java 动态代理深度学习[转]

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()
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
(详见api文档)

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。
(详见api文档)

 

二.源代码:

 

被代理对象的接口及实现类:
package com.ml.test;

 

public interface Manager {
public void modify();
}

 

package com.ml.test;

 

public class ManagerImpl implements Manager {

 

@Override
public void modify() {
System.out.println("*******modify()方法被调用");
}
}

 

业务代理类:
package com.ml.test;

 

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;
}

 

@Override
public 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:
package com.ml.test;
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);
}

 

@Override
public final boolean equals(Object obj) {
try {
return ((Boolean) super.h.invoke(this, m1, new Object[] { obj }))
.booleanValue();
} catch (Throwable throwable) {
throw new UndeclaredThrowableException(throwable);
}
}

 

@Override
public 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);
}
}

 

@Override
public 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();