代理模式

1 用于实现代理。静态代理 

如果不用InvocationHandler接口实现代理的话，我们写代码是这样的： 
定义一个接口：

Java code

interface Greet { void sayHello(String name); void goodBye(); }

实现这个接口：

Java code

class GreetImpl implements Greet { public void sayHello(String name) { System.out.println("Hello " + name); } public void goodBye() { System.out.println("Good bye."); } }

实现一个代理类

Java code

public class SimpleProxy implements Greet { private Greet greet = null; SimpleProxy(Greet greet) { this.greet = greet; } public void sayHello(String name) { System.out.println("--before method sayHello"); greet.sayHello(name); System.out.println("--after method sayHello"); } public void goodBye() { System.out.println("--before method goodBye"); greet.goodBye(); System.out.println("--after method goodBye"); } /** * @param args */ public static void main(String[] args) { Greet greet = new SimpleProxy(new GreetImpl());//生成代理 greet.sayHello("walter"); greet.goodBye(); } }

2  动态代理
代理其实没什么的，再看看如果实现了InvocationHandler接口， 
我们怎样实现代理。 
还是要实现原来的Greet接口。 
接口的实现还是GreetImpl。

Java code

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class DebugProxy implements java.lang.reflect.InvocationHandler
{
    private Object obj;

    public static Object newInstance(Object obj)
    {
        return java.lang.reflect.Proxy.newProxyInstance(obj.getClass().getClassLoader(),
                obj.getClass().getInterfaces(), new DebugProxy(obj));
    }

    private DebugProxy(Object obj)
    {
        //Greet接口的實現：GreetImpl
        this.obj = obj;
    }

    //Method m：調用的方法
    //Object[] args：方法要傳入的參數
    public Object invoke(Object proxy, Method m, Object[] args) throws Throwable
    {
        Object result;
        try
        {
            //自定義的處理
            System.out.println("--before method " + m.getName());
            //調用GreetImpl中方法
            result = m.invoke(obj, args);
        }
        catch(InvocationTargetException e)
        {
            throw e.getTargetException();
        }
        catch(Exception e)
        {
            throw new RuntimeException("unexpected invocation exception: " + e.getMessage());
        }
        finally
        {
            System.out.println("--after method " + m.getName());
        }
        return result;
    }

    /**
     * @param args
     */
    public static void main(String[] args)
    {
        Greet tmp = new GreetImpl();
        
        Greet greet = (Greet) DebugProxy.newInstance(tmp);
        //生成的greet和tmp有相同的hashCode
        
        greet.sayHello("walter");
        greet.goodBye();
    }
}