一、介绍:
注解（也被称为元数据）为我们在代码中添加信息提供了一种形式化的方式，使我们可以在稍后的某个时刻更容易的使用这些数据。
注解是 Java 5 所引入的众多语言变化之一。它们提供了 Java 无法表达的但是你需要完整表述程序所需的信息。因此，注解使得我们可以以编译器验证的格式存储程序的额外信息。注解可以生成描述符文件，甚至是新的类定义，并且有助于减轻编写“样板”代码的负担。通过使用注解，你可以将元数据保存在 Java 源代码中。并拥有如下优势：简单易读的代码，编译器类型检查，使用 annotation API 为自己的注解构造处理工具。即使 Java 定义了一些类型的元数据，但是一般来说注解类型的添加和如何使用完全取决于你。
二、Jdk中常见的注解:
- @Override： 表示当前的方法定义将覆盖基类的方法。如果你不小心拼写错误，或者方法签名被错误拼写的时候，编译器就会发出错误提示。
- @Deprecated： 如果使用该注解的元素被调用，编译器就会发出警告信息。
- @SuppressWarnings： 关闭不当的编译器警告信息。
- @SafeVarargs： 在 Java 7 中加入用于禁止对具有泛型varargs参数的方法或构造函数的调用方
- @FunctionalInterface： Java 8 中加入用于表示类型声明为函数式接口
每当创建涉及重复工作的类或接口时，你通常可以使用注解来自动化和简化流程。例如在 Enterprise JavaBean（EJB）中的许多额外工作就是通过注解来消除的。
三、元注解(本文并没有实操所有元注解，想学习可自查)
Java 语言中目前有 5 种标准注解（前面介绍过），以及 5 种元注解。元注解用于注解其他的注解
四、定义注解：
|
1 |
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>annotation<span class="token punctuation">.</span></span><span class="token class-name">ElementType</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>annotation<span class="token punctuation">.</span></span><span class="token class-name">Retention</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>annotation<span class="token punctuation">.</span></span><span class="token class-name">RetentionPolicy</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>annotation<span class="token punctuation">.</span></span><span class="token class-name">Target</span><span class="token punctuation">;</span> <span class="token annotation punctuation">@Target</span><span class="token punctuation">(</span><span class="token punctuation">{<!-- --></span><span class="token class-name">ElementType</span><span class="token punctuation">.</span>METHOD<span class="token punctuation">,</span><span class="token class-name">ElementType</span><span class="token punctuation">.</span>FIELD<span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token annotation punctuation">@Retention</span><span class="token punctuation">(</span><span class="token class-name">RetentionPolicy</span><span class="token punctuation">.</span>RUNTIME<span class="token punctuation">)</span> <span class="token keyword">public</span> <span class="token annotation punctuation">@interface</span> <span class="token class-name">AnimalAnnotation</span> <span class="token punctuation">{<!-- --></span> <span class="token class-name">String</span> <span class="token function">value</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">default</span> <span class="token string">" "</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token function">id</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">default</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token class-name">String</span> <span class="token function">hairColor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">default</span> <span class="token string">" "</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
除了 @ 符号之外， @AnimalAnnotation的定义看起来更像一个空接口。注解的定义也需要一些元注解（meta-annotation），比如 @Target 和 @Retention。@Target 定义你的注解可以应用在哪里（例如是方法还是字段）。@Retention 定义了注解在哪里可用，在源代码中（SOURCE），class文件（CLASS）中或者是在运行时（RUNTIME）。
注解通常会包含一些表示特定值的元素。当分析处理注解的时候，程序或工具可以利用这些值。注解的元素看起来就像接口的方法，但是可以为其指定默认值。
不包含任何元素的注解称为标记注解（marker annotation），例如上例中的 @AnimalAnnotation 就是标记注解。
五、使用注解
|
1 |
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Dog</span> <span class="token punctuation">{<!-- --></span> <span class="token annotation punctuation">@AnimalAnnotation</span><span class="token punctuation">(</span><span class="token string">"小黄"</span><span class="token punctuation">)</span> <span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span> <span class="token annotation punctuation">@AnimalAnnotation</span><span class="token punctuation">(</span>hairColor <span class="token operator">=</span> <span class="token string">"黄色"</span><span class="token punctuation">)</span> <span class="token keyword">private</span> <span class="token class-name">String</span> color<span class="token punctuation">;</span> <span class="token annotation punctuation">@AnimalAnnotation</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">getDog</span><span class="token punctuation">(</span><span class="token class-name">String</span> dogData<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>dogData<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> |
六、处理器
如果没有用于读取注解的工具，那么注解不会比注释更有用。使用注解中一个很重要的部分就是，创建与使用注解处理器。Java 拓展了反射机制的 API 用于帮助你创造这类工具。同时他还提供了 javac 编译器钩子在编译时使用注解。
下面是一个非常简单的注解处理器，我们用它来读取被注解的 Dog 类，并且使用反射机制来寻找 @AnimalAnnotation标记。
|
1 |
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>reflect<span class="token punctuation">.</span></span><span class="token class-name">Field</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>reflect<span class="token punctuation">.</span></span><span class="token class-name">InvocationTargetException</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span>reflect<span class="token punctuation">.</span></span><span class="token class-name">Method</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">Arrays</span><span class="token punctuation">;</span> <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">AnnotationProcessor</span> <span class="token punctuation">{<!-- --></span> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">operationAnnotation</span><span class="token punctuation">(</span><span class="token class-name">Class</span><span class="token generics"><span class="token punctuation"><</span><span class="token operator">?</span><span class="token punctuation">></span></span> cl<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">NoSuchMethodException</span><span class="token punctuation">,</span> <span class="token class-name">InstantiationException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalAccessException</span><span class="token punctuation">,</span> <span class="token class-name">InvocationTargetException</span> <span class="token punctuation">{<!-- --></span> <span class="token class-name">Field</span><span class="token punctuation">[</span><span class="token punctuation">]</span> fields <span class="token operator">=</span> cl<span class="token punctuation">.</span><span class="token function">getDeclaredFields</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">StringBuffer</span> stringBuffer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuffer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span>fields<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>p<span class="token operator">-></span><span class="token punctuation">{<!-- --></span> <span class="token class-name">AnimalAnnotation</span> annotation <span class="token operator">=</span> p<span class="token punctuation">.</span><span class="token function">getAnnotation</span><span class="token punctuation">(</span><span class="token class-name">AnimalAnnotation</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span><span class="token punctuation">(</span>annotation<span class="token punctuation">.</span><span class="token function">value</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">></span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span> stringBuffer<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"我家小狗叫:"</span><span class="token operator">+</span>annotation<span class="token punctuation">.</span><span class="token function">value</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">if</span><span class="token punctuation">(</span>annotation<span class="token punctuation">.</span><span class="token function">hairColor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">></span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span> stringBuffer<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"\t它的毛发是:"</span><span class="token operator">+</span>annotation<span class="token punctuation">.</span><span class="token function">hairColor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">Method</span> method <span class="token operator">=</span> cl<span class="token punctuation">.</span><span class="token function">getDeclaredMethod</span><span class="token punctuation">(</span><span class="token string">"getDog"</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">AnimalAnnotation</span> annotation <span class="token operator">=</span> method<span class="token punctuation">.</span><span class="token function">getAnnotation</span><span class="token punctuation">(</span><span class="token class-name">AnimalAnnotation</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span><span class="token punctuation">(</span>annotation<span class="token punctuation">.</span><span class="token function">id</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span> stringBuffer<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"\t编号为:"</span><span class="token operator">+</span>annotation<span class="token punctuation">.</span><span class="token function">id</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> method<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>cl<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>stringBuffer<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InvocationTargetException</span><span class="token punctuation">,</span> <span class="token class-name">NoSuchMethodException</span><span class="token punctuation">,</span> <span class="token class-name">InstantiationException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalAccessException</span> <span class="token punctuation">{<!-- --></span> <span class="token function">operationAnnotation</span><span class="token punctuation">(</span><span class="token class-name">Dog</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> |
处理器肯定能做很多事情、我只是想简单一点，或者说我了解的比较少，你可以对加了注解的方法加一个监听器，然后获取其参数去执行你想执行的方法，这个蛮有用的，其实注解本身并没有什么作用，它只是一个标识或描述，具体行为由你写的处理器去进行执行，我这个处理器是运行期间进行的，有的还可以在编译期间进行操作，然后对外进行输出文件，比如说mybatis-plus的生成工具或者类似的框架，哪些算是复杂的处理器了。
微信扫一扫打赏
支付宝扫一扫打赏
