[置顶] 译 Programming with typesafe enums and annotations in Java 5

原文为：

Java 101: The next generation: The essential Java language features tour, Part 2

Annotations――Java注解

你可能会遇到这类需求：通过与元数据（描写其他数据的数据）关联来注解你Java利用程序。1直以来，Java通过transient关键字提供了1种即时注解（ad hoc annotation）机制，这允许你标识1个成员变量在序列化进程中应当被疏忽。但是，直到java5才正式地引入了1种标准方式去注解程序。

Java5平台提供了下面4种注解机制：

1. 声明注解类型：@interface；

2. 元注解类型：可以辨认利用于注解类型的程序元素和注解的生命周期（注解类型的1个实例）等；

3．通过Java反射API的1种扩大方式来支持注解处理，你可以用来发现1个程序的运行时注解，并引入1个泛型Annotation处理工具（APT）；

4. 标准的注解类型；

我将通过下面的实例解释如何应用这些方式并阐明注解的难点。

Annotations

You’veprobably encountered the need to annotate elements of your Java applicationsbyassociating metadata (data that describes other data) with them.Java has always provided an ad hoc annotation mechanism via the transientreserved word, which lets you annotate fields that are to be excluded duringserialization. But it didn’t offer a standard way to annotate program elementsuntil Java 5.

Java5’s general annotation mechanism consists of four components:

1.An @interface mechanism for declaring annotation types.
2. Meta-annotation types, which you can use to identify the applicationelements to which an annotation type applies; to identify the lifetime of anannotation (an instance of an annotation type); and more.
3. Support for annotation processing via an extension to the Java ReflectionAPI, which you can use to discover a program’s runtime annotations, and theintroduction of a generalized tool for processing annotations.
4. Standard annotation types.

I’ll explain how to use these componentsand point out some of the challenges of annotations in the examples thatfollow.

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