spring-core的注解工具提供的方法 AnnotatedElementUtils.findMergedRepeatableAnnotations用于從AnnotatedElement 對象獲取可重復的注解。但如果注解本身也是可以定義在其他注解之上的元注解(meta-annotation),且該注解也是可重復注解。這個方法就可能會失效。這就是我最近在使用spring注解工具時遇到的問題。示例如下。

注解定義

先定義一組注解：

	@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.ANNOTATION_TYPE, ElementType.METHOD })@Repeatable(LevelAs.class)public @interface LevelA {String value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.METHOD })public @interface LevelAs {LevelA[] value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.ANNOTATION_TYPE,ElementType.METHOD })@LevelA("shadows")@Repeatable(LevelBs.class)public @interface LevelB {@AliasFor(annotation = LevelA.class,attribute = "value")String value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.METHOD })public @interface LevelBs {LevelB[] value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.METHOD })@LevelB("shadows")public @interface LevelC {@AliasFor(annotation = LevelB.class,attribute = "value")String value();}

上面定義了@LevalA,@LevalB,@LevalC三個注解，其中@LeveA,@LevelB是可以重復注解(參見 @LevelAs,@LevelBs),而且還是元注解。@LeveA是@LevelB的元注解，@LevelB是@LevelC的元注解。它們三個層級關系是這樣的。

@LevelA				可重復 @LevelAs└─@LevelB		可重復 @LevelBs└─@LevelC

如下定義了測試這些注解的類

	public static class TestClass{@LevelB("hello")@LevelB("world")@LevelA("top1")@LevelA("top2")@LevelC("jerry")public void foo() {};@LevelB("hello")@LevelA("top1")@LevelA("top2")@LevelC("jerry")public void tar() {};}

findMergedRepeatableAnnotations

如下調用AnnotatedElementUtils.findMergedRepeatableAnnotations方法讀取TestClass.tar上定義的所有@LevelA注解

	@Testpublic void test1() {try {Method method = TestClass.class.getMethod("tar");Set<LevelA> set = AnnotatedElementUtils.findMergedRepeatableAnnotations(method,LevelA.class);set.stream().forEach(a->{System.out.printf("%s\n", a);});} catch (Throwable e) {e.printStackTrace();fail();}}

確實可以獲取正確的結果：

@RepeatableAnnotTest$LevelA(value=top1)
@RepeatableAnnotTest$LevelA(value=top2)
@RepeatableAnnotTest$LevelA(value=hello)
@RepeatableAnnotTest$LevelA(value=jerry)

那是因為TestClass.tar上只定義了一個@LevelB注解,如果上面的測試代碼只是把方法名換為定義了多個@LevelB的方法foo,結果就是這樣的：

@RepeatableAnnotTest$LevelA(value=top1)
@RepeatableAnnotTest$LevelA(value=top2)
@RepeatableAnnotTest$LevelA(value=jerry)

因為這里foo方法定義了超過一個@LevelB注解，實際定義在foo上的注解就成了@LeveBs({@LevelB("hello"),@LevelB("world")})，就導致AnnotatedElementUtils.findMergedRepeatableAnnotations方法不能正確獲取。
分析AnnotatedElementUtils的源碼，找到findMergedRepeatableAnnotations方法實際的核心執行代碼如下：

RepeatableContainers repeatableContainers = RepeatableContainers.of(annotationType, containerType);
return MergedAnnotations.from(element, SearchStrategy.TYPE_HIERARCHY, repeatableContainers);

RepeatableContainers

進一查看RepeatableContainers的代碼發現它還有另一個方法standardRepeatables()

創建一個使用Java的@Repeatable注釋進行搜索的RepeatableContainers實例。

	/*** Create a {@link RepeatableContainers} instance that searches using Java's* {@link Repeatable @Repeatable} annotation.* @return a {@link RepeatableContainers} instance*/public static RepeatableContainers standardRepeatables() {return StandardRepeatableContainers.INSTANCE;}

于是照著上面的代碼將測試代碼修改如下：

	@Testpublic void test3() {try {Method method = TestClass.class.getMethod("foo");RepeatableContainers repeatableContainers = RepeatableContainers.standardRepeatables();MergedAnnotations.from(method, SearchStrategy.TYPE_HIERARCHY, repeatableContainers).stream(LevelA.class).forEach(a->{System.out.printf("%s\n", a.synthesize());});} catch (Throwable e) {e.printStackTrace();fail();}}

則結果正確：

@LevelA(value=top1)
@LevelA(value=top2)
@LevelA(value=hello)
@LevelA(value=world)
@LevelA(value=jerry)

完整測試代碼

import static org.junit.Assert.*;import java.lang.annotation.ElementType;
import java.lang.annotation.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Method;
import java.util.Set;
import org.junit.Test;
import org.springframework.core.annotation.AliasFor;
import org.springframework.core.annotation.AnnotatedElementUtils;
import org.springframework.core.annotation.MergedAnnotations;
import org.springframework.core.annotation.MergedAnnotations.SearchStrategy;
import org.springframework.core.annotation.RepeatableContainers;public class RepeatableAnnotTest {@Testpublic void test1() {try {Method method = TestClass.class.getMethod("foo");Set<LevelA> set = AnnotatedElementUtils.findMergedRepeatableAnnotations(method,LevelA.class);set.stream().forEach(a->{System.out.printf("%s\n", a);});} catch (Throwable e) {e.printStackTrace();fail();}}@Testpublic void test2() {try {Method method = TestClass.class.getMethod("foo");RepeatableContainers repeatableContainers = RepeatableContainers.of(LevelA.class,null);MergedAnnotations.from(method, SearchStrategy.TYPE_HIERARCHY,repeatableContainers) .stream(LevelA.class) /* .stream() .sorted((o1,o2)->Integer.compare(o1.getDistance(), o2.getDistance()))*/.forEach(a->{System.out.printf("%s\n", a.synthesize());});} catch (Throwable e) {e.printStackTrace();fail();}}@Testpublic void test3() {try {Method method = TestClass.class.getMethod("foo");RepeatableContainers repeatableContainers = RepeatableContainers.standardRepeatables();MergedAnnotations.from(method, SearchStrategy.TYPE_HIERARCHY, repeatableContainers).stream(LevelA.class).forEach(a->{System.out.printf("%s\n", a.synthesize());});} catch (Throwable e) {e.printStackTrace();fail();}}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.ANNOTATION_TYPE, ElementType.METHOD })@Repeatable(LevelAs.class)public @interface LevelA {String value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.METHOD })public @interface LevelAs {LevelA[] value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.ANNOTATION_TYPE,ElementType.METHOD })@LevelA("shadows")@Repeatable(LevelBs.class)public @interface LevelB {@AliasFor(annotation = LevelA.class,attribute = "value")String value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.METHOD })public @interface LevelBs {LevelB[] value();}@Retention(RetentionPolicy.RUNTIME)@Target({ ElementType.METHOD })@LevelB("shadows")public @interface LevelC {@AliasFor(annotation = LevelB.class,attribute = "value")String value();}public static class TestClass{@LevelB("hello")@LevelB("world")@LevelA("top1")@LevelA("top2")@LevelC("jerry")public void foo() {};@LevelB("hello")@LevelA("top1")@LevelA("top2")@LevelC("jerry")public void tar() {};}
}