元注解--@Target和@Retention

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元注解

作用:用来修饰注解
@Target ----- 作用对象

@Retention ----作用时期
作用范围@Reatention:枚举类型 ElementType
1: Source:注解只保留在源文件,当Java文件编译成class文件的时候,注解被遗弃;
源码: Annotations are to be discarded by the compiler.
2: Class:注解被保留到class文件,但jvm加载class文件时候被遗弃,这是默认的生命周期
源码: 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.

  1. 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。
    源码: Annotations are to be recorded in the class file by the compiler andretained by the VM at run time, so they may be read reflectively.
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}
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
}

三个作用时期分别可以对应文件的
Source ---- 源文件(.java文件)
Class ---- 编译文件(.class文件)
Runtime ---- 运行文件(字节码)

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