代理模式

代理模式（Proxy Pattern）是程序设计中的一种设计模式。所谓的代理者是指一个类别可以作为其它东西的接口。当希望使用某个对象或者模块的功能，但是因为种种原因，我们不能直接访问，这时就可以考虑使用代理。

UML图

代码举例

import java.util.*;

interface Image {
    public void displayImage();
}

//on System A 
class RealImage implements Image {
    private String filename;
    public RealImage(String filename) { 
        this.filename = filename;
        loadImageFromDisk();
    }

    private void loadImageFromDisk() {
        System.out.println("Loading   " + filename);
    }

    public void displayImage() { 
        System.out.println("Displaying " + filename); 
    }
}

//on System B 
class ProxyImage implements Image {
    private String filename;
    private Image image;

    public ProxyImage(String filename) { 
        this.filename = filename; 
    }
    public void displayImage() {
        if(image == null)
              image = new RealImage(filename);
        image.displayImage();
    }
}

class ProxyExample {
    public static void main(String[] args) {
        Image image1 = new ProxyImage("HiRes_10MB_Photo1");
        Image image2 = new ProxyImage("HiRes_10MB_Photo2");     

        image1.displayImage(); // loading necessary
        image2.displayImage(); // loading necessary
    }
}

代码举例2

有个接口Italk，people对象实现这个接口的talk()方法，有些想另外加入sing()方法，可以增加代理类talkProxy

public interface ITalk
{
    public void talk(String msg);
}

public class People implements ITalk
{
    @Override
    public void talk(final String msg)
    {
        System.out.println(msg);
    }
}

public class TalkProxy implements ITalk
{
    ITalk italk;

    public TalkProxy(final ITalk italk)
    {
        this.italk = italk;
    }

    @Override
    public void talk(final String msg)
    {
        this.italk.talk(msg);
    }

    public void sing(final String songName)
    {
        System.out.println("Song Name:" + songName);
    }
}

public class Client
{
    public static void main(final String[] args)
    {
        final People people = new People();
        people.talk("I can't sing");

        final TalkProxy proxy = new TalkProxy(people);
        proxy.talk("I can talk");
        proxy.sing("I can sing");
    }
}

运行结果

I can't sing
I can talk
Song Name:I can sing