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java元注解@Inherited的使用詳解

更新時間：2019年11月27日 08:30:48 作者：lele

這篇文章主要介紹了java元注解@Inherited的使用詳解，文中通過示例代碼介紹的非常詳細，對大家的學習或者工作具有一定的參考學習價值，需要的朋友們下面隨著小編來一起學習學習吧

1.先看源碼文檔

/**
 * 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 {
}

上述代碼注釋部分可以用谷歌翻譯大法大致意思是

表示注釋類型自動繼承。如果在注釋類型聲明中存在繼承的元注釋，并且用戶在類聲明上查詢注釋類型，并且類聲明沒有此類型的注釋，則該類的超類將自動查詢注釋類型。將重復(fù)此過程，直到找到此類型的注釋，或者達到類層次結(jié)構(gòu)（Object）的頂部。如果沒有超類具有此類型的注釋，則查詢將指示所討論的類沒有這樣的注釋。

請注意，如果使用注釋類型來注釋除類之外的任何內(nèi)容，則此元注釋類型不起作用。還要注意，這個元注釋只會導(dǎo)致從超類繼承注釋; 已實現(xiàn)的接口上的注釋無效。

通過上述描述可知，使用該注解的注解父類的子類可以繼承父類的注解。

2.代碼測試

2.1擁有@Inherited注解

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

  String value();
}

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


  public void method(){
  }


  public void method2(){
  }
}

public class Student extends Person {
}

測試：

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());
    }


  }
}

輸出：

2.2未擁有@Inherited注解

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

@IsNotInherited("未擁有Inherited")
public class Person {


  public void method(){
  }


  public void method2(){
  }
}

public class Student extends Person {
}

測試:

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());
    }


  }
}