元注解--@Target和@Retention
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元注解
作用:用来修饰注解
一 @Target ----- 作用对象
二 @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.
- 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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