java元注解@Inherited的使用详解

1.先看源码文档

/**
 * 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 {
}

上述代码注释部分可以用谷歌翻译大法大致意思是

表示注释类型自动继承。 如果在注释类型声明中存在继承的元注释，并且用户在类声明上查询注释类型，并且类声明没有此类型的注释，则该类的超类将自动查询注释类型。 将重复此过程，直到找到此类型的注释，或者达到类层次结构（Object）的顶部。 如果没有超类具有此类型的注释，则查询将指示所讨论的类没有这样的注释。

请注意，如果使用注释类型来注释除类之外的任何内容，则此元注释类型不起作用。 还要注意，这个元注释只会导致从超类继承注释; 已实现的接口上的注释无效。

通过上述描述可知，使用该注解的注解父类的子类可以继承父类的注解。

2.代码测试

2.1拥有@Inherited注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Inherited
public @interface InheritedTest {

  String value();
}

@InheritedTest("拥有Inherited")
public class Person {

  public void method(){
  }

  public void method2(){
  }
}

public class Student extends Person {
}

测试：

public class TestInherited {

  public static void main(String[] args) {
    Class<Student> studentClass = Student.class;
    if (studentClass.isAnnotationPresent(InheritedTest.class)){
      System.out.println(studentClass.getAnnotation(InheritedTest.class).value());
    }

  }
}

输出：

2.2未拥有@Inherited注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface IsNotInherited {
  String value();
}

@IsNotInherited("未拥有Inherited")
public class Person {

  public void method(){
  }

  public void method2(){
  }
}

public class Student extends Person {
}

测试:

public class TestInherited {

  public static void main(String[] args) {
    Class<Student> studentClass = Student.class;
    if (studentClass.isAnnotationPresent(IsNotInherited.class)){
      System.out.println(studentClass.getAnnotation(IsNotInherited.class).value());
    }

  }
}

没有输出任何任容，由此可知未拥有@Inherited注解的注解的类的子类不会被继承该注解。

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