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元注解 @Retention 的作用机制

发表于 更新于 分类于 阅读次数： Valine：

一、@Retention 简介

@Retention 是用来定义注解的注解，这类注解也叫 元注解。@Retention 定义了注解的生命周期，生命周期的长短取决于 RetentionPolicy 属性值。

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@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}
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public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     */
    SOURCE,

    /**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.
     */
    CLASS,

    /**
     * Annotations are to be recorded in the class file by the compiler and
     * retained by the VM at run time, so they may be read reflectively.
     *
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

归纳如下：

属性值 描述 使用场景
RetentionPolicy.SOURCE 注解只保留在源文件，当被编译成class文件，注解就会消失 代码预检，如 @Override、@SuppressWarnings
RetentionPolicy.CLASS 注解被保留到class文件，但当jvm加载class文件时被遗弃 编译时进行一些预处理操作，如生成辅助信息（META-INF/services）
RetentionPolicy.RUNTIME 注解不仅被保存到class文件中，jvm加载class文件之后，仍然存在 运行时动态获取注解信息，如 @Autowired、@Required

二、通过字节码认识 @Retention

定义三种生命周期的注解：SourcePolicy、ClassPolicy、RuntimePolicy
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@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.METHOD)
public @interface SourcePolicy {
}

@Retention(RetentionPolicy.CLASS)
@Target(ElementType.METHOD)
public @interface ClassPolicy {
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface RuntimePolicy {
}

使用以上注解：

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public class RetentionPolicyTest {
    @SourcePolicy
    public void sourcePolicy() {
    }

    @ClassPolicy
    public void classPolicy() {
    }

    @RuntimePolicy
    public void runtimePolicy() {
    }
}

javap -v RetentionPolicyTest 查看字节码：

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{
  public RetentionPolicyTest();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0

  public void sourcePolicy();
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 7: 0

  public void classPolicy();
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 11: 0
    RuntimeInvisibleAnnotations:
      0: #11()

  public void runtimePolicy();
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 15: 0
    RuntimeVisibleAnnotations:
      0: #14()
}

可以看到：

  • 编译之后，@SourcePolicy 信息不存在；
  • 编译之后，@ClassPolicy 具有属性：RuntimeInvisibleAnnotations；
  • 编译之后，@RuntimePolicy 具有属性：RuntimeVisibleAnnotations；