自定义注解

一

例1：

@Target({ElementType.FIELD, ElementType.METHOD})//可以运用到字段及方法上
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = ThresholdValidator.class)
@Documented
public @interface Threshold {

    String regexp();

    String message() default "";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

}

public enum ElementType {
    /** Class, interface (including annotation type), or enum declaration */
    TYPE,

    /** Field declaration (includes enum constants) */
    FIELD,

    /** Method declaration */
    METHOD,

    /** Parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation type declaration */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE

public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     *注释会在编译期丢弃
     */
    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. 
     *  注释会在编译期保留在class中但是会被jvm忽视
     */
    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
     * 注释会保留在class中而且也会被jvm读取
     */
    RUNTIME
}