java.lang.annotation.RetentionPolicy 学习记录(翻译解析)

注意：以下文章仅作为个人学习记录。相关翻译和个人理解描述正确性仍需斟酌，文章仅供参考。

1.概念

概念：注解保留策略。枚举类型常量描述了保留注解的一些策略。它们被用来与元注解类型组合使用，标明注解将会被保留多久。

SOURCE:
注解将会被编译器丢弃。(编译完成丢弃)

CLASS：
注解将会被编译器记录在类文件中。但是在运行时，不会被虚拟机保留。这是默认行为（编译完成保留，运行时丢弃）

RUNTIME：
注解将会被编译器记录在类文件中，同时在运行时会被虚拟机保留，所以它们可以被反射地读取。（编译，运行都保留，可反射获取）

2.用途

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {
    boolean required() default true;
}

源码：

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.
     */
    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
}