為另一個對象提供一個替身或占位符以控制對這個對象的訪問，簡而言之就是用一個對象來代表另一個對象。
類圖：
簡單例子：有個接口Italk,people對象實現這個接口的talk()方法，有些想另外加入sing()方法，所以增加代理類talkProxy，實現兩個方法.
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");
}
}
結果：
I can't sing
I can talk
Song Name:I can sing
常見的代理:
1. 遠程代理(Remote Proxy)：為一個位於不同的地址空間的對象提供一個本地的代理對象。這個不同的地址空間可以是在同一台主機中，也
可是在另一台主機中，遠程代理又叫做大使(Ambassador)。
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface MyRemote extends Remote
{
public String getUserName(String userId) throws RemoteException;
}
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
public class MyRemoteImp extends UnicastRemoteObject implements MyRemote
{
protected MyRemoteImp() throws RemoteException
{
}
public String getUserName(String userId) throws RemoteException
{
return userId + "/myName is Cherry";
}
}
在cmd中輸入:
javac MyRemote.java
javac MyRemoteImp.java
rmic MyRemoteImp
生成類的class文件及MyRemoteImp_Stub.class文件
import java.net.MalformedURLException;
import java.rmi.Naming;
import java.rmi.RemoteException;
public class MyRemoteServer
{
public static void main(final String[] args)
{
// YTODO Auto-generated method stub
MyRemote remote;
try
{
remote = new MyRemoteImp();
Naming.rebind("RemoteService", remote);
}
catch (final RemoteException e)
{
// YTODO Auto-generated catch block
e.printStackTrace();
}
catch (final MalformedURLException e)
{
// YTODO Auto-generated catch block
e.printStackTrace();
}
}
}
import java.net.MalformedURLException;
import java.rmi.Naming;
import java.rmi.NotBoundException;
import java.rmi.RemoteException;
public class MyRemoteClient
{
public static void main(final String[] args)
{
try
{
final MyRemote service = (MyRemote) Naming.lookup("rmi://127.0.0.1/RemoteService");
final String name = service.getUserName("my id is 001");
System.out.println(name);
}
catch (final MalformedURLException e)
{
e.printStackTrace();
}
catch (final RemoteException e)
{
e.printStackTrace();
}
catch (final NotBoundException e)
{
e.printStackTrace();
}
}
}
在dos下運行rmiregistry, 這個命令是開啟RMI注冊服務, 開啟以后我們的server程序才能調用rebing方法發布我們的類,然后運行server程序
.
javac MyRemoteServer.java
java MyRemoteServer
start rmiregistry
javac MyRemoteServer.java
java MyRemoteServer
再打開一個dos運行客戶端代碼
javac MyRemoteClient.java
java MyRemoteClient
輸出:my id is 001/myName is Cherry
這里一共有三個dos窗口, 當執行start rmiregistry時,會彈出它的窗口,當執行java MyRemoteServer的服務器端窗口,執行java
MyRemoteClient的客戶端窗口(這里的rmiregistry.exe的窗口需要一直打開,否則無法調用服務器端).
2. 虛擬代理(Virtual Proxy)：根據需要創建開銷很大的對象。如果需要創建一個資源消耗較大的對象，先創建一個消耗相對較小的對象來表
示，真實對象只會在需要時才會被真正創建。
public interface Image
{
public void show();
}
public class BigImage implements Image
{
public BigImage()
{
//Thread.sleep(3000);//for simulating to load the big images
System.out.println("create the big images");
}
@Override
public void show()
{
System.out.println("show the big images");
}
}
public class ImageProxy implements Image
{
Image image;
public ImageProxy()
{
}
public ImageProxy(final Image image)
{
this.image = image;
}
@Override
public void show()
{
if (this.image == null)
{
this.image = new BigImage();
}
this.image.show();
}
}
public class Client
{
public static void main(final String[] args)
{
System.out.println("big image:");
final Image bigImage = new BigImage();
bigImage.show();
System.out.println("image proxy:");
final Image imageProxy = new ImageProxy();
imageProxy.show();
}
}
結果:
create the big images
show the big images
image proxy:
create the big images
show the big images
3. Copy-on-Write代理: 虛擬代理的一種，把復制（克隆）拖延到只有在客戶端需要時，才真正采取行動。
4. 保護代理(Protection or Access Proxy)：控制對原始對象的訪問。保護代理用於對象應該有不同的訪問權限的時候。
5. 智能指引(Smart Reference)：取代了簡單的指針,它在訪問對象時執行一些附加操作.