Java Proxy InvocationHandler invoke理解

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一.相关类及其方法:

java.lang.reflect.Proxy,
Proxy 提供用于创建动态代理类和实例的静态方法.
newProxyInstance()

Returns an instance of a proxy class for the specified interfaces that dispatches method invocations to the specified invocation handler. This method is equivalent to:      Proxy.getProxyClass(loader, interfaces).         getConstructor(new Class[] { InvocationHandler.class }).         newInstance(new Object[] { handler }); Proxy.newProxyInstance throws IllegalArgumentException for the same reasons that Proxy.getProxyClass does.

返回一个指定接口的代理类实例，该接口可以将方法调用指派到指定的调用处理程序。

java.lang.reflect.InvocationHandler,
InvocationHandler 是代理实例的调用处理程序 实现的接口。 
invoke()
在代理实例上处理方法调用并返回结果。在与方法关联的代理实例上调用方法时，将在调用处理程序上调用此方法。

二.源代码:

被代理对象的接口及实现类:

public interface Manager {public void modify();}

//接口实现类public class ManagerImpl implements Manager {@Overridepublic void modify() {System.out.println("*******modify()方法被调用");}}

业务代理类

import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;public class BusinessHandler implements InvocationHandler {private Object object = null;public BusinessHandler(Object object) {this.object = object;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args)throws Throwable {System.out.println("do something before method");Object ret = method.invoke(this.object, args);System.out.println("do something after method");return ret;}}

客户端类

import java.lang.reflect.Proxy;public class Client {public static void main(String[] args) {// 元对象(被代理对象)ManagerImpl managerImpl = new ManagerImpl();// 业务代理类BusinessHandler securityHandler = new BusinessHandler(managerImpl);// 获得代理类($Proxy0 extends Proxy implements Manager)的实例.Manager managerProxy = (Manager) Proxy.newProxyInstance(managerImpl.getClass().getClassLoader(), managerImpl.getClass().getInterfaces(), securityHandler);managerProxy.modify();}}

三.执行结果:

do something before method*******modify()方法被调用do something after method

四.机制分析:

Proxy.(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)做了以下几件事.
(1)根据参数loader和interfaces调用方法 getProxyClass(loader, interfaces)创建代理类$Proxy.
$Proxy0类实现了interfaces的接口,并继承了Proxy类.
(2)实例化$Proxy0并在构造方法中把BusinessHandler传过去,接着$Proxy0调用父类Proxy的构造器,为h赋值,如下:
class Proxy{
   InvocationHandler h=null;
   protected Proxy(InvocationHandler h) {
    this.h = h;
   }
   ...
}

下面是本例的$Proxy0类的源码(好不容易才把它提出来):

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 Manager {private static Method m1;private static Method m0;private static Method m3;private static Method m2;static {try {m1 = Class.forName("java.lang.Object").getMethod("equals",new Class[] { Class.forName("java.lang.Object") });m0 = Class.forName("java.lang.Object").getMethod("hashCode",new Class[0]);m3 = Class.forName("com.ml.test.Manager").getMethod("modify",new Class[0]);m2 = Class.forName("java.lang.Object").getMethod("toString",new Class[0]);} catch (NoSuchMethodException nosuchmethodexception) {throw new NoSuchMethodError(nosuchmethodexception.getMessage());} catch (ClassNotFoundException classnotfoundexception) {throw new NoClassDefFoundError(classnotfoundexception.getMessage());}}public $Proxy0(InvocationHandler invocationhandler) {super(invocationhandler);}@Overridepublic final boolean equals(Object obj) {try {return ((Boolean) super.h.invoke(this, m1, new Object[] { obj })).booleanValue();} catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}@Overridepublic final int hashCode() {try {return ((Integer) super.h.invoke(this, m0, null)).intValue();} catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}public final void modify() {try {super.h.invoke(this, m3, null);return;} catch (Error e) {} catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}@Overridepublic final String toString() {try {return (String) super.h.invoke(this, m2, null);} catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}}

接着把得到的$Proxy0实例强制转换成Manager.
当执行managerProxy.modify()方法时,就调用了$Proxy0类中的modify()方法.
在modify方法中,调用父类Proxy中的h的invoke()方法.
即InvocationHandler.invoke();