自定义注解

一

例1：

 1 @Target({ElementType.FIELD, ElementType.METHOD})//可以运用到字段及方法上
 2 @Retention(RetentionPolicy.RUNTIME)
 3 @Constraint(validatedBy = ThresholdValidator.class)
 4 @Documented
 5 public @interface Threshold {
 6 
 7     String regexp();
 8 
 9     String message() default "";
10 
11     Class<?>[] groups() default {};
12 
13     Class<? extends Payload>[] payload() default {};
14 
15 }

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@Target: 是指定义的注解可以应用在哪些程序上面

看一下ElementType 的源码

 1 public enum ElementType {
 2     /** Class, interface (including annotation type), or enum declaration */
 3     TYPE,
 4 
 5     /** Field declaration (includes enum constants) */
 6     FIELD,
 7 
 8     /** Method declaration */
 9     METHOD,
10 
11     /** Parameter declaration */
12     PARAMETER,
13 
14     /** Constructor declaration */
15     CONSTRUCTOR,
16 
17     /** Local variable declaration */
18     LOCAL_VARIABLE,
19 
20     /** Annotation type declaration */
21     ANNOTATION_TYPE,
22 
23     /** Package declaration */
24     PACKAGE

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@Retention: 是指java编译期如何对待注解的

看下RetentionPolicy的源码

 1 public enum RetentionPolicy {
 2     /**
 3      * Annotations are to be discarded by the compiler.
 4      *注释会在编译期丢弃
 5      */
 6     SOURCE,
 7 
 8     /**
 9      * Annotations are to be recorded in the class file by the compiler
10      * but need not be retained by the VM at run time.  This is the default
11      * behavior. 
12      *  注释会在编译期保留在class中但是会被jvm忽视
13      */
14     CLASS,
15 
16     /**
17      * Annotations are to be recorded in the class file by the compiler and
18      * retained by the VM at run time, so they may be read reflectively.
19      *
20      * @see java.lang.reflect.AnnotatedElement
21      * 注释会保留在class中而且也会被jvm读取
22      */
23     RUNTIME
24 }

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@Documented

指被定义的注解被认作为所熟悉的程序单元的公开API之一，会在javadoc中显示

很多事情不是看到希望才去坚持，而是坚持了才会看到希望