这怎么可能，一个注释可以是一个注释本身？ [英] How is it possible that an annotation can be an annotation to itself?

问题描述

我在浏览通过JDK 7的文档时，我注意到 @Target 在呼包注释 java.lang.annotation中的。这个类的标题是

I was browsing through the documentation of JDK 7 when I noticed an annotation called @Target in package java.lang.annotation. The header of that class is

@Documented
@Retention(value=RUNTIME)
@Target(value=ANNOTATION_TYPE)
public @interface Target

现在， @Target 用作注释本身。这怎么可能？ @Target 在声明它甚至在头部。我想这与我写的注释，和它的工作为好。任何人能解释这里发生了什么？

Now, @Target is used as an annotation to itself. How is this possible? @Target is used in the header even before it is declared. I tried this with annotations I had written, and it worked as well. Can anyone explain what's happening here?

推荐答案

在JLS明确预期这一点，的部分9.6注释类型的：

The JLS specifically anticipates this, in section 9.6 Annotation Types:

如果一个注解 A
  （§9.7）
  在注释类型声明对应一个注释类型
   T 和 T 有（​​间位）注释 M 对应于
   java.lang.annotation.Target ，那么 M 必须有一个元素
  其值 java.lang.annotation.ElementType.ANNOTATION_TYPE 或
  一个元素，其值 java.lang.annotation.ElementType.TYPE 或
  一个发生编译时错误。

If an annotation a (§9.7) on an annotation type declaration corresponds to an annotation type T, and T has a (meta-)annotation m that corresponds to java.lang.annotation.Target, then m must have either an element whose value is java.lang.annotation.ElementType.ANNOTATION_TYPE, or an element whose value is java.lang.annotation.ElementType.TYPE, or a compile-time error occurs.

9.6节或9.7的任何其他部分说，大约它是非法的注释声明与所声明的注释的参考注释东西。

No other part of section 9.6 or 9.7 says anything about it being illegal for an annotation declaration to be annotated with a reference to the annotation being declared.

这篇关于这怎么可能，一个注释可以是一个注释本身？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！