作用是定义被它所注解的注解保留多久，一共有三种策略，定义在RetentionPolicy枚举中:

package java.lang.annotation;
/**
 * Annotation retention policy.  The constants of this enumerated type
 * describe the various policies for retaining annotations.  They are used
 * in conjunction with the {@link Retention} meta-annotation type to specify
 * how long annotations are to be retained.
 *
 * @author  Joshua Bloch
 * @since 1.5
 */
public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
　　　* 注解在编译时就被忽略
     */
    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.
　　　* 默认是该策略,注解被编译器编译进class文件;但是不被VM运行时保留
     */
    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.
     * 一直保留到运行时 可以通过反射获取注解信息
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}