Proxy pattern

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Proxy pattern

代理模式为另一个对象提供一个代理对象以控制对这个对象的访问，被代理的对象可以是远程对象、创建开销大的对象或者需要安全控制的对象。

远程代理

远程代理即是我们希望通过调用一个本地对象，然后其能将每个请求转发到远程对象上进行。换句话说，客户对象意味它调用的是远程服务上的方法，实际上是代理对象假装有客户要调用的方法，真正执行方法的在远程的对象。

Design_pattern_proxy_1

通过Java RMI我们可以简单实现远程调用，而无需处理细节的网络通信。

Design_pattern_proxy_2

使用Java RMI的具体步骤为：

  1. 创建远程接口
  2. 实现远程接口的类Impl（main方法需要注册到RMI中）
  3. 使用Java rmic生成Impl的stud和skeleton，也就是客户和服务的辅助类。
  4. 在远程机器上启动RMI registry(倾听Impl的注册)
  5. 运行主方法(可以是启动类)将Impl注册到RMI服务上。
  6. 客户端调用Naming方法取得stub对象。
  7. 客户调用哦该stub方法，就像stub就是真正的服务对象一样。

Design_pattern_proxy_3

Design_pattern_proxy_4

服务端实现：

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public interface GumballMachineRemote extends Remote {
	public int getCount() throws RemoteException;
	public String getLocation() throws RemoteException;
	public State getState() throws RemoteException;
}
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public interface State extends Serializable {
	public void insertQuarter();
	public void ejectQuarter();
	public void turnCrank();
	public void dispense();
}

public class NoQuarterState implements State {
    transient GumballMachine gumballMachine;
 
    public NoQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
 
	public void insertQuarter() {
		System.out.println("You inserted a quarter");
		gumballMachine.setState(gumballMachine.getHasQuarterState());
	}
 
	public void ejectQuarter() {
		System.out.println("You haven't inserted a quarter");
	}
 
	public void turnCrank() {
		System.out.println("You turned, but there's no quarter");
	 }
 
	public void dispense() {
		System.out.println("You need to pay first");
	} 
 
	public String toString() {
		return "waiting for quarter";
	}
}

...
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public class GumballMachine
		extends UnicastRemoteObject implements GumballMachineRemote 
{
	State soldOutState;
	State noQuarterState;
	State hasQuarterState;
	State soldState;
	State winnerState;
 
	State state = soldOutState;
	int count = 0;
 	String location;

	public GumballMachine(String location, int numberGumballs) throws RemoteException {
		soldOutState = new SoldOutState(this);
		noQuarterState = new NoQuarterState(this);
		hasQuarterState = new HasQuarterState(this);
		soldState = new SoldState(this);
		winnerState = new WinnerState(this);

		this.count = numberGumballs;
 		if (numberGumballs > 0) {
			state = noQuarterState;
		} 
		this.location = location;
	}
 
 
	public void insertQuarter() {
		state.insertQuarter();
	}
 
	public void ejectQuarter() {
		state.ejectQuarter();
	}
 
	public void turnCrank() {
		state.turnCrank();
		state.dispense();
	}

	void setState(State state) {
		this.state = state;
	}
 
	void releaseBall() {
		System.out.println("A gumball comes rolling out the slot...");
		if (count != 0) {
			count = count - 1;
		}
	}

	public void refill(int count) {
		this.count = count;
		state = noQuarterState;
	}
 
	public int getCount() {
		return count;
	}
 
    public State getState() {
        return state;
    }
 
    public String getLocation() {
        return location;
    }
  
    public State getSoldOutState() {
        return soldOutState;
    }

    public State getNoQuarterState() {
        return noQuarterState;
    }

    public State getHasQuarterState() {
        return hasQuarterState;
    }

    public State getSoldState() {
        return soldState;
    }

    public State getWinnerState() {
        return winnerState;
    }
 
	public String toString() {
		StringBuffer result = new StringBuffer();
		result.append("\nMighty Gumball, Inc.");
		result.append("\nJava-enabled Standing Gumball Model #2004");
		result.append("\nInventory: " + count + " gumball");
		if (count != 1) {
			result.append("s");
		}
		result.append("\n");
		result.append("Machine is " + state + "\n");
		return result.toString();
	}
}

在RMI registry中注册

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public class GumballMachineTestDrive {
 
	public static void main(String[] args) {
		GumballMachineRemote gumballMachine = null;
		int count;

		if (args.length < 2) {
			System.out.println("GumballMachine <name> <inventory>");
 			System.exit(1);
		}

		try {
			count = Integer.parseInt(args[1]);

			gumballMachine = 
				new GumballMachine(args[0], count);
			Naming.rebind("//" + args[0] + "/gumballmachine", gumballMachine);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

客户端

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public class GumballMonitor {
	GumballMachineRemote machine;
 
	public GumballMonitor(GumballMachineRemote machine) {
		this.machine = machine;
	}
 
	public void report() {
		try {
			System.out.println("Gumball Machine: " + machine.getLocation());
			System.out.println("Current inventory: " + machine.getCount() + " gumballs");
			System.out.println("Current state: " + machine.getState());
		} catch (RemoteException e) {
			e.printStackTrace();
		}
	}
}
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public class GumballMonitorTestDrive {
 
	public static void main(String[] args) {
		String[] location = {"rmi://santafe.mightygumball.com/gumballmachine",
		                     "rmi://boulder.mightygumball.com/gumballmachine",
		                     "rmi://seattle.mightygumball.com/gumballmachine"}; 
 
		GumballMonitor[] monitor = new GumballMonitor[location.length];
 
		for (int i=0;i < location.length; i++) {
			try {
           		GumballMachineRemote machine = 
						(GumballMachineRemote) Naming.lookup(location[i]);
           		monitor[i] = new GumballMonitor(machine);
				System.out.println(monitor[i]);
        	} catch (Exception e) {
            	e.printStackTrace();
        	}
		}
 
		for(int i=0; i < monitor.length; i++) {
			monitor[i].report();
		}
	}
}

虚拟代理

远程代理可以作为另一个JVM上对象的本地代表。调用代理的方法，会被代理利用网络转发到远程执行，并且结果会通过网络返回给代理，再有代理将结果转给客户。

Design_pattern_proxy_5

而虚拟代理通常作为创建开销大的对象的代表。虚拟代理经常直到我们真正需要一个对象的时候才创建它。当对象在创建前和创建中时，由虚拟代理来扮演对象的替身。对象创建后，代理就会将请求委托给对象。

比如我们需要创建一个CD封面对象，这个封面图片需要从网上下载。在下外完成前，我们最好也显式一些内容，这时我们就可以使用虚拟代理。

Design_pattern_proxy_6

  • ImageProxy首先创建一个ImageIcon，然后开始从网络上加载图像。
  • 在加载过程中，ImageProxy显式”加载中…”
  • 当图像加载完毕，ImageProxy把所有方法掉用委托给真正的ImageIcon。
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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();
			}
		}
	}
}
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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);
	}
}
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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;
		}
	}
}

使用Java API代理实现保护代理

Java在java.lang.reflect包中有自己的代理支持，利用哦该这个包我们可以在运行时动态的创建一个代理类，实现一个或多个接口，并将方法的调用转发到我们所指定的类。因为实际的代理是在运行时创建的，所以这个技术称为动态代理。

Design_pattern_proxy_7

我们不需要自己创建Proxy类，也不能往里面添加代码。我们希望处理代码应该放在InvocationHandler中。InvocationHandler的工作是响应代理的任何调用，可以认为是代理收到方法调用后实际执行请求的对象。

假设我们正在实现一个信息管理服务，我们希望当前用户可以更改自己的基本信息，但不是不能更改自己的客观信息，同时其他用户也不能更改当前用户的基本信息。

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public interface PersonBean {
 
	String getName();
	String getGender();
	String getInterests();
	int getHotOrNotRating();
 
    void setName(String name);
    void setGender(String gender);
    void setInterests(String interests);
    void setHotOrNotRating(int rating); 
 
}

public class PersonBeanImpl implements PersonBean {
	String name;
	String gender;
	String interests;
	int rating;
	int ratingCount = 0;
  
	public String getName() {
		return name;	
	} 
  
	public String getGender() {
		return gender;
	}
  
	public String getInterests() {
		return interests;
	}
   
	public int getHotOrNotRating() {
		if (ratingCount == 0) return 0;
		return (rating/ratingCount);
	}
  
 
	public void setName(String name) {
		this.name = name;
	}
 
	public void setGender(String gender) {
		this.gender = gender;
	} 
  
	public void setInterests(String interests) {
		this.interests = interests;
	} 
  
	public void setHotOrNotRating(int rating) {
		this.rating += rating;	
		ratingCount++;
	}
}

这时我们可以为PersonBean创建动态代理，以过滤对这个类的请求：

  1. 创建InvocationHandler(如果有多种模式，需要创建多个，这里我们分为两类，操作用户是用户本人或是其他人，所以需要两个InvocationHandler)。
  2. 写代码创建动态代理。
  3. 利用适当的代理保证PersonBean对象，并将这个代理提供给客户，而不是对象本身。
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public class OwnerInvocationHandler implements InvocationHandler { 
	PersonBean person;
 
	public OwnerInvocationHandler(PersonBean person) {
		this.person = person;
	}
 
	public Object invoke(Object proxy, Method method, Object[] args) 
			throws IllegalAccessException {
  
		try {
			if (method.getName().startsWith("get")) {
				return method.invoke(person, args);
   			} else if (method.getName().equals("setHotOrNotRating")) {
				throw new IllegalAccessException();
			} else if (method.getName().startsWith("set")) {
				return method.invoke(person, args);
			} 
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } 
		return null;
	}
}
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public class NonOwnerInvocationHandler implements InvocationHandler { 
	PersonBean person;
 
	public NonOwnerInvocationHandler(PersonBean person) {
		this.person = person;
	}
 
	public Object invoke(Object proxy, Method method, Object[] args) 
			throws IllegalAccessException {
  
		try {
			if (method.getName().startsWith("get")) {
				return method.invoke(person, args);
   			} else if (method.getName().equals("setHotOrNotRating")) {
				return method.invoke(person, args);
			} else if (method.getName().startsWith("set")) {
				throw new IllegalAccessException();
			} 
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } 
		return null;
	}
}
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public class MatchMakingTestDrive {
	Hashtable datingDB = new Hashtable();
 	
	public static void main(String[] args) {
		MatchMakingTestDrive test = new MatchMakingTestDrive();
		test.drive();
	}
 
	public MatchMakingTestDrive() {
		initializeDatabase();
	}

	public void drive() {
		PersonBean joe = getPersonFromDatabase("Joe Javabean"); 
		PersonBean ownerProxy = getOwnerProxy(joe);
		System.out.println("Name is " + ownerProxy.getName());
		ownerProxy.setInterests("bowling, Go");
		System.out.println("Interests set from owner proxy");
		try {
			ownerProxy.setHotOrNotRating(10);
		} catch (Exception e) {
			System.out.println("Can't set rating from owner proxy");
		}
		System.out.println("Rating is " + ownerProxy.getHotOrNotRating());

		PersonBean nonOwnerProxy = getNonOwnerProxy(joe);
		System.out.println("Name is " + nonOwnerProxy.getName());
		try {
			nonOwnerProxy.setInterests("bowling, Go");
		} catch (Exception e) {
			System.out.println("Can't set interests from non owner proxy");
		}
		nonOwnerProxy.setHotOrNotRating(3);
		System.out.println("Rating set from non owner proxy");
		System.out.println("Rating is " + nonOwnerProxy.getHotOrNotRating());
	}

	PersonBean getOwnerProxy(PersonBean person) {
 		
        return (PersonBean) Proxy.newProxyInstance( 
            	person.getClass().getClassLoader(),
            	person.getClass().getInterfaces(),
                new OwnerInvocationHandler(person));
	}

	// 获得这个对象的代理对象
	PersonBean getNonOwnerProxy(PersonBean person) {
		
        return (PersonBean) Proxy.newProxyInstance(
            	person.getClass().getClassLoader(),
            	person.getClass().getInterfaces(),
                new NonOwnerInvocationHandler(person));
	}

	PersonBean getPersonFromDatabase(String name) {
		return (PersonBean)datingDB.get(name);
	}

	void initializeDatabase() {
		PersonBean joe = new PersonBeanImpl();
		joe.setName("Joe Javabean");
		joe.setInterests("cars, computers, music");
		joe.setHotOrNotRating(7);
		datingDB.put(joe.getName(), joe);

		PersonBean kelly = new PersonBeanImpl();
		kelly.setName("Kelly Klosure");
		kelly.setInterests("ebay, movies, music");
		kelly.setHotOrNotRating(6);
		datingDB.put(kelly.getName(), kelly);
	}
}