（1）概述：
Annotation：主要用于对其他元素进行描述。包括Annotation，Class ， Method， Field 等。
（2）Demo：
package x.demo.spring.core.anno; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Inherited @interface Desc { /** * 描述 * * @return 描述信息 */ String value() default ""; } //被注解的bean @Desc("simple bean") class Bean { } public class SimpleAnnotationDemo { public static void main(String[] args) { Desc desc = Bean.class.getAnnotation(Desc.class); System.out.println("bean desc : " + desc.value()); } }
（3）Meta Annotation 介绍：
（a）@Target：Annotation可应用于哪种Java 元素
其中ElementType说明：
TYPE：Class ，Interface（包括Annotation），枚举 FIELD：属性 METHOD：方法 PARAMETER：参数 CONSTRUCTOR：构造函数 LOCAL_VARIABLE：局部变量 ANNOTATION_TYPE：Annotation PACKAGE:包 TYPE_PARAMETER：类型参数，1.8新增 TYPE_USE：类型使用：1.8新增
（b）@Retention：Annotation能保存到什么时候
其中RetentionPolicy说明：
SOURCE：源文件有效 CLASS：Class文件有效 RUNTIME：运行时有效，可以通过反射读取
（c）@Inherited：类能否继承到父类的Annotation
（1）概述
Java 运行时读取Annotation 需要通过反射，Spring 提供AnnotationUtils ， AnnotationElementUtils 用于简化操作，其他特点如下：
查询Meta Annotation（注解的注解）对@AliasFor 解析，生成指定注解的代理，并赋值。（注：定义其他Annotation 的别名）
（2）Demo
(a) Dependency
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <parent> <artifactId>spring</artifactId> <groupId>x.demo.spring</groupId> <version>1.0-SNAPSHOT</version> <relativePath>../../pom.xml</relativePath> </parent> <modelVersion>4.0.0</modelVersion> <artifactId>spring-core</artifactId> <packaging>jar</packaging> <name>spring-core</name> <url>http://maven.apache.org</url> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> </properties> <dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> </dependency> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-beans</artifactId> </dependency> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> </dependency> </dependencies> </project>
(b) Annotation
package x.demo.spring.core.anno.define; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import org.springframework.context.annotation.Scope; import org.springframework.core.annotation.AliasFor; import org.springframework.stereotype.Component; /** * 单例的组件 * 这里组合了@Component ， @Scope 所有spring 扫描时具有了这两个Annotation的能力： * * 单例，被扫描到并被容器实例化 */ @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Scope("singleton") @Component @Inherited public @interface SingletonComponent { @AliasFor(annotation = Component.class, attribute = "value") String value() default ""; }
(c) 使用Annotation
package x.demo.spring.core.anno.define; /** * 单例，被扫描到并被容器实例化 */ @SingletonComponent("simpleService") public class SimpleSingletonService { }
(d)解析
package x.demo.spring.core.anno; import java.util.Map; import org.springframework.context.annotation.AnnotationConfigApplicationContext; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.Scope; import org.springframework.core.annotation.AnnotatedElementUtils; import org.springframework.core.annotation.AnnotationUtils; import org.springframework.stereotype.Component; import x.demo.spring.core.anno.define.SimpleSingletonService; import x.demo.spring.core.anno.define.SingletonComponent; @ComponentScan public class AnnotationUtilsDemo { private static void annotationUtilsDemo() { //获取类注解 SingletonComponent singletonComponentAnnocation = AnnotationUtils.findAnnotation(SimpleSingletonService.class, SingletonComponent .class); System.out.println("@SingletonComponent : " + singletonComponentAnnocation); System.out.println("@SingletonComponent value: " + AnnotationUtils.getValue(singletonComponentAnnocation, "value")); System.out.println("----------------------------------------------"); Scope scopeAnnocation = AnnotationUtils.findAnnotation(SimpleSingletonService.class, Scope.class); System.out.println("@Scope : " + scopeAnnocation); System.out.println("@Scope value: " + AnnotationUtils.getValue(scopeAnnocation, "scopeName")); System.out.println("----------------------------------------------"); //获取@AliasFor Marge 后的注解，直接调用 AnnotationUtils的方法不会组合@AliasFor的值 Component componentAnnocation = AnnotatedElementUtils.findMergedAnnotation(SimpleSingletonService.class, Component.class); System.out.println("@Component : " + componentAnnocation); System.out.println("@Component value: " + AnnotationUtils.getValue(componentAnnocation, "value")); } //获取所有Annotation注解的类对象，获取Meta Annotation private static void getAllAnnocations() { try (AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();) { context.register(AnnotationUtilsDemo.class); context.refresh(); //@SingletonComponent 继承了 @Component 所以存在实例 Map<String, Object> beans = context.getBeansWithAnnotation(SingletonComponent.class); for (Object bean : beans.values()) { System.out.println("bean : " + bean); Component componentAnnocation = AnnotatedElementUtils.findMergedAnnotation(bean.getClass(), Component.class); System.out.println(componentAnnocation); } } } public static void main(String[] args) { AnnotationUtilsDemo.annotationUtilsDemo(); System.out.println("----------------------------------------------"); AnnotationUtilsDemo.getAllAnnocations(); } }
(e) 结果
@SingletonComponent : @x.demo.spring.core.anno.define.SingletonComponent(value=simpleService) @SingletonComponent value: simpleService ---------------------------------------------- @Scope : @org.springframework.context.annotation.Scope(value=singleton, scopeName=singleton, proxyMode=DEFAULT) @Scope value: singleton ---------------------------------------------- @Component : @org.springframework.stereotype.Component(value=simpleService) @Component value: simpleService ---------------------------------------------- bean : x.demo.spring.core.anno.define.SimpleSingletonService@74a10858 @org.springframework.stereotype.Component(value=simpleService)（3）说明
（a）Utils 调用涉及的元素：
Annotation: Annotation 元数据AnnotatedElement: 被Annotation注解的对象，包括annotation ， class ， method等Method: 类和接口中的方法信息
（b）常用方法：
getAnnotation: 从某个类获取某个annotationfindAnnotation: 从类或方法中查找某个annotation。isAnnotationDeclaredLocally: 验证annotation是否直接注释在类上而不是集成来的。isAnnotationInherited: 验证annotation是否继承于另一个class。getAnnotationAttributes: 获取annotation的所有属性。getValue: 获取指定annotation的值. getDefaultValue: 获取指定annotation或annotation 属性的默认值
转自:http://shihlei.iteye.com/blog/2406149
