java8新特性——重複註解

java8對註解提供了兩點改進：

• 可重複的註解
• 可用於類型的註解

可重複的註解

先定義一個註解

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnno {
	String value() default "";
}

此時在成員變量上定義兩個註解會報錯

@MyAnno("哈哈")
@MyAnno("heh")
public void name(){

}

報錯內容：Duplicate annotation. The declaration of 'repeatAnnotation.MyAnno' does not have a valid java.lang.annotation.Repeatable annotation

如何定義重複註解？

1. 先定義一個父容器容器

2. 用MyAnno作爲變量類型，聲明一個數組

   @Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
   @Retention(RetentionPolicy.RUNTIME)
   public @interface MyAnnos{
   	MyAnno[] value();
   }
   

3. 然後把原來註解上加個新註解@Repeatable，入參父容器類對象

   @Repeatable(value = MyAnnos.class)
   @Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
   @Retention(RetentionPolicy.RUNTIME)
   public @interface MyAnno {
   	String value() default "";
   }
   

4. 此時就不報錯了

   @MyAnno("哈哈")
   @MyAnno("heh")
   public void name(){
   
   }
   

5. 獲取註解內容

   由於設置了@Retention(RetentionPolicy.RUNTIME)，所以可以通過反射獲取註解值

   @Test
   public void test() throws NoSuchMethodException {
       Class<demo> clazz = demo.class; //獲取目標類的Class對象
       Method name = clazz.getMethod("name");
       MyAnno[] annos = name.getAnnotationsByType(MyAnno.class);
       for (MyAnno anno : annos) {
           System.out.println("anno = " + anno.value());
       }
   }
   /*
   結果：
       anno = 哈哈
       anno = heh
   */  
   

類型的註解

• 多了一個 TYPE_PARAMETER
  

@Repeatable(value = MyAnnos.class)
@Target({
    TYPE, FIELD, METHOD, PARAMETER, 
         CONSTRUCTOR, LOCAL_VARIABLE,TYPE_PARAMETER}
       )
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnno {
	String value() default "";
}