一、Java 自定义注解详解

1. 定义注解：

注解的定义很像接口的定义。事实上与其他 java 接口一样，注解也会被编译成 class 文件。定义注解时需要一些元注解。

2. 元注解介绍

@Target 详细介绍

@Relation 详细介绍

@Documented 介绍

将此注解包含在 javadoc 中

@Inherited 介绍

允许子类继承父类中的注解，千万不要误解是注解的嵌套，是 Class 类继承的时候，是否拥有父类的注解

3. 注解可用的类型

• 所有基本类型（int、float、double，boolean）

• String

• Class

• enum

• Annotaion

• 以上类型的数组

  注解可以嵌套注解

4. 默认值限制

元素必须具有默认值，默认值不能是 null

注解不支持继承，不像类一样可以通过 extends 继承

5. 创建一个简单的自定义注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface CustomAnnotation {
}

二、读取注解上的信息

通过反射机制去获取注解上的信息

• 创建各种类型的注解

  @Target(ElementType.TYPE)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Inherited
  public @interface AnnotationForClass {
      String value();
  }
  @Target(ElementType.CONSTRUCTOR)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Inherited
  public @interface AnnotationForConstructor {
  String value();
  }
  @Target(ElementType.FIELD)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Inherited
  public @interface AnnotationForField {
  String value();
  }
  @Target(ElementType.METHOD)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Inherited
  public @interface AnnotationForMethod {
  String value();
  }
  @Target(ElementType.PARAMETER)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Inherited
  public @interface AnnotationForParam {
  String value();
  }
  

• 带有各种注解的测试类

  @AnnotationForClass(value = "AnnotationForClassValue")
  public class Test {
      @AnnotationForField(value = "AnnotationForFieldValue")
      private String name;
  <span class="hljs-meta">@AnnotationForMethod</span>(value = <span class="hljs-string">"AnnotationForMethodValue"</span>)
  <span class="hljs-keyword">public</span> <span class="hljs-title class_">String</span> <span class="hljs-title function_">getName</span>(<span class="hljs-params"></span>) {
      <span class="hljs-keyword">return</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">name</span>;
  }
  
  <span class="hljs-keyword">public</span> <span class="hljs-built_in">void</span> <span class="hljs-title function_">setName</span>(<span class="hljs-params"><span class="hljs-meta">@AnnotationForParam</span>(value = <span class="hljs-string">"AnnotationForParamValue"</span>) <span class="hljs-built_in">String</span> name</span>) {
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">name</span> = name;
  }
  
  <span class="hljs-meta">@AnnotationForConstructor</span>(value = <span class="hljs-string">"AnnotationForConstructorValue"</span>)
  <span class="hljs-keyword">public</span> <span class="hljs-title class_">Test</span>() {
  }
  
  }
  

• 获取注解以及注解上的信息

  public class Main {
  <span class="hljs-comment">// getDeclaredAnnotations 获取所有的注解</span>
  <span class="hljs-comment">// getDeclaredAnnotation 获取指定的注解</span>
  <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title function_">main</span><span class="hljs-params">(String[] args)</span> <span class="hljs-keyword">throws</span> NoSuchMethodException, NoSuchFieldException {
      Class&lt;Test&gt; testClazz = Test.class;
  
      <span class="hljs-comment">// 获取(类、接口、枚举、注解)上注解的</span>
      <span class="hljs-type">AnnotationForClass</span> <span class="hljs-variable">annotationForClass</span> <span class="hljs-operator">=</span> testClazz.getDeclaredAnnotation(AnnotationForClass.class);
      System.out.println(<span class="hljs-string">"类上的注解的值\t"</span> + annotationForClass.value());
  
      <span class="hljs-comment">// 获取类中构造器的注解</span>
      Constructor&lt;Test&gt; constructor = testClazz.getConstructor();
      <span class="hljs-type">AnnotationForConstructor</span> <span class="hljs-variable">annotationForConstructor</span> <span class="hljs-operator">=</span> constructor.getDeclaredAnnotation(AnnotationForConstructor.class);
      System.out.println(<span class="hljs-string">"类中构造器的注解的值\t"</span> + annotationForConstructor.value());
  
      <span class="hljs-comment">// 获取类中字段的注解</span>
      <span class="hljs-type">Field</span> <span class="hljs-variable">nameField</span> <span class="hljs-operator">=</span> testClazz.getDeclaredField(<span class="hljs-string">"name"</span>);
      <span class="hljs-type">AnnotationForField</span> <span class="hljs-variable">annotationForField</span> <span class="hljs-operator">=</span> nameField.getDeclaredAnnotation(AnnotationForField.class);
      System.out.println(<span class="hljs-string">"类中字段的注解的值\t"</span> + annotationForField.value());
  
      <span class="hljs-comment">// 获取类中方法的注解</span>
      <span class="hljs-type">Method</span> <span class="hljs-variable">getNameMethod</span> <span class="hljs-operator">=</span> testClazz.getDeclaredMethod(<span class="hljs-string">"getName"</span>);
      <span class="hljs-type">AnnotationForMethod</span> <span class="hljs-variable">annotationForMethod</span> <span class="hljs-operator">=</span> getNameMethod.getDeclaredAnnotation(AnnotationForMethod.class);
      System.out.println(<span class="hljs-string">"类中方法的注解的值\t"</span> + annotationForMethod.value());
  
      <span class="hljs-comment">// 获取方法上参数的注解</span>
      <span class="hljs-type">Method</span> <span class="hljs-variable">setNameMethod</span> <span class="hljs-operator">=</span> testClazz.getDeclaredMethod(<span class="hljs-string">"setName"</span>, String.class);
      <span class="hljs-type">AnnotationForParam</span> <span class="hljs-variable">annotationForParam</span> <span class="hljs-operator">=</span> setNameMethod.getDeclaredAnnotation(AnnotationForParam.class);
      System.out.println(<span class="hljs-string">"类中方法的参数的注解的值\t"</span> + annotationForParam.value());
  }
  
  }
  

• 运行结果

  类上的注解的值 AnnotationForClassValue
  类中构造器的注解的值 AnnotationForConstructorValue
  类中字段的注解的值 AnnotationForFieldValue
  类中方法的注解的值 AnnotationForMethodValue
  类中方法的参数的注解的值 AnnotationForParamValue