spring-core:获取类/方法/字段/字段上直接定义的注解

AnnotatedElement.getAnnotation

如何获取一个类上定义的注解？
这个问题似乎不应该问，我们知道如果要获取一个类/方法/字段/字段上直接定义的注解是很方便的，如Class.getAnnotation(Class<CasbanScan>)就能实现,
只要实现了java.lang.reflect.AnnotatedElement接口的对象(Class,Method,Field)都有这个方法，
但是在Spring框架下，如果注解中有@AliasFor定义了别名的字段，要区别对待。
如下示例,
Class.getAnnotation获取注解对象，basePackages字段是空的，value字段才能获取正确的值。

	@Testpublic void test2GetDirectlyAnnotation() {try {{System.out.printf("-----> For %s\n",UserSummy.class.getSimpleName());System.out.println("\t==== USING Class.getAnnotation ====");CasbanScan annot = UserSummy.class.getAnnotation(CasbanScan.class);System.out.println("\t"+annot.toString());assertArrayEquals(new String[] {"hello"}, annot.value());System.out.printf("\tvalue() size:%d\n",annot.value().length);System.out.printf("\tbasePackages() size:%d\n",annot.basePackages().length);assertEquals(0, annot.basePackages().length);}} catch (Throwable e) {e.printStackTrace();fail();}}@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.TYPE)public @interface CasbanScan {@AliasFor("basePackages")String[] value() default {};@AliasFor("value")String[] basePackages() default {};}@CasbanScan("hello")public static class UserSummy{}

Spring框架将注解(Annotation)用到了极致，对注解定义扩展了很多灵活了定义，@AliasFor就是一种灵活扩展。所以在Spring框架下，要获取正确直接定义在类上的注解。Class.getAnnotation(Class<CasbanScan>)方法并不一定合适。

Spring-core

SearchStrategy

为适应各种场景，Spring通过枚举类型SearchStrategy来定义搜索策略,
其中SearchStrategy.DIRECT 定义为仅查找直接声明的注释，而不考虑@Inherited注释，也不搜索父类或实现的接口。
所以只要使用SearchStrategy.DIRECT 作为搜索策略就只查找直接声明的注释。

MergedAnnotations

MergedAnnotations.from(AnnotatedElement element)方法就是使用SearchStrategy.DIRECT 作为搜索策略创建一个新的MergedAnnotations实例，其中包含来自指定元素的所有注释和元注释
如下是MergedAnnotations.from(AnnotatedElement element)方法的实现

	static MergedAnnotations from(AnnotatedElement element) {return from(element, SearchStrategy.DIRECT);}

所以我们可以使用MergedAnnotations.from(AnnotatedElement element)方法来获取直接注解定义，

MergedAnnotation<CasbanScan> mergedAnnotation = MergedAnnotations.from(UserSummy.class).get(CasbanScan.class);
if(mergedAnnotation.isDirectlyPresent()){System.out.println("FOUND @CasbanScan in UserSummy");/** 将 MergedAnnotation转为方便输出的 AnnotationAttributes 对象 */System.out.println(mergedAnnotation.asAnnotationAttributes());/** 直接从MergedAnnotation对象读取字段 */System.out.println(mergedAnnotation.getStringArray("basePackages")[0]);System.out.println(mergedAnnotation.getStringArray("value")[0]);
}

完整测试代码如下：

import static org.junit.Assert.*;import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.junit.Test;import org.springframework.core.annotation.AliasFor;
import org.springframework.core.annotation.AnnotationUtils;
import org.springframework.core.annotation.MergedAnnotation;
import org.springframework.core.annotation.MergedAnnotations;public class SpringAnnotationUtilsTest {@Testpublic void test2GetDirectlyAnnotation() {try {System.out.printf("-----> For %s\n",UserSummy.class.getSimpleName());{/*** 直接调用 Class.getAnnotation方法可以读取到注解，但对于用@AliasFor定义了别名的字段，要区别对待。* 如下,basePackages字段是空的，value字段才能获取正确的值 */System.out.println("\t==== USING Class.getAnnotation ====");CasbanScan annot = UserSummy.class.getAnnotation(CasbanScan.class);System.out.println("\t"+annot.toString());assertArrayEquals(new String[] {"hello"}, annot.value());System.out.printf("\tvalue() size:%d\n",annot.value().length);System.out.printf("\tbasePackages() size:%d\n",annot.basePackages().length);assertEquals(0, annot.basePackages().length);}{System.out.println("\t==== USING AnnotationUtils.isAnnotationDeclaredLocally  ====");boolean declaredLocally = AnnotationUtils.isAnnotationDeclaredLocally(CasbanScan.class, UserSummy.class);System.out.printf("\t@%s declaredLocally in %s:%b\n",CasbanScan.class.getSimpleName(),UserSummy.class.getSimpleName(), declaredLocally);assertTrue(declaredLocally);System.out.println("\t==== USING Spring ====");MergedAnnotations mergedAnnotations= MergedAnnotations.from(UserSummy.class);assertTrue(mergedAnnotations.isDirectlyPresent(CasbanScan.class));MergedAnnotation<CasbanScan> mergedAnnotation= mergedAnnotations.get(CasbanScan.class);assertTrue(mergedAnnotation.isDirectlyPresent());/** 将 MergedAnnotation转为方便输出的 AnnotationAttributes 对象 */System.out.println("\t"+mergedAnnotation.asAnnotationAttributes());/** 直接从MergedAnnotation对象读取字段 */assertArrayEquals(new String[] {"hello"}, mergedAnnotation.getStringArray("basePackages"));assertArrayEquals(new String[] {"hello"}, mergedAnnotation.getStringArray("value"));System.out.println("\t==== USING Annotation synthesized ====");/*** 获取合成注解对象 */CasbanScan annot = mergedAnnotation.synthesize();/** 调用AnnotationUtils.getAnnotationAttributes将 Annotation对象转为方便输出的Map  */System.out.printf("\t%s\n", AnnotationUtils.getAnnotationAttributes(annot));/** 对于用@AliasFor定义了别名的字段不论用读取哪个字段都是有正确的值*/assertArrayEquals(new String[] {"hello"}, annot.value());assertArrayEquals(new String[] {"hello"}, annot.basePackages());}{System.out.printf("-----> For %s\n",VipSummy.class.getSimpleName());MergedAnnotations mergedAnnotations= MergedAnnotations.from(VipSummy.class);boolean declaredLocally = mergedAnnotations.isDirectlyPresent(CasbanScan.class);System.out.printf("\t%s FOUND directly @%s in %s\n", declaredLocally?"":"NOT",CasbanScan.class.getSimpleName(),VipSummy.class.getSimpleName());assertFalse(mergedAnnotations.isDirectlyPresent(CasbanScan.class));MergedAnnotation<CasbanScan> mergedAnnotation= mergedAnnotations.get(CasbanScan.class);assertFalse(mergedAnnotation.isDirectlyPresent());}} catch (Throwable e) {e.printStackTrace();fail();}}@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.TYPE)public @interface CasbanScan {@AliasFor("basePackages")String[] value() default {};@AliasFor("value")String[] basePackages() default {};}@CasbanScan("hello")public static class UserSummy{}public static class VipSummy extends UserSummy{}
}