Java注解之Retention与RetentionPolicy

Retention注解与RetentionPolicy配合使用，用于标注一个注解在什么阶段有效。

如果一个注解没有使用@Retention则默认使用RetentionPolicy.CLASS策略

/**
 * Annotations are to be discarded by the compiler.
 */
SOURCE

注解在源码时有效，将会被编译器抛弃。

/**
 * 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

注解在编译时有效，但在运行时没有保留。这也是默认行为。

/**
 * 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

运行时有效，并且可以通过反射获取。