【Java】关于获取注解的问题发现

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同事设置了个注解，想用Spring获取的Bean来找到Class获取注解

但是发现是空的，在查看的Spring返回Bean之后，发现这个Bean对象并不是原生的实例

而是被Spring代理增强的代理对象

为了复现这个问题，这里我写了个样例：

首先有两个样例注解（一个叫A 一个叫B，B也是一样的就不写了）：

package cn.cloud9.test.reflect.annotation;

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)
public @interface AnnoA {
    String value() default "";
}

再写一个A类，里面就是一个方法没别的

打上A和B注解

package cn.cloud9.test.reflect.sample;

import cn.cloud9.test.reflect.annotation.AnnoA;
import cn.cloud9.test.reflect.annotation.AnnoB;

@AnnoA
@AnnoB
public class A {

    public void fun() {
        System.out.println("A.fun executed... ");
    }
}

为了模拟Spring代理的场景，这里要引入CGLIB组件

        <dependency>
            <groupId>cglib</groupId>
            <artifactId>cglib</artifactId>
            <version>2.2.2</version>
        </dependency>

编写样例代码：

package cn.cloud9.test.reflect;

import cn.cloud9.test.reflect.sample.A;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
import org.junit.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

public class ReflectAndAnnotation {


    @Test
    public void simpleGet() throws Exception {

        Class<?> aClass = Class.forName("cn.cloud9.test.reflect.sample.A");

        Annotation[] annotations = aClass.getAnnotations();

        Class<?> superclass = aClass.getSuperclass();
        System.out.println(superclass);

        for (Annotation annotation : annotations) {
            System.out.println(annotation.toString());
        }
    }

}

执行之后，自然而然的就能得到我们设置的注解：

"C:\Program Files\Java\jdk1.8.0_301\bin\java.exe" -ea -Didea.test.cyclic.buffer.size=1048576 "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\lib\idea_rt.jar=57548:C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\lib\idea_rt.jar;C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\plugins\junit\lib\junit5-rt.jar;C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\plugins\junit\lib\junit-rt.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\rt.jar;C:\Users\Cloud9\IdeaProjects\EveryThing-Framework\target\test-classes;C:\Users\Cloud9\IdeaProjects\EveryThing-Framework\target\classes;D:\Maven Local Repository\org\apache\curator\curator-framework\5.2.0\curator-framework-5.2.0.jar;D:\Maven Local Repository\org\apache\curator\curator-client\5.2.0\curator-client-5.2.0.jar;D:\Maven Local Repository\org\apache\zookeeper\zookeeper\3.6.3\zookeeper-3.6.3.jar;D:\Maven Local Repository\org\apache\zookeeper\zookeeper-jute\3.6.3\zookeeper-jute-3.6.3.jar;D:\Maven Local Repository\org\apache\yetus\audience-annotations\0.5.0\audience-annotations-0.5.0.jar;D:\Maven Local Repository\io\netty\netty-handler\4.1.63.Final\netty-handler-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-common\4.1.63.Final\netty-common-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-resolver\4.1.63.Final\netty-resolver-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-buffer\4.1.63.Final\netty-buffer-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-transport\4.1.63.Final\netty-transport-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-codec\4.1.63.Final\netty-codec-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-transport-native-epoll\4.1.63.Final\netty-transport-native-epoll-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-transport-native-unix-common\4.1.63.Final\netty-transport-native-unix-common-4.1.63.Final.jar;D:\Maven Local Repository\org\apache\curator\curator-recipes\5.2.0\curator-recipes-5.2.0.jar;D:\Maven Local Repository\commons-io\commons-io\2.11.0\commons-io-2.11.0.jar;D:\Maven Local Repository\commons-beanutils\commons-beanutils\1.7.0\commons-beanutils-1.7.0.jar;D:\Maven Local Repository\commons-logging\commons-logging\1.0.3\commons-logging-1.0.3.jar;D:\Maven Local Repository\org\javassist\javassist\3.26.0-GA\javassist-3.26.0-GA.jar;D:\Maven Local Repository\com\alibaba\fastjson\1.2.78\fastjson-1.2.78.jar;D:\Maven Local Repository\javax\servlet\javax.servlet-api\4.0.1\javax.servlet-api-4.0.1.jar;D:\Maven Local Repository\javax\servlet\jstl\1.2\jstl-1.2.jar;D:\Maven Local Repository\javax\servlet\jsp\javax.servlet.jsp-api\2.3.3\javax.servlet.jsp-api-2.3.3.jar;D:\Maven Local Repository\org\reflections\reflections\0.9.11\reflections-0.9.11.jar;D:\Maven Local Repository\com\google\guava\guava\20.0\guava-20.0.jar;D:\Maven Local Repository\mysql\mysql-connector-java\8.0.25\mysql-connector-java-8.0.25.jar;D:\Maven Local Repository\com\google\protobuf\protobuf-java\3.11.4\protobuf-java-3.11.4.jar;D:\Maven Local Repository\io\jsonwebtoken\jjwt\0.9.1\jjwt-0.9.1.jar;D:\Maven Local Repository\com\fasterxml\jackson\core\jackson-databind\2.9.6\jackson-databind-2.9.6.jar;D:\Maven Local Repository\com\fasterxml\jackson\core\jackson-annotations\2.9.0\jackson-annotations-2.9.0.jar;D:\Maven Local Repository\com\fasterxml\jackson\core\jackson-core\2.9.6\jackson-core-2.9.6.jar;D:\Maven Local Repository\org\apache\httpcomponents\httpclient\4.5.13\httpclient-4.5.13.jar;D:\Maven Local Repository\org\apache\httpcomponents\httpcore\4.4.13\httpcore-4.4.13.jar;D:\Maven Local Repository\commons-codec\commons-codec\1.11\commons-codec-1.11.jar;D:\Maven Local Repository\org\apache\httpcomponents\httpmime\4.5.13\httpmime-4.5.13.jar;D:\Maven Local Repository\junit\junit\4.12\junit-4.12.jar;D:\Maven Local Repository\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar;D:\Maven Local Repository\org\slf4j\slf4j-api\1.7.32\slf4j-api-1.7.32.jar;D:\Maven Local Repository\org\slf4j\slf4j-log4j12\1.7.32\slf4j-log4j12-1.7.32.jar;D:\Maven Local Repository\log4j\log4j\1.2.17\log4j-1.2.17.jar;D:\Maven Local Repository\cglib\cglib\2.2.2\cglib-2.2.2.jar;D:\Maven Local Repository\asm\asm\3.3.1\asm-3.3.1.jar" com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 cn.cloud9.test.reflect.ReflectAndAnnotation,simpleGet
class java.lang.Object
@cn.cloud9.test.reflect.annotation.AnnoA(value=)
@cn.cloud9.test.reflect.annotation.AnnoB(value=)

Process finished with exit code 0

那么在调用CGLIB的增强代理之后：

package cn.cloud9.test.reflect;

import cn.cloud9.test.reflect.sample.A;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
import org.junit.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

public class ReflectAndAnnotation {


    @Test
    public void simpleGet() throws Exception {

        Class<?> aClass = Class.forName("cn.cloud9.test.reflect.sample.A");

        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(aClass);
        enhancer.setCallback(new MethodInterceptor() {
            @Override
            public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
                System.out.println("before method run...");
                Object result = proxy.invokeSuper(obj, args);
                System.out.println("after method run...");
                return result;
            }
        });
        A proxyA = (A) enhancer.create();
        proxyA.fun();

        Class<? extends A> proxyAClass = proxyA.getClass();

        System.out.println(proxyAClass);

        for (Annotation annotation : proxyAClass.getAnnotations()) {
            System.out.println(annotation);
        }

    }

}

执行发现，我们注解获取不到了

"C:\Program Files\Java\jdk1.8.0_301\bin\java.exe" -ea -Didea.test.cyclic.buffer.size=1048576 "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\lib\idea_rt.jar=57581:C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\lib\idea_rt.jar;C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\plugins\junit\lib\junit5-rt.jar;C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2\plugins\junit\lib\junit-rt.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_301\jre\lib\rt.jar;C:\Users\Cloud9\IdeaProjects\EveryThing-Framework\target\test-classes;C:\Users\Cloud9\IdeaProjects\EveryThing-Framework\target\classes;D:\Maven Local Repository\org\apache\curator\curator-framework\5.2.0\curator-framework-5.2.0.jar;D:\Maven Local Repository\org\apache\curator\curator-client\5.2.0\curator-client-5.2.0.jar;D:\Maven Local Repository\org\apache\zookeeper\zookeeper\3.6.3\zookeeper-3.6.3.jar;D:\Maven Local Repository\org\apache\zookeeper\zookeeper-jute\3.6.3\zookeeper-jute-3.6.3.jar;D:\Maven Local Repository\org\apache\yetus\audience-annotations\0.5.0\audience-annotations-0.5.0.jar;D:\Maven Local Repository\io\netty\netty-handler\4.1.63.Final\netty-handler-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-common\4.1.63.Final\netty-common-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-resolver\4.1.63.Final\netty-resolver-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-buffer\4.1.63.Final\netty-buffer-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-transport\4.1.63.Final\netty-transport-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-codec\4.1.63.Final\netty-codec-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-transport-native-epoll\4.1.63.Final\netty-transport-native-epoll-4.1.63.Final.jar;D:\Maven Local Repository\io\netty\netty-transport-native-unix-common\4.1.63.Final\netty-transport-native-unix-common-4.1.63.Final.jar;D:\Maven Local Repository\org\apache\curator\curator-recipes\5.2.0\curator-recipes-5.2.0.jar;D:\Maven Local Repository\commons-io\commons-io\2.11.0\commons-io-2.11.0.jar;D:\Maven Local Repository\commons-beanutils\commons-beanutils\1.7.0\commons-beanutils-1.7.0.jar;D:\Maven Local Repository\commons-logging\commons-logging\1.0.3\commons-logging-1.0.3.jar;D:\Maven Local Repository\org\javassist\javassist\3.26.0-GA\javassist-3.26.0-GA.jar;D:\Maven Local Repository\com\alibaba\fastjson\1.2.78\fastjson-1.2.78.jar;D:\Maven Local Repository\javax\servlet\javax.servlet-api\4.0.1\javax.servlet-api-4.0.1.jar;D:\Maven Local Repository\javax\servlet\jstl\1.2\jstl-1.2.jar;D:\Maven Local Repository\javax\servlet\jsp\javax.servlet.jsp-api\2.3.3\javax.servlet.jsp-api-2.3.3.jar;D:\Maven Local Repository\org\reflections\reflections\0.9.11\reflections-0.9.11.jar;D:\Maven Local Repository\com\google\guava\guava\20.0\guava-20.0.jar;D:\Maven Local Repository\mysql\mysql-connector-java\8.0.25\mysql-connector-java-8.0.25.jar;D:\Maven Local Repository\com\google\protobuf\protobuf-java\3.11.4\protobuf-java-3.11.4.jar;D:\Maven Local Repository\io\jsonwebtoken\jjwt\0.9.1\jjwt-0.9.1.jar;D:\Maven Local Repository\com\fasterxml\jackson\core\jackson-databind\2.9.6\jackson-databind-2.9.6.jar;D:\Maven Local Repository\com\fasterxml\jackson\core\jackson-annotations\2.9.0\jackson-annotations-2.9.0.jar;D:\Maven Local Repository\com\fasterxml\jackson\core\jackson-core\2.9.6\jackson-core-2.9.6.jar;D:\Maven Local Repository\org\apache\httpcomponents\httpclient\4.5.13\httpclient-4.5.13.jar;D:\Maven Local Repository\org\apache\httpcomponents\httpcore\4.4.13\httpcore-4.4.13.jar;D:\Maven Local Repository\commons-codec\commons-codec\1.11\commons-codec-1.11.jar;D:\Maven Local Repository\org\apache\httpcomponents\httpmime\4.5.13\httpmime-4.5.13.jar;D:\Maven Local Repository\junit\junit\4.12\junit-4.12.jar;D:\Maven Local Repository\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar;D:\Maven Local Repository\org\slf4j\slf4j-api\1.7.32\slf4j-api-1.7.32.jar;D:\Maven Local Repository\org\slf4j\slf4j-log4j12\1.7.32\slf4j-log4j12-1.7.32.jar;D:\Maven Local Repository\log4j\log4j\1.2.17\log4j-1.2.17.jar;D:\Maven Local Repository\cglib\cglib\2.2.2\cglib-2.2.2.jar;D:\Maven Local Repository\asm\asm\3.3.1\asm-3.3.1.jar" com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 cn.cloud9.test.reflect.ReflectAndAnnotation,simpleGet
before method run...
A.fun executed... 
after method run...
class cn.cloud9.test.reflect.sample.A$$EnhancerByCGLIB$$fe34985a

Process finished with exit code 0

为什么获取不到？

因为这个是代理的对象，那么代理对象也应该由一个类来创建

这个类是代理对象类 class cn.cloud9.test.reflect.sample.A$$EnhancerByCGLIB$$fe34985a ，那么代理对象类会有我们设置的注解吗？

结果是没有的，但是根据CGLIB代理的实现原理，CGLIB代理原理是通过继承实现的，所以我们是不是可以通过代理类向上找父类来获取？

下面就是对猜想的测试：

package cn.cloud9.test.reflect;

import cn.cloud9.test.reflect.sample.A;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
import org.junit.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

public class ReflectAndAnnotation {


    @Test
    public void simpleGet() throws Exception {

        Class<?> aClass = Class.forName("cn.cloud9.test.reflect.sample.A");

        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(aClass);
        enhancer.setCallback(new MethodInterceptor() {
            @Override
            public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
                System.out.println("before method run...");
                Object result = proxy.invokeSuper(obj, args);
                System.out.println("after method run...");
                return result;
            }
        });
        A proxyA = (A) enhancer.create();
        proxyA.fun();

        Class<? extends A> proxyAClass = proxyA.getClass();

        System.out.println(proxyAClass);

        for (Annotation annotation : proxyAClass.getAnnotations()) {
            System.out.println(annotation);
        }
        
        // 获取父类
        Class<?> superclass1 = proxyAClass.getSuperclass();
        System.out.println(superclass1);
        for (Annotation annotation : superclass1.getAnnotations()) {
            System.out.println(annotation);
        }
    }
}

执行结果发现，代理对象的父类就是我们原来的类

从原类再次获取注解，结果是可以的

before method run...
A.fun executed... 
after method run...
class cn.cloud9.test.reflect.sample.A$$EnhancerByCGLIB$$fe34985a
class cn.cloud9.test.reflect.sample.A
@cn.cloud9.test.reflect.annotation.AnnoA(value=)
@cn.cloud9.test.reflect.annotation.AnnoB(value=)

由此可以推断JDK的接口实现代理的方式

对象要增强就需要通过接口实现处理，也就是说实际的接口对象，是实现类的对象

注解在接口上，要通过实现类找到对应的接口，通过接口实例，再找到标注的注解

注解在实现类上，就可以直接通过实现类，获取标注的注解

这篇关于【Java】关于获取注解的问题发现的文章就介绍到这儿，希望我们推荐的文章对大家有所帮助，也希望大家多多支持为之网！

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