• 自定义注解


    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 }
    View Code
    @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
    View Code
    @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 }
    View Code
    @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
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  • 原文地址:https://www.cnblogs.com/isisbenben/p/6054120.html
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