关于Java动态代理的疑问？

Question

以下是几个简单的类：
[code="java"]
//接口
public interface ItemMgr {
void add();
}
//实现
public class ItemMgrImpl implements ItemMgr {

public void add() {
    System.out.println("Add an Item");
}

}
//处理日志
public class LogHandler implements InvocationHandler {

private Object target;

public LogHandler(Object target) {
    this.target = target;
}

public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable {
    System.out.println("Log Start Write...");
    Object res = method.invoke(target, args);
    System.out.println("Log End Write...");
    return res;
}

}
//计算方法运行时间
public class TimeHandler implements InvocationHandler {

private Object target;

public TimeHandler(Object target) {
    this.target = target;
}

public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable {

    long start = System.currentTimeMillis();
    System.out.println("starttime : " + start);

    Object res = method.invoke(target, args);

    long end = System.currentTimeMillis();
    System.out.println("time : " + (end-start));

    return res;
}

}

//测试方法
public class TestProxy {

public static void main(String[] args) throws Exception {
    ItemMgr mgr = new ItemMgrImpl();

    TimeHandler th = new TimeHandler(mgr);
    LogHandler lh = new LogHandler(th);

    ItemMgr m = (ItemMgr)Proxy.newProxyInstance(mgr.getClass().getClassLoader(), 
                                                        mgr.getClass().getInterfaces(), 
                                                        th);
    m.add();
}

}

[/code]

[size=small]运行测试方法会抛出java.lang.IllegalArgumentException: object is not an instance of declaring class异常

我只是在计算时间外面套了一层处理日志，望指教。[/size]

Answer 1

//处理日志

public class LogHandler implements InvocationHandler {

//被代理对象
private Object target;

public LogHandler(Object target) {   
    this.target = target;   
}   

public Object invoke(Object proxy, Method method, Object[] args)   
        throws Throwable {   
    System.out.println("Log Start Write...");  
    //方法参数target是被代理对象 
    Object res = method.invoke(target, args);   
    System.out.println("Log End Write...");   
    return res;   
}   

}

//计算方法运行时间

public class TimeHandler implements InvocationHandler {

//被代理对象
private Object target;

public TimeHandler(Object target) {   
    this.target = target;   
}   

public Object invoke(Object proxy, Method method, Object[] args)   
        throws Throwable {   

    long start = System.currentTimeMillis();   
    System.out.println("starttime : " + start);   
    //方法参数target是被代理对象 
    Object res = method.invoke(target, args);   

    long end = System.currentTimeMillis();   
    System.out.println("time : " + (end-start));   

    return res;   
}   

}

ItemMgr mgr = new ItemMgrImpl();

//参数传递正确，mgr实现了ItemMgr 接口
TimeHandler th = new TimeHandler(mgr);

//参数传递错误， th实现了ItemMgr接口吗？
LogHandler lh = new LogHandler(th);