java annotation注解

java 注解，从名字上看是注释，解释。但功能却不仅仅是注释那么简单。注解（Annotation） 为我们在代码中添加信息提供了一种形式化的方法，是我们可以在稍后某个时刻方便地使用这些数据（通过 解析注解 来使用这些数据），常见的作用有以下几种：

• 生成文档。这是最常见的，也是java 最早提供的注解。常用的有@see @param @return 等
• 跟踪代码依赖性，实现替代配置文件功能。比较常见的是spring 2.5 开始的基于注解配置。作用就是减少配置。现在的框架基本都使用了这种配置来减少配置文件的数量。也是
• 在编译时进行格式检查。如@override 放在方法前，如果你这个方法并不是覆盖了超类方法，则编译时就能检查出。

包 java.lang.annotation 中包含所有定义自定义注解所需用到的原注解和接口。如接口 java.lang.annotation.Annotation 是所有注解继承的接口,并且是自动继承，不需要定义时指定，类似于所有类都自动继承Object。

该包同时定义了四个元注解，Documented,Inherited,Target(作用范围，方法，属性，构造方法等),Retention(生命范围，源代码，class,runtime)。下面将在实例中逐个讲解他们的作用，及使用方法。

Inherited : 在您定义注解后并使用于程序代码上时，预设上父类别中的注解并不会被继承至子类别中，您可以在定义注解时加上java.lang.annotation.Inherited 限定的Annotation，这让您定义的Annotation型别被继承下来。注意注解继承只针对class 级别注解有效（这段建议看完全文后在来回顾）。 多说无益，下面就一步步从零开始建一个我们自己的注解。

   建第一个注解

<ol class="linenums"><li class="L0"><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L1"><span class="com">/**</span></li><li class="L2"><span class="com"> *</span></li><li class="L3"><span class="com"> *@interface用来声明一个注解，其中的每一个方法实际上是声明了一个配置参数。</span></li><li class="L4"><span class="com"> *方法的名称就是参数的名称，返回值类型就是参数的类型。</span></li><li class="L5"><span class="com"> *可以通过default来声明参数的默认值。</span></li><li class="L6"><span class="com"> *在这里可以看到@Retention和@Target这样的元注解，用来声明注解本身的行为。</span></li><li class="L7"><span class="com"> *@Retention用来声明注解的保留策略，有CLASS、RUNTIME和SOURCE这三种，</span></li><li class="L8"><span class="com"> *分别表示注解保存在类文件、JVM运行时刻和源代码中。</span></li><li class="L9"><span class="com"> *只有当声明为RUNTIME的时候，才能够在运行时刻通过反射API来获取到注解的信息。</span></li><li class="L0"><span class="com"> *@Target用来声明注解可以被添加在哪些类型的元素上，如类型、方法和域等。</span></li><li class="L1"><span class="com"> *就可以定义一个注解了，它将自动继承Annotation</span></li><li class="L2"><span class="com"> */</span></li><li class="L3"><span class="kwd">public</span><span class="pln"> </span><span class="lit">@interface</span><span class="pln"> </span><span class="typ">TestA</span><span class="pln"> </span><span class="pun">{</span></li><li class="L4"><span class="com">//这里定义了一个空的注解，它能干什么呢。我也不知道，但他能用。</span></li><li class="L5"><span class="pun">}</span></li></ol>

在下面这个程序中使用它：

<ol class="linenums"><li class="L0"><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L1"><span class="pln"> </span></li><li class="L2"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">util</span><span class="pun">.</span><span class="typ">HashMap</span><span class="pun">;</span></li><li class="L3"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">util</span><span class="pun">.</span><span class="typ">Map</span><span class="pun">;</span></li><li class="L4"><span class="pln"> </span></li><li class="L5"><span class="com">/**</span></li><li class="L6"><span class="com"> * 这个类专门用来测试注解使用</span></li><li class="L7"><span class="com"> * @author tmser</span></li><li class="L8"><span class="com"> */</span></li><li class="L9"><span class="lit">@TestA</span><span class="pln">    </span><span class="com">//使用了类注解</span></li><li class="L0"><span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">UserAnnotation</span><span class="pln"> </span><span class="pun">{</span></li><li class="L1"><span class="pln">    </span></li><li class="L2"><span class="pln">    </span><span class="lit">@TestA</span><span class="pln"> </span><span class="com">//使用了类成员注解</span></li><li class="L3"><span class="pln">    </span><span class="kwd">private</span><span class="pln"> </span><span class="typ">Integer</span><span class="pln"> age</span><span class="pun">;</span></li><li class="L4"><span class="pln">    </span></li><li class="L5"><span class="pln">    </span><span class="lit">@TestA</span><span class="pln"> </span><span class="com">//使用了构造方法注解</span></li><li class="L6"><span class="pln">    </span><span class="kwd">public</span><span class="pln"> </span><span class="typ">UserAnnotation</span><span class="pun">(){</span></li><li class="L7"><span class="pln">        </span></li><li class="L8"><span class="pln">    </span><span class="pun">}</span></li><li class="L9"><span class="pln">    </span><span class="lit">@TestA</span><span class="pln"> </span><span class="com">//使用了类方法注解</span></li><li class="L0"><span class="pln">    </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> a</span><span class="pun">(){</span></li><li class="L1"><span class="pln">        </span><span class="lit">@TestA</span><span class="pln"> </span><span class="com">//使用了局部变量注解</span></li><li class="L2"><span class="pln">        </span><span class="typ">Map</span><span class="pln"> m </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HashMap</span><span class="pun">(</span><span class="lit">0</span><span class="pun">);</span></li><li class="L3"><span class="pln">    </span><span class="pun">}</span></li><li class="L4"><span class="pln">    </span></li><li class="L5"><span class="pln">    </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> b</span><span class="pun">(</span><span class="lit">@TestA</span><span class="pln"> </span><span class="typ">Integer</span><span class="pln"> a</span><span class="pun">){</span><span class="pln"> </span><span class="com">//使用了方法参数注解</span></li><li class="L6"><span class="pln">        </span></li><li class="L7"><span class="pln">    </span><span class="pun">}</span></li><li class="L8"><span class="pun">}</span></li></ol>

编译没有报错，ok，一个注解实验完成。这个注解也太简单了吧，好像什么信息也不能传递。别急下面就来一步步完善它，也该四位元注解依次开始上场了。

四个元注解分别是：@Target,@Retention,@Documented,@Inherited ，再次强调下元注解是java API提供，是专门用来定义注解的注解，其作用分别如下。
   
        @Target 表示该注解用于什么地方，可能的值在枚举类 ElemenetType 中，包括： 
          ElemenetType.CONSTRUCTOR 构造器声明 
          ElemenetType.FIELD 域声明（包括 enum 实例） 
          ElemenetType.LOCAL_VARIABLE 局部变量声明 
          ElemenetType.METHOD 方法声明 
          ElemenetType.PACKAGE 包声明 
          ElemenetType.PARAMETER 参数声明 
          ElemenetType.TYPE 类，接口（包括注解类型）或enum声明 
           
     @Retention 表示在什么级别保存该注解信息。可选的参数值在枚举类型 RetentionPolicy 中，包括： 
          RetentionPolicy.SOURCE 注解将被编译器丢弃 
          RetentionPolicy.CLASS 注解在class文件中可用，但会被VM丢弃 
          RetentionPolicy.RUNTIME VM将在运行期也保留注释，因此可以通过反射机制读取注解的信息。 
           
      @Documented 将此注解包含在 javadoc 中 ，它代表着此注解会被javadoc工具提取成文档。在doc文档中的内容会因为此注解的信息内容不同而不同。相当与@see,@param 等。
       
      @Inherited 允许子类继承父类中的注解，例子中补充。

   

学习最忌好高骛远，我们就一个一个来实验。

第一个：@Target，动手在前面我们编写的注解上加上元注解。

<ol class="linenums"><li class="L0"><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L1"><span class="pln"> </span></li><li class="L2"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">.</span><span class="typ">ElementType</span><span class="pun">;</span></li><li class="L3"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">.</span><span class="typ">Target</span><span class="pun">;</span></li><li class="L4"><span class="com">/* </span></li><li class="L5"><span class="com"> * 定义注解 Test </span></li><li class="L6"><span class="com"> * 首先使用ElementType.TYPE</span></li><li class="L7"><span class="com"> */</span><span class="pln"> </span></li><li class="L8"><span class="lit">@Target</span><span class="pun">(</span><span class="typ">ElementType</span><span class="pun">.</span><span class="pln">PACKAGE</span><span class="pun">)</span><span class="pln">  </span></li><li class="L9"><span class="kwd">public</span><span class="pln"> </span><span class="lit">@interface</span><span class="pln"> </span><span class="typ">TestA</span><span class="pln"> </span><span class="pun">{</span></li><li class="L0"><span class="pln"> </span></li><li class="L1"><span class="pun">}</span></li></ol>

ctrl+ s 保存，今天电脑比较给力，我们的测试类那边立马出现了一堆错误，除了类注解。我想到这，聪明的你立刻明白了这个元注解的意义了。是不是想当然的偷起懒来了。？难道还有意外？细心的朋友应该发现了，我们的测试类少了一个属性没用，就是ElemenetType.PACKAGE。在我们的注解加上这个属性的元注解后，我们测试程序的元注解全部阵亡，不对，还有一个没加呢，好加上。package 包，想当然是加载 package 前面。即

<ol class="linenums"><li class="L0"><span class="lit">@TestA</span><span class="pln"> </span><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li></ol>

什么也报错。这就搞不明白了，不加在这加哪去呢。我也不知道了，不过这是编译错误，我们的eclipse 将错误给我们指出了，就是

  Package annotations must be in file package-info.java ，e 文虽然不好，但这个简单的还是难不倒几个人的，package 注解必须定义在 package-info.java 中。package-info 又是什么东西，好了为节省你们的时间帮你百度好了，传送门。ok,到此 target 元注解就全部完成了。

第二个元注解： @Retention 参数 RetentionPolicy。有了前面的经验这个注解理解起来就简单多了，并且幸运的是这个注解还没有特殊的属性值。 简单演示下如何使用：

<ol class="linenums"><li class="L0"><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L1"><span class="pln"> </span></li><li class="L2"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">.</span><span class="typ">ElementType</span><span class="pun">;</span></li><li class="L3"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">.</span><span class="typ">Target</span><span class="pun">;</span></li><li class="L4"><span class="com">/* </span></li><li class="L5"><span class="com"> * 定义注解 Test </span></li><li class="L6"><span class="com"> * 首先使用ElementType.TYPE</span></li><li class="L7"><span class="com"> * 运行级别定为 运行时，以便后面测试解析</span></li><li class="L8"><span class="com"> */</span><span class="pln"> </span></li><li class="L9"><span class="lit">@Target</span><span class="pun">(</span><span class="typ">ElementType</span><span class="pun">.</span><span class="pln">PACKAGE</span><span class="pun">)</span></li><li class="L0">
</li><li class="L1"><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px;"><span class="pln">	</span><span class="lit">@Retention</span><span class="pun">(</span><span class="typ">RetentionPolicy</span><span class="pun">.</span><span class="pln">RUNTIME</span><span class="pun">)</span></p></li><li class="L2"><span class="pln"> </span></li><li class="L3"><span class="kwd">public</span><span class="pln"> </span><span class="lit">@interface</span><span class="pln"> </span><span class="typ">TestA</span><span class="pln"> </span><span class="pun">{</span></li><li class="L4"><span class="pln"> </span></li><li class="L5"><span class="pun">}</span></li></ol>

第三和第四个元注解就不再举例了。比较简单，也没有值，相信看过上面的解释也就清楚了。下面我们还是继续来深入的探讨下注解的使用。上面的例子都非常简单，注解连属性都没有。ok，下面我们就来定义一个有属性的注解，并在例子程序中获取都注解中定义的值。

开始之前将下定义属性的规则：

        @interface用来声明一个注解，其中的每一个方法实际上是声明了一个配置参数。方法的名称就是参数的名称，返回值类型就是参数的类型（返回值类型只能是基本类型、Class、String、enum）。可以通过default来声明参数的默认值。

代码：

<ol class="linenums"><li class="L0"><span class="com">/* </span></li><li class="L1"><span class="com"> * 定义注解 Test </span></li><li class="L2"><span class="com"> * 为方便测试：注解目标为类 方法，属性及构造方法 </span></li><li class="L3"><span class="com"> * 注解中含有三个元素 id ,name和 gid; </span></li><li class="L4"><span class="com"> * id 元素 有默认值 0</span></li><li class="L5"><span class="com"> */</span><span class="pln"> </span></li><li class="L6"><span class="lit">@Target</span><span class="pun">({</span><span class="pln">TYPE</span><span class="pun">,</span><span class="pln">METHOD</span><span class="pun">,</span><span class="pln">FIELD</span><span class="pun">,</span><span class="pln">CONSTRUCTOR</span><span class="pun">})</span></li><li class="L7"><span class="lit">@Retention</span><span class="pun">(</span><span class="typ">RetentionPolicy</span><span class="pun">.</span><span class="pln">RUNTIME</span><span class="pun">)</span></li><li class="L8"><span class="kwd">public</span><span class="pln"> </span><span class="lit">@interface</span><span class="pln"> </span><span class="typ">TestA</span><span class="pln"> </span><span class="pun">{</span></li><li class="L9"><span class="pln">	</span><span class="typ">String</span><span class="pln"> name</span><span class="pun">();</span></li><li class="L0"><span class="pln">	</span><span class="kwd">int</span><span class="pln"> id</span><span class="pun">()</span><span class="pln"> </span><span class="kwd">default</span><span class="pln"> </span><span class="lit">0</span><span class="pun">;</span></li><li class="L1"><span class="pln">	</span><span class="typ">Class</span><span class="pun"><</span><span class="typ">Long</span><span class="pun">></span><span class="pln"> gid</span><span class="pun">();</span></li><li class="L2"><span class="pun">}</span></li></ol>

下面改下我们的测试类：

<ol class="linenums"><li class="L0"><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L1"><span class="pln"> </span></li><li class="L2"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">util</span><span class="pun">.</span><span class="typ">HashMap</span><span class="pun">;</span></li><li class="L3"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">util</span><span class="pun">.</span><span class="typ">Map</span><span class="pun">;</span></li><li class="L4"><span class="pln"> </span></li><li class="L5"><span class="com">/**</span></li><li class="L6"><span class="com"> * 这个类专门用来测试注解使用</span></li><li class="L7"><span class="com"> * @author tmser</span></li><li class="L8"><span class="com"> */</span></li><li class="L9"><span class="pln"> </span></li><li class="L0"><span class="lit">@TestA</span><span class="pun">(</span><span class="pln">name</span><span class="pun">=</span><span class="str">"type"</span><span class="pun">,</span><span class="pln">gid</span><span class="pun">=</span><span class="typ">Long</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">)</span><span class="pln"> </span><span class="com">//类成员注解</span></li><li class="L1"><span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">UserAnnotation</span><span class="pln"> </span><span class="pun">{</span></li><li class="L2"><span class="pln">	</span></li><li class="L3"><span class="pln">	</span><span class="lit">@TestA</span><span class="pun">(</span><span class="pln">name</span><span class="pun">=</span><span class="str">"param"</span><span class="pun">,</span><span class="pln">id</span><span class="pun">=</span><span class="lit">1</span><span class="pun">,</span><span class="pln">gid</span><span class="pun">=</span><span class="typ">Long</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">)</span><span class="pln"> </span><span class="com">//类成员注解</span></li><li class="L4"><span class="pln">	</span><span class="kwd">private</span><span class="pln"> </span><span class="typ">Integer</span><span class="pln"> age</span><span class="pun">;</span></li><li class="L5"><span class="pln">	</span></li><li class="L6"><span class="pln">	</span><span class="lit">@TestA</span><span class="pln"> </span><span class="pun">(</span><span class="pln">name</span><span class="pun">=</span><span class="str">"construct"</span><span class="pun">,</span><span class="pln">id</span><span class="pun">=</span><span class="lit">2</span><span class="pun">,</span><span class="pln">gid</span><span class="pun">=</span><span class="typ">Long</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">)</span><span class="com">//构造方法注解</span></li><li class="L7"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="typ">UserAnnotation</span><span class="pun">(){</span></li><li class="L8"><span class="pln">		</span></li><li class="L9"><span class="pln">	</span><span class="pun">}</span></li><li class="L0"><span class="pln">	</span><span class="lit">@TestA</span><span class="pun">(</span><span class="pln">name</span><span class="pun">=</span><span class="str">"public method"</span><span class="pun">,</span><span class="pln">id</span><span class="pun">=</span><span class="lit">3</span><span class="pun">,</span><span class="pln">gid</span><span class="pun">=</span><span class="typ">Long</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">)</span><span class="pln"> </span><span class="com">//类方法注解</span></li><li class="L1"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> a</span><span class="pun">(){</span></li><li class="L2"><span class="pln">		</span><span class="typ">Map</span><span class="pun"><</span><span class="typ">String</span><span class="pun">,</span><span class="typ">String</span><span class="pun">></span><span class="pln"> m </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HashMap</span><span class="pun"><</span><span class="typ">String</span><span class="pun">,</span><span class="typ">String</span><span class="pun">>(</span><span class="lit">0</span><span class="pun">);</span></li><li class="L3"><span class="pln">	</span><span class="pun">}</span></li><li class="L4"><span class="pln">	</span></li><li class="L5"><span class="pln">	</span><span class="lit">@TestA</span><span class="pun">(</span><span class="pln">name</span><span class="pun">=</span><span class="str">"protected method"</span><span class="pun">,</span><span class="pln">id</span><span class="pun">=</span><span class="lit">4</span><span class="pun">,</span><span class="pln">gid</span><span class="pun">=</span><span class="typ">Long</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">)</span><span class="pln"> </span><span class="com">//类方法注解</span></li><li class="L6"><span class="pln">	</span><span class="kwd">protected</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> b</span><span class="pun">(){</span></li><li class="L7"><span class="pln">		</span><span class="typ">Map</span><span class="pun"><</span><span class="typ">String</span><span class="pun">,</span><span class="typ">String</span><span class="pun">></span><span class="pln"> m </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HashMap</span><span class="pun"><</span><span class="typ">String</span><span class="pun">,</span><span class="typ">String</span><span class="pun">>(</span><span class="lit">0</span><span class="pun">);</span></li><li class="L8"><span class="pln">	</span><span class="pun">}</span></li><li class="L9"><span class="pln">	</span></li><li class="L0"><span class="pln">	</span><span class="lit">@TestA</span><span class="pun">(</span><span class="pln">name</span><span class="pun">=</span><span class="str">"private method"</span><span class="pun">,</span><span class="pln">id</span><span class="pun">=</span><span class="lit">5</span><span class="pun">,</span><span class="pln">gid</span><span class="pun">=</span><span class="typ">Long</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">)</span><span class="pln"> </span><span class="com">//类方法注解</span></li><li class="L1"><span class="pln">	</span><span class="kwd">private</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> c</span><span class="pun">(){</span></li><li class="L2"><span class="pln">		</span><span class="typ">Map</span><span class="pun"><</span><span class="typ">String</span><span class="pun">,</span><span class="typ">String</span><span class="pun">></span><span class="pln"> m </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HashMap</span><span class="pun"><</span><span class="typ">String</span><span class="pun">,</span><span class="typ">String</span><span class="pun">>(</span><span class="lit">0</span><span class="pun">);</span></li><li class="L3"><span class="pln">	</span><span class="pun">}</span></li><li class="L4"><span class="pln">	</span></li><li class="L5"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> b</span><span class="pun">(</span><span class="typ">Integer</span><span class="pln"> a</span><span class="pun">){</span><span class="pln"> </span></li><li class="L6"><span class="pln">		</span></li><li class="L7"><span class="pln">	</span><span class="pun">}</span></li><li class="L8"><span class="pun">}</span></li><li class="L9"><span class="pln"> </span></li></ol>

下面到了最重要的一步了，就是如何读取我们在类中定义的注解。只要读取出来了使用的话就简单了。

<ol class="linenums"><li class="L0"><span class="kwd">package</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L1"><span class="pln"> </span></li><li class="L2"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">.</span><span class="typ">Annotation</span><span class="pun">;</span></li><li class="L3"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">reflect</span><span class="pun">.</span><span class="typ">Constructor</span><span class="pun">;</span></li><li class="L4"><span class="kwd">import</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="pln">reflect</span><span class="pun">.</span><span class="typ">Method</span><span class="pun">;</span></li><li class="L5"><span class="pln"> </span></li><li class="L6"><span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">ParseAnnotation</span><span class="pln"> </span><span class="pun">{</span></li><li class="L7"><span class="pln"> </span></li><li class="L8"><span class="pln">	</span><span class="com">/**</span></li><li class="L9"><span class="com">	 * 简单打印出UserAnnotation 类中所使用到的类注解</span></li><li class="L0"><span class="com">	 * 该方法只打印了 Type 类型的注解</span></li><li class="L1"><span class="com">	 * @throws ClassNotFoundException</span></li><li class="L2"><span class="com">	 */</span></li><li class="L3"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> parseTypeAnnotation</span><span class="pun">()</span><span class="pln"> </span><span class="kwd">throws</span><span class="pln"> </span><span class="typ">ClassNotFoundException</span><span class="pln"> </span><span class="pun">{</span><span class="pln">  </span></li><li class="L4"><span class="pln">        </span><span class="typ">Class</span><span class="pln"> clazz </span><span class="pun">=</span><span class="pln"> </span><span class="typ">Class</span><span class="pun">.</span><span class="pln">forName</span><span class="pun">(</span><span class="str">"com.tmser.annotation.UserAnnotation"</span><span class="pun">);</span><span class="pln"> </span></li><li class="L5"><span class="pln">        </span></li><li class="L6"><span class="pln">        </span><span class="typ">Annotation</span><span class="pun">[]</span><span class="pln"> annotations </span><span class="pun">=</span><span class="pln"> clazz</span><span class="pun">.</span><span class="pln">getAnnotations</span><span class="pun">();</span><span class="pln">  </span></li><li class="L7"><span class="pln">        </span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Annotation</span><span class="pln"> annotation </span><span class="pun">:</span><span class="pln"> annotations</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">  </span></li><li class="L8"><span class="pln">        	</span><span class="typ">TestA</span><span class="pln"> testA </span><span class="pun">=</span><span class="pln"> </span><span class="pun">(</span><span class="typ">TestA</span><span class="pun">)</span><span class="pln">annotation</span><span class="pun">;</span></li><li class="L9"><span class="pln">            </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"id= \""</span><span class="pun">+</span><span class="pln">testA</span><span class="pun">.</span><span class="pln">id</span><span class="pun">()+</span><span class="str">"\"; name= \""</span><span class="pun">+</span><span class="pln">testA</span><span class="pun">.</span><span class="pln">name</span><span class="pun">()+</span><span class="str">"\"; gid = "</span><span class="pun">+</span><span class="pln">testA</span><span class="pun">.</span><span class="pln">gid</span><span class="pun">());</span><span class="pln">  </span></li><li class="L0"><span class="pln">        </span><span class="pun">}</span><span class="pln">  </span></li><li class="L1"><span class="pln">    </span><span class="pun">}</span><span class="pln"> </span></li><li class="L2"><span class="pln">	</span></li><li class="L3"><span class="pln">	</span><span class="com">/**</span></li><li class="L4"><span class="com">	 * 简单打印出UserAnnotation 类中所使用到的方法注解</span></li><li class="L5"><span class="com">	 * 该方法只打印了 Method 类型的注解</span></li><li class="L6"><span class="com">	 * @throws ClassNotFoundException</span></li><li class="L7"><span class="com">	 */</span></li><li class="L8"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> parseMethodAnnotation</span><span class="pun">(){</span></li><li class="L9"><span class="pln">		</span><span class="typ">Method</span><span class="pun">[]</span><span class="pln"> methods </span><span class="pun">=</span><span class="pln"> </span><span class="typ">UserAnnotation</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">.</span><span class="pln">getDeclaredMethods</span><span class="pun">();</span><span class="pln">  </span></li><li class="L0"><span class="pln">        </span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Method</span><span class="pln"> method </span><span class="pun">:</span><span class="pln"> methods</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">  </span></li><li class="L1"><span class="pln">            </span><span class="com">/* </span></li><li class="L2"><span class="com">             * 判断方法中是否有指定注解类型的注解 </span></li><li class="L3"><span class="com">             */</span><span class="pln">  </span></li><li class="L4"><span class="pln">            </span><span class="kwd">boolean</span><span class="pln"> hasAnnotation </span><span class="pun">=</span><span class="pln"> method</span><span class="pun">.</span><span class="pln">isAnnotationPresent</span><span class="pun">(</span><span class="typ">TestA</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">);</span><span class="pln">  </span></li><li class="L5"><span class="pln">            </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">hasAnnotation</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">  </span></li><li class="L6"><span class="pln">                </span><span class="com">/* </span></li><li class="L7"><span class="com">                 * 根据注解类型返回方法的指定类型注解 </span></li><li class="L8"><span class="com">                 */</span><span class="pln">  </span></li><li class="L9"><span class="pln">            	</span><span class="typ">TestA</span><span class="pln"> annotation </span><span class="pun">=</span><span class="pln"> method</span><span class="pun">.</span><span class="pln">getAnnotation</span><span class="pun">(</span><span class="typ">TestA</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">);</span><span class="pln">  </span></li><li class="L0"><span class="pln">                </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"method = "</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> method</span><span class="pun">.</span><span class="pln">getName</span><span class="pun">()</span><span class="pln">  </span></li><li class="L1"><span class="pln">                        </span><span class="pun">+</span><span class="pln"> </span><span class="str">" ; id = "</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> annotation</span><span class="pun">.</span><span class="pln">id</span><span class="pun">()</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="str">" ; description = "</span><span class="pln">  </span></li><li class="L2"><span class="pln">                        </span><span class="pun">+</span><span class="pln"> annotation</span><span class="pun">.</span><span class="pln">name</span><span class="pun">()</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="str">"; gid= "</span><span class="pun">+</span><span class="pln">annotation</span><span class="pun">.</span><span class="pln">gid</span><span class="pun">());</span><span class="pln">  </span></li><li class="L3"><span class="pln">            </span><span class="pun">}</span><span class="pln">  </span></li><li class="L4"><span class="pln">        </span><span class="pun">}</span><span class="pln">  </span></li><li class="L5"><span class="pln">	</span><span class="pun">}</span></li><li class="L6"><span class="pln">	</span></li><li class="L7"><span class="pln">	</span><span class="com">/**</span></li><li class="L8"><span class="com">	 * 简单打印出UserAnnotation 类中所使用到的方法注解</span></li><li class="L9"><span class="com">	 * 该方法只打印了 Method 类型的注解</span></li><li class="L0"><span class="com">	 * @throws ClassNotFoundException</span></li><li class="L1"><span class="com">	 */</span></li><li class="L2"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> parseConstructAnnotation</span><span class="pun">(){</span></li><li class="L3"><span class="pln">		</span><span class="typ">Constructor</span><span class="pun">[]</span><span class="pln"> constructors </span><span class="pun">=</span><span class="pln"> </span><span class="typ">UserAnnotation</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">.</span><span class="pln">getConstructors</span><span class="pun">();</span><span class="pln">  </span></li><li class="L4"><span class="pln">        </span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Constructor</span><span class="pln"> constructor </span><span class="pun">:</span><span class="pln"> constructors</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"> </span></li><li class="L5"><span class="pln">        	</span><span class="com">/* </span></li><li class="L6"><span class="com">             * 判断构造方法中是否有指定注解类型的注解 </span></li><li class="L7"><span class="com">             */</span><span class="pln">  </span></li><li class="L8"><span class="pln">            </span><span class="kwd">boolean</span><span class="pln"> hasAnnotation </span><span class="pun">=</span><span class="pln"> constructor</span><span class="pun">.</span><span class="pln">isAnnotationPresent</span><span class="pun">(</span><span class="typ">TestA</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">);</span><span class="pln">  </span></li><li class="L9"><span class="pln">            </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">hasAnnotation</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">  </span></li><li class="L0"><span class="pln">                </span><span class="com">/* </span></li><li class="L1"><span class="com">                 * 根据注解类型返回方法的指定类型注解 </span></li><li class="L2"><span class="com">                 */</span><span class="pln">  </span></li><li class="L3"><span class="pln">            	</span><span class="typ">TestA</span><span class="pln"> annotation </span><span class="pun">=(</span><span class="typ">TestA</span><span class="pun">)</span><span class="pln"> constructor</span><span class="pun">.</span><span class="pln">getAnnotation</span><span class="pun">(</span><span class="typ">TestA</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">);</span><span class="pln">  </span></li><li class="L4"><span class="pln">                </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"constructor = "</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> constructor</span><span class="pun">.</span><span class="pln">getName</span><span class="pun">()</span><span class="pln">  </span></li><li class="L5"><span class="pln">                        </span><span class="pun">+</span><span class="pln"> </span><span class="str">" ; id = "</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> annotation</span><span class="pun">.</span><span class="pln">id</span><span class="pun">()</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="str">" ; description = "</span><span class="pln">  </span></li><li class="L6"><span class="pln">                        </span><span class="pun">+</span><span class="pln"> annotation</span><span class="pun">.</span><span class="pln">name</span><span class="pun">()</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="str">"; gid= "</span><span class="pun">+</span><span class="pln">annotation</span><span class="pun">.</span><span class="pln">gid</span><span class="pun">());</span><span class="pln">  </span></li><li class="L7"><span class="pln">            </span><span class="pun">}</span><span class="pln">  </span></li><li class="L8"><span class="pln">        </span><span class="pun">}</span><span class="pln">  </span></li><li class="L9"><span class="pln">	</span><span class="pun">}</span></li><li class="L0"><span class="pln">	</span></li><li class="L1"><span class="pln">	</span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> main</span><span class="pun">(</span><span class="typ">String</span><span class="pun">[]</span><span class="pln"> args</span><span class="pun">)</span><span class="pln"> </span><span class="kwd">throws</span><span class="pln"> </span><span class="typ">ClassNotFoundException</span><span class="pln"> </span><span class="pun">{</span></li><li class="L2"><span class="pln">		parseTypeAnnotation</span><span class="pun">();</span></li><li class="L3"><span class="pln">		parseMethodAnnotation</span><span class="pun">();</span></li><li class="L4"><span class="pln">		parseConstructAnnotation</span><span class="pun">();</span></li><li class="L5"><span class="pln">	</span><span class="pun">}</span></li><li class="L6"><span class="pun">}</span></li></ol>

先别说话，运行：

<ol class="linenums"><li class="L0"><span class="pln">id</span><span class="pun">=</span><span class="pln"> </span><span class="str">"0"</span><span class="pun">;</span><span class="pln"> name</span><span class="pun">=</span><span class="pln"> </span><span class="str">"type"</span><span class="pun">;</span><span class="pln"> gid </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="typ">Long</span></li><li class="L1"><span class="pln">method </span><span class="pun">=</span><span class="pln"> c </span><span class="pun">;</span><span class="pln"> id </span><span class="pun">=</span><span class="pln"> </span><span class="lit">5</span><span class="pln"> </span><span class="pun">;</span><span class="pln"> description </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">private</span><span class="pln"> method</span><span class="pun">;</span><span class="pln"> gid</span><span class="pun">=</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="typ">Long</span></li><li class="L2"><span class="pln">method </span><span class="pun">=</span><span class="pln"> a </span><span class="pun">;</span><span class="pln"> id </span><span class="pun">=</span><span class="pln"> </span><span class="lit">3</span><span class="pln"> </span><span class="pun">;</span><span class="pln"> description </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">public</span><span class="pln"> method</span><span class="pun">;</span><span class="pln"> gid</span><span class="pun">=</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="typ">Long</span></li><li class="L3"><span class="pln">method </span><span class="pun">=</span><span class="pln"> b </span><span class="pun">;</span><span class="pln"> id </span><span class="pun">=</span><span class="pln"> </span><span class="lit">4</span><span class="pln"> </span><span class="pun">;</span><span class="pln"> description </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">protected</span><span class="pln"> method</span><span class="pun">;</span><span class="pln"> gid</span><span class="pun">=</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="typ">Long</span></li><li class="L4"><span class="pln">constructor </span><span class="pun">=</span><span class="pln"> com</span><span class="pun">.</span><span class="pln">tmser</span><span class="pun">.</span><span class="pln">annotation</span><span class="pun">.</span><span class="typ">UserAnnotation</span><span class="pln"> </span><span class="pun">;</span><span class="pln"> id </span><span class="pun">=</span><span class="pln"> </span><span class="lit">2</span><span class="pln"> </span><span class="pun">;</span><span class="pln"> description </span><span class="pun">=</span><span class="pln"> construct</span><span class="pun">;</span><span class="pln"> gid</span><span class="pun">=</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> java</span><span class="pun">.</span><span class="pln">lang</span><span class="pun">.</span><span class="typ">Long</span></li></ol>

看到了吧，我们定义的注解都完整的输出了，你要使用哪个，直接拿去用就好了。

为了不让这篇文章打开太慢，我省略了类属性注解，及参数注解的解析。其实都大同小异。

另外，我也没有举使用例子。因为我认为好的教程是讲的详细的同时，还会留有扩展。如果我全部写出来，而你只是学习的话，那基本不会自己去动脑了，而是复制粘贴运行一遍完事。

   最后提醒下：

     1. 要用好注解，必须熟悉java 的反射机制，从上面的例子可以看出，注解的解析完全依赖于反射。

     2. 不要滥用注解。平常我们编程过程很少接触和使用注解，只有做设计，且不想让设计有过多的配置时。