定义：为对象提供一个代理以控制其它对象对它的访问

使用场景： 1）远程代理：访问远程对象 2）虚拟代理：根据需要创建开销大的对象 3）保护代理：控制对原对象的访问

类图：

代码样例：

package headfirst.proxy.virtualproxy;

import java.awt.*;
import javax.swing.*;

class ImageComponent extends JComponent {
	private Icon icon;

	public ImageComponent(Icon icon) {
		this.icon = icon;
	}

	public void setIcon(Icon icon) {
		this.icon = icon;
	}

	public void paintComponent(Graphics g) {
		super.paintComponent(g);
		int w = icon.getIconWidth();
		int h = icon.getIconHeight();
		int x = (800 - w)/2;
		int y = (600 - h)/2;
		icon.paintIcon(this, g, x, y);
	}
}

package headfirst.proxy.virtualproxy;

import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class ImageProxy implements Icon {
	ImageIcon imageIcon;
	URL imageURL;
	Thread retrievalThread;
	boolean retrieving = false;
     
	public ImageProxy(URL url) { imageURL = url; }
     
	public int getIconWidth() {
		if (imageIcon != null) {
            return imageIcon.getIconWidth();
        } else {
			return 800;
		}
	}
 
	public int getIconHeight() {
		if (imageIcon != null) {
            return imageIcon.getIconHeight();
        } else {
			return 600;
		}
	}
     
	public void paintIcon(final Component c, Graphics  g, int x,  int y) {
		if (imageIcon != null) {
			imageIcon.paintIcon(c, g, x, y);
		} else {
			g.drawString("Loading CD cover, please wait...", x+300, y+190);
			if (!retrieving) {
				retrieving = true;

				retrievalThread = new Thread(new Runnable() {
					public void run() {
						try {
							imageIcon = new ImageIcon(imageURL, "CD Cover");
							c.repaint();
						} catch (Exception e) {
							e.printStackTrace();
						}
					}
				});
				retrievalThread.start();
			}
		}
	}
}

package headfirst.proxy.virtualproxy;

import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;

public class ImageProxyTestDrive {
	ImageComponent imageComponent;
	JFrame frame = new JFrame("CD Cover Viewer");
    JMenuBar menuBar;
    JMenu menu;
	Hashtable cds = new Hashtable();
 
	public static void main (String[] args) throws Exception {
		ImageProxyTestDrive testDrive = new ImageProxyTestDrive();
	}
 
	public ImageProxyTestDrive() throws Exception{
		cds.put("Ambient: Music for Airports","http://images.amazon.com/images/P/B000003S2K.01.LZZZZZZZ.jpg");
        cds.put("Buddha Bar","http://images.amazon.com/images/P/B00009XBYK.01.LZZZZZZZ.jpg");
        cds.put("Ima","http://images.amazon.com/images/P/B000005IRM.01.LZZZZZZZ.jpg");
        cds.put("Karma","http://images.amazon.com/images/P/B000005DCB.01.LZZZZZZZ.gif");
        cds.put("MCMXC A.D.","http://images.amazon.com/images/P/B000002URV.01.LZZZZZZZ.jpg");
        cds.put("Northern Exposure","http://images.amazon.com/images/P/B000003SFN.01.LZZZZZZZ.jpg");
        cds.put("Selected Ambient Works, Vol. 2","http://images.amazon.com/images/P/B000002MNZ.01.LZZZZZZZ.jpg");

		URL initialURL = new URL((String)cds.get("Selected Ambient Works, Vol. 2"));
		menuBar = new JMenuBar();
		menu = new JMenu("Favorite CDs");
        menuBar.add(menu);
        frame.setJMenuBar(menuBar);

		for(Enumeration e = cds.keys(); e.hasMoreElements();) {
			String name = (String)e.nextElement();
        	JMenuItem menuItem = new JMenuItem(name);
        	menu.add(menuItem); 
        	menuItem.addActionListener(new ActionListener() {
          		  public void actionPerformed(ActionEvent event) {
           		     imageComponent.setIcon(new ImageProxy(getCDUrl(event.getActionCommand())));
					frame.repaint();
           	      }
        	});
		}
 		
		// set up frame and menus
 
		Icon icon = new ImageProxy(initialURL);
		imageComponent = new ImageComponent(icon);
		frame.getContentPane().add(imageComponent);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(800,600);
		frame.setVisible(true);

	}

	URL getCDUrl(String name) {
		try {
			return new URL((String)cds.get(name));
		} catch (MalformedURLException e) {
			e.printStackTrace();
			return null;
		}
	}
}

优点：1）见使用场景

缺点：1）性能不如直接对象

类似的设计模式：1）装饰者模式：强调增加行为，而代理模式强调的是代理但是不改变其主要行为

配套的内功心法：1）

代理模式——Head First Design Patterns