBoot中的 @SpringBootApplication注解，@SpringBootApplication注解类就定义了@Inherited注解，看下源码中的红色部分：
@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 {
// .....省略
}
那么现在有一个我们自己开发的类使用了这个注解，例如：
@SpringBootApplication
@Service
public class Person {
}
然后有个类Employee继承了Person
public class Employee extends Person{
}
那么现在在判断Employee类上有没有@SpringBootApplication时，通过代码验证：
@Test
public void test1(){
Class clazz = Employee.class ;
if(clazz.isAnnotationPresent(SpringBootApplication.class)){
System.out.println("true");
}
}
上面这个测试用例执行将输出true，也就是子类中能查找到@SpringBootApplication ，但同样，你用上述代码查找Employee类上是否有Spring的@Service注解时，会输出false，至此你应该明白@Inherited注解的用意了吧。
经过这样的分析，我们再来读一下JDK的文档，就会比较容易理解了，否则会觉的有些绕，下面列出 @interface注解的中文文档：
指示注释类型被自动继承。如果在注释类型声明中存在 Inherited 元注释，并且用户在某一类声明中查询该注释类型，同时该类声明中没有此类型的注释，则将在该类的超类中自动查询该注释类型。此过程会重复进行，直到找到此类型的注释或到达了该类层次结构的顶层 (Object) 为止。如果没有超类具有该类型的注释，则查询将指示当前类没有这样的注释。
注意，如果使用注释类型注释类以外的任何事物，此元注释类型都是无效的。还要注意，此元注释仅促成从超类继承注释；对已实现接口的注释无效。
