注解 java.lang.annotation.Inherited 介绍

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。