51工具盒子英雄非无泪，不洒敌人前。男儿七尺躯，愿为祖国捐。------陈辉
在java中如果我们需要一个注解能被重复使用
例如这个
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | hljs java package com.ruben.annotation; import java.lang.annotation.*; /** * @ClassName: BeanFieldSort * @Description: * @Date: 2020/9/11 22:18 * * * @author: achao<achao1441470436 @ gmail.com> * @version: 1.0 * @since: JDK 1.8 */ @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) public @interface BeanFieldSort { /** * 序号 * * @return */ int order(); } |
如果我们直接重复注解，会发现编译错误
我们需要在注解上加上@Repeatable注解，里面参数放另外一个注解，作为它的承载
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | hljs java package com.ruben.annotation; import java.lang.annotation.*; /** * @ClassName: BeanFieldSort * @Description: * @Date: 2020/9/11 22:18 * * * @author: achao<achao1441470436 @ gmail.com> * @version: 1.0 * @since: JDK 1.8 */ @Repeatable(BeanFieldSort.BeanFieldSorts.class) @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) public @interface BeanFieldSort { /** * 序号 * * @return */ int order(); @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) @interface BeanFieldSorts { BeanFieldSort[] value(); } } |
这样就可以重复注解了
如果我们需要取出注解里面的order
使用之前的方式就会报空指针了，运行结果也打印出来为true
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| 1 2 3 4 5 | hljs java Field field = UserInfo.class.getDeclaredField("serialVersionUID"); BeanFieldSort empty = field.getAnnotation(BeanFieldSort.class); System.out.println("---"); System.out.println(Objects.isNull(empty)); // empty.order(); NPE |
正确方式是使用
|-------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 1 2 3 4 5 | hljs java Field field = UserInfo.class.getDeclaredField("serialVersionUID"); BeanFieldSort.BeanFieldSorts annotation = field.getAnnotation(BeanFieldSort.BeanFieldSorts.class); for (BeanFieldSort beanFieldSort : annotation.value()) { System.out.println(beanFieldSort.order()); } |
即可
