元注解--@Target和@Retention

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.
3. 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）