Spring Boot 2 实践记录之 组合注解原理
Spring 的组合注解功能,网上有很多文章介绍,不过都是介绍其使用方法,鲜有其原理解析。
组合注解并非 Java 的原生能力。就是说,想通过用「注解A」来注解「注解B」,再用「注解B」 来注解 C(类或方法),就能够使 C 同时拥有「注解A」和「注解B」是行不通的。
示例如下:
先定义注解 SuperAnno:
import java.lang.annotation.*; @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) public @interface SuperAnno { }
再定义注解 SubAnno,并使用 SuperAnno 注解 SubAnno:
import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @SuperAnno @SomeAnno public @interface SubAnno { }
定义 Test,使用 SubAnno 注解 Test:
@SubAnno public class Test { }
测试一下,看看我们能不能拿到 Test 的 SuperAnno 注解:
import java.lang.annotation.Annotation; public class Main { public static void main(String[] args) { // write your code here Test test = new Test(); Class<?> cl = test.getClass(); Annotation[] annotations = cl.getAnnotations(); for(Annotation annotation: annotations) { System.out.println(annotation.annotationType()); } } }
打印出来的结果为:
interface com.company.SubAnno
唔,SuperAnno 没有出现。
怎么拿到合并的注解呢?
核心的思路就是拿到注解后,递归获取其上的注解,见下例:
import javax.annotation.*; import java.lang.annotation.*; import java.util.ArrayList; public class Main { private ArrayList<Annotation> annos = new ArrayList<>(); public static void main(String[] args) { // write your code here Test test = new Test(); Class<?> cl = test.getClass(); Main main = new Main(); main.getAnnos(cl); for(Annotation anno: main.annos) { System.out.println(anno.annotationType()); } } private void getAnnos(Class<?> cl) { Annotation[] annotations = cl.getAnnotations(); for(Annotation annotation: annotations) { if (annotation.annotationType() != Deprecated.class && annotation.annotationType() != SuppressWarnings.class && annotation.annotationType() != Override.class && annotation.annotationType() != PostConstruct.class && annotation.annotationType() != PreDestroy.class && annotation.annotationType() != Resource.class && annotation.annotationType() != Resources.class && annotation.annotationType() != Generated.class && annotation.annotationType() != Target.class && annotation.annotationType() != Retention.class && annotation.annotationType() != Documented.class && annotation.annotationType() != Inherited.class ) { annos.add(annotation); getAnnos(annotation.annotationType()); } } } }