Java中代理(Proxy)的实现机制

Question

看TIJ的代理看得有点晕，其中有一段代码如下。（print是包装了下的System.out.println）

[code="java"]
package TypeInfo;
// typeinfo/SimpleDynamicProxy23.java
// TIJ4 Chapter Typeinfo, Exercise 23, page 598
// Inside invoke() in SimpleDynamicProxy.java, try to print the proxy argument and explain
// what happens.
import java.lang.reflect.*;

interface Interface {
void doSomething();
void somethingElse(String arg);
}

class RealObject implements Interface {
public void doSomething() { print("doSomething"); }
public void somethingElse(String arg) {
print("somethingElse " + arg);
}
}

class DynamicProxyHandler implements InvocationHandler {
private Object proxied;
public DynamicProxyHandler(Object proxied) {
this.proxied = proxied;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
/*
* trying to print proxy leads to:
* StackOverFlowError
* at AbstractStringBuilder.(Unknown Source)
* at StringBuilder.(Unknown Source)
* at DynamicProxyHandler.invoke(SimpleDynamicProxy23.java)
* at $Proxy0.toString(Unknown Source)
* at String.valueOf(Unknown Source)
* at StrinbBuilcer.append(Unknown Source)
* at DynamicProxyHandler.invoke(SimpleDynamicProxy23.java), etc,
* probably due to infinite recursion because calls to toString()
* are passed repeatedly back to this invoke method
/
// System.out.println("proxy: " + proxy); // error
System.out.println("*** proxy: " + proxy.getClass() +
", method: " + method + ", args: " + args);
if(args != null)
for(Object arg : args)
System.out.println(" " + args);
return method.invoke(proxied, args);
}
}

class SimpleDynamicProxy23 {
public static void consumer(Interface iface) {
iface.doSomething();
iface.somethingElse("bonobo");
}
public static void main(String[] args) {
RealObject real = new RealObject();
consumer(real);
// Insert a proxy and call again:
Interface proxy = (Interface)Proxy.newProxyInstance(
Interface.class.getClassLoader(),
new Class[]{ Interface.class },
new DynamicProxyHandler(real));
consumer(proxy);

}
}
[/code]
代码比较容易看懂，但是其中的内在机制就很隐晦。调试时发现，consumer中调用Interface的方法后程序流直接就跳入了invoke。这是由什么机制决定的？换言之，在consumer中对接口Interface的方法的调用是如何转交给DynamicProxyHandler的invoke方法的？
书中有一句话：“在invoke()内部，在代理上调用方法时需要格外当心，因为对接口的调用将被重定向为对代理的调用。”这句解释了注释掉的“// System.out.println("proxy: " + proxy); // error”的错误是因为引起了无限递归（很空洞的解释，具体怎样引起？）invoke的第一个参数proxy是什么类型？
希望大家不吝赐教。3Q!

Answer 1

推荐你看下
[url]http://hllvm.group.iteye.com/group/topic/36843[/url]

然后dump出代理类出来 然后观察

我这里给你一份我dump出的
[code="java"]
package TypeInfo;

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 Interface
{
private static Method m1;
private static Method m0;
private static Method m3;
private static Method m4;
private static Method m2;

public final void doSomething()
{
try
{
this.h.invoke(this, m3, null);
return;
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}

public final void somethingElse(String paramString)
{
try
{
this.h.invoke(this, m4, new Object[] { paramString });
return;
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}

public $Proxy0(InvocationHandler paramInvocationHandler)
{
super(paramInvocationHandler);
}

public final int hashCode()
{
try
{
return ((Integer)this.h.invoke(this, m0, null)).intValue();
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}

public final boolean equals(Object paramObject)
{
try
{
return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}

public final String toString()
{
try
{
return ((String)this.h.invoke(this, m2, null));
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}

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("TypeInfo.Interface").getMethod("doSomething", new Class[0]);
m4 = Class.forName("TypeInfo.Interface").getMethod("somethingElse", new Class[] { Class.forName("java.lang.String") });
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
return;
}
catch (NoSuchMethodException localNoSuchMethodException)
{
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
}
catch (ClassNotFoundException localClassNotFoundException)
{
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}
}[/code]