动态代理（JDK 动态代理）-蒲公英云

动态代理：代理类在程序运行时被创建，这种代理方式称为动态代理

动态代理实现有两种方式，JDK 动态代理和 CGlib 动态代理，本文介绍JDK 动态代理

JDK 动态代理：创建动态代理类，实现 InvocationHandler 接口，重写 invoke 方法，在 invoke 方法中通过反射调用被代理类的方法，实现代理。jdk 动态代理只能代理接口，即只能代理接口的实现类

JDK 动态代理示例代码

定义一个接口

package com.jdkdynamicproxy.service;
public interface Subject {
    void proxyMethod();
}

被代理类，实现接口

package com.jdkdynamicproxy.service.impl;
import com.jdkdynamicproxy.service.Subject;
/**
 * 被代理类
* @author Administrator
*/
public class AgentTarget implements Subject {
    @Override
    public void proxyMethod() {
        System.out.println("被代理的方法执行了");
    }
}

动态代理类,实现 InvocationHandler 接口

package com.jdkdynamicproxy.jdkproxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import com.jdkdynamicproxy.service.impl.AgentTarget;
/**
   * 动态代理类,实现 InvocationHandler接口
 * @author Administrator
 */
public class JdkDynamicProxySubject implements InvocationHandler {
    private AgentTarget agentTarget;
    public JdkDynamicProxySubject(AgentTarget agentTarget) {
        this.agentTarget = agentTarget;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            //动态代理逻辑
            System.out.println("before");
            Object res = null;
            try {
                 //用反射调用被代理类的方法
                 res = method.invoke(agentTarget, args);
            }catch (Exception ex){
                System.out.println("ex:" + ex.getMessage());
                throw ex;
            }finally {
                System.out.println("after");
            }
            return res;
    }
}

测试类

package com.jdkdynamicproxy;
import java.lang.reflect.Proxy;
import com.jdkdynamicproxy.jdkproxy.JdkDynamicProxySubject;
import com.jdkdynamicproxy.service.Subject;
import com.jdkdynamicproxy.service.impl.AgentTarget;
public class JdkDynamicProxyMain {
    public static void main(String[] args) {
          //配置系统属性sun.misc.ProxyGenerator.saveGeneratedFile为true，代理类生成时会自动将生成的代理类写入硬盘
              System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
              Subject subject = (Subject) Proxy.newProxyInstance(
                      JdkDynamicProxyMain.class.getClassLoader(), //类加载器
                          new Class[]{Subject.class} ,  //代理接口
                          new JdkDynamicProxySubject(new AgentTarget()) //InvocationHandler对象
              );
              subject.proxyMethod();
    }
}

运行如下

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程序运行后，在项目目录下生成了代理类的 class 文件

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对生成的 $Proxy0.class 文件进行反编译

package com.sun.proxy;
import com.jdkdynamicproxy.service.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements Subject {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;
    public $Proxy0(InvocationHandler var1) throws  {
      super(var1);
   }
    public final boolean equals(Object var1) throws  {
      try {
         return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
      } catch (RuntimeException | Error var3) {
         throw var3;
      } catch (Throwable var4) {
         throw new UndeclaredThrowableException(var4);
      }
   }
    public final String toString() throws  {
      try {
         return (String)super.h.invoke(this, m2, (Object[])null);
      } catch (RuntimeException | Error var2) {
         throw var2;
      } catch (Throwable var3) {
         throw new UndeclaredThrowableException(var3);
      }
   }
    public final void proxyMethod() throws  {
      try {
         super.h.invoke(this, m3, (Object[])null);
      } catch (RuntimeException | Error var2) {
         throw var2;
      } catch (Throwable var3) {
         throw new UndeclaredThrowableException(var3);
      }
   }
    public final int hashCode() throws  {
      try {
         return (Integer)super.h.invoke(this, m0, (Object[])null);
      } catch (RuntimeException | Error var2) {
         throw var2;
      } catch (Throwable var3) {
         throw new UndeclaredThrowableException(var3);
      }
   }
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.jdkdynamicproxy.service.Subject").getMethod("proxyMethod");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以看到生成的 $Proxy0.class 实现了 Subject 接口，重写 proxyMethod 方法。在重写的 proxyMethod 方法中，去调用 JdkDynamicProxySubject 的 invoke(Object proxy, Method method, Object[] args) 方法，实现代理

整个程序的运行流程：Proxy.newProxyInstance(JdkDynamicProxyMain.class.getClassLoader(), new Class[]{Subject.class} , new JdkDynamicProxySubject(new AgentTarget()) ) 创建对象 subject 时，生成了代理类 $Proxy0.class；subject 对象调用proxyMethod 方法时，proxyMethod 方法调用 JdkDynamicProxySubject 的 invoke 方法，invoke 方法中通过反射调用被代理方法，实现动态代理