设计模式之Proxy(代理)
代理模式是比较有用途的一种模式,而且变种较多,应用场合覆盖从小结构到整个系统的大结构,Proxy是代理的意思,我们也许有代理服务器等概念,代理概念可以解释为:在出发点到目的地之间有一道中间层,意为代理.
设计模式中定义: 为其他对象提供一种代理以控制对这个对象的访问.
为什么要使用Proxy?
1.授权机制 不同级别的用户对同一对象拥有不同的访问权利,如Jive论坛系统中,就使用Proxy进行授权机制控制,访问论坛有两种人:注册用户和游客(未注册用户),Jive中就通过类似ForumProxy这样的代理来控制这两种用户对论坛的访问权限.
2.某个客户端不能直接操作到某个对象,但又必须和那个对象有所互动.
举例两个具体情况:
(1)如果那个对象是一个是很大的图片,需要花费很长时间才能显示出来,那么当这个图片包含在文档中时,使用编辑器或浏览器打开这个文档,打开文档必须很迅速,不能等待大图片处理完成,这时需要做个图片Proxy来代替真正的图片.
(2)如果那个对象在Internet的某个远端服务器上,直接操作这个对象因为网络速度原因可能比较慢,那我们可以先用Proxy来代替那个对象.
总之原则是,对于开销很大的对象,只有在使用它时才创建,这个原则可以为我们节省很多宝贵的Java内存. 所以,有些人认为Java耗费资源内存,我以为这和程序编制思路也有一定的关系.
如何使用Proxy?
以Jive论坛系统为例,访问论坛系统的用户有多种类型:注册普通用户 论坛管理者 系统管理者 游客,注册普通用户才能发言;论坛管理者可以管理他被授权的论坛;系统管理者可以管理所有事务等,这些权限划分和管理是使用Proxy完成的.
Forum是Jive的核心接口,在Forum中陈列了有关论坛操作的主要行为,如论坛名称 论坛描述的获取和修改,帖子发表删除编辑等.
在ForumPermissions中定义了各种级别权限的用户:public class ForumPermissions implements Cacheable {
/**
* Permission to read object.
*/
public static final int READ = 0;
/**
* Permission to administer the entire sytem.
*/
public static final int SYSTEM_ADMIN = 1;
/**
* Permission to administer a particular forum.
*/
public static final int FORUM_ADMIN = 2;
/**
* Permission to administer a particular user.
*/
public static final int USER_ADMIN = 3;
/**
* Permission to administer a particular group.
*/
public static final int GROUP_ADMIN = 4;
/**
* Permission to moderate threads.
*/
public static final int MODERATE_THREADS = 5;
/**
* Permission to create a new thread.
*/
public static final int CREATE_THREAD = 6;
/**
* Permission to create a new message.
*/
public static final int CREATE_MESSAGE = 7;
/**
* Permission to moderate messages.
*/
public static final int MODERATE_MESSAGES = 8;
.....
public boolean isSystemOrForumAdmin() {
return (values[FORUM_ADMIN] || values[SYSTEM_ADMIN]);
}
.....
}
因此,Forum中各种操作权限是和ForumPermissions定义的用户级别有关系的,作为接口Forum的实现:ForumProxy正是将这种对应关系联系起来.比如,修改Forum的名称,只有论坛管理者或系统管理者可以修改,代码如下:
public class ForumProxy implements Forum {
private ForumPermissions permissions;
private Forum forum;
this.authorization = authorization;
public ForumProxy(Forum forum, Authorization authorization,
ForumPermissions permissions)
{
this.forum = forum;
this.authorization = authorization;
this.permissions = permissions;
}
.....
public void setName(String name) throws UnauthorizedException,
ForumAlreadyExistsException
{
//只有是系统或论坛管理者才可以修改名称
if (permissions.isSystemOrForumAdmin()) {
forum.setName(name);
}
else {
throw new UnauthorizedException();
}
}
...
}
而DbForum才是接口Forum的真正实现,以修改论坛名称为例:
public class DbForum implements Forum, Cacheable {
...
public void setName(String name) throws ForumAlreadyExistsException {
....
this.name = name;
//这里真正将新名称保存到数据库中
saveToDb();
....
}
...
}
凡是涉及到对论坛名称修改这一事件,其他程序都首先得和ForumProxy打交道,由ForumProxy决定是否有权限做某一样事情,ForumProxy是个名副其实的"网关","安全代理系统".
在平时应用中,无可避免总要涉及到系统的授权或安全体系,不管你有无意识的使用Proxy,实际你已经在使用Proxy了.
我们继续结合Jive谈入深一点,下面要涉及到工厂模式了,如果你不了解工厂模式,请看我的另外一篇文章:设计模式之Factory
我们已经知道,使用Forum需要通过ForumProxy,Jive中创建一个Forum是使用Factory模式,有一个总的抽象类ForumFactory,在这个抽象类中,调用ForumFactory是通过getInstance()方法实现,这里使用了Singleton(也是设计模式之一,由于介绍文章很多,我就不写了,看这里),getInstance()返回的是ForumFactoryProxy.
为什么不返回ForumFactory,而返回ForumFactory的实现ForumFactoryProxy?
原因是明显的,需要通过代理确定是否有权限创建forum.
在ForumFactoryProxy中我们看到代码如下:public class ForumFactoryProxy extends ForumFactory {
protected ForumFactory factory;
protected Authorization authorization;
protected ForumPermissions permissions;
public ForumFactoryProxy(Authorization authorization, ForumFactory factory,
ForumPermissions permissions)
{
this.factory = factory;
this.authorization = authorization;
this.permissions = permissions;
}
public Forum createForum(String name, String description)
throws UnauthorizedException, ForumAlreadyExistsException
{
//只有系统管理者才可以创建forum
if (permissions.get(ForumPermissions.SYSTEM_ADMIN)) {
Forum newForum = factory.createForum(name, description);
return new ForumProxy(newForum, authorization, permissions);
}
else {
throw new UnauthorizedException();
}
}
方法createForum返回的也是ForumProxy, Proxy就象一道墙,其他程序只能和Proxy交互操作.
注意到这里有两个Proxy:ForumProxy和ForumFactoryProxy. 代表两个不同的职责:使用Forum和创建Forum;
至于为什么将使用对象和创建对象分开,这也是为什么使用Factory模式的原因所在:是为了"封装" "分派";换句话说,尽可能功能单一化,方便维护修改.
Jive论坛系统中其他如帖子的创建和使用,都是按照Forum这个思路而来的.
以上我们讨论了如何使用Proxy进行授权机制的访问,Proxy还可以对用户隐藏另外一种称为copy-on-write的优化方式.拷贝一个庞大而复杂的对象是一个开销很大的操作,如果拷贝过程中,没有对原来的对象有所修改,那么这样的拷贝开销就没有必要.用代理延迟这一拷贝过程.
比如:我们有一个很大的Collection,具体如hashtable,有很多客户端会并发同时访问它.其中一个特别的客户端要进行连续的数据获取,此时要求其他客户端不能再向hashtable中增加或删除 东东.
最直接的解决方案是:使用collection的lock,让这特别的客户端获得这个lock,进行连续的数据获取,然后再释放lock.
public void foFetches(Hashtable ht){
synchronized(ht){
//具体的连续数据获取动作..
}
}
但是这一办法可能锁住Collection会很长时间,这段时间,其他客户端就不能访问该Collection了.
第二个解决方案是clone这个Collection,然后让连续的数据获取针对clone出来的那个Collection操作.这个方案前提是,这个Collection是可clone的,而且必须有提供深度clone的方法.Hashtable就提供了对自己的clone方法,但不是Key和value对象的clone,关于Clone含义可以参考专门文章.
public void foFetches(Hashtable ht){
Hashttable newht=(Hashtable)ht.clone();
}
问题又来了,由于是针对clone出来的对象操作,如果原来的母体被其他客户端操作修改了, 那么对clone出来的对象操作就没有意义了.
最后解决方案:我们可以等其他客户端修改完成后再进行clone,也就是说,这个特别的客户端先通过调用一个叫clone的方法来进行一系列数据获取操作.但实际上没有真正的进行对象拷贝,直至有其他客户端修改了这个对象Collection.
使用Proxy实现这个方案.这就是copy-on-write操作.
Proxy应用范围很广,现在流行的分布计算方式RMI和Corba等都是Proxy模式的应用.
c#的proxy模式
using System;
namespace DoFactory.GangOfFour.Proxy.Structural
{
// MainApp test application
class MainApp
{
static void Main()
{
// Create proxy and request a service
Proxy proxy = new Proxy();
proxy.Request();
// Wait for user
Console.Read();
}
}
// "Subject"
abstract class Subject
{
public abstract void Request();
}
// "RealSubject"
class RealSubject : Subject
{
public override void Request()
{
Console.WriteLine("Called RealSubject.Request()");
}
}
// "Proxy"
class Proxy : Subject
{
RealSubject realSubject;
public override void Request()
{
// Use 'lazy initialization'
if (realSubject == null)
{
realSubject = new RealSubject();
}
realSubject.Request();
}
}
}
------------------------------------------------
In computer programming, a proxy pattern is a software design pattern.
A proxy, in its most general form, is a class functioning as an interface to another thing. The other thing could be anything, a network connection, a large object in memory, a file, or other resource that is expensive or impossible to duplicate.
A well-known example of the proxy pattern is a reference counting pointer object, also known as an auto pointer.
The proxy pattern can be used in situations where multiple copies of a complex object must exist. In order to reduce the application's memory footprint in such situations, one instance of the complex object is created, and multiple proxy objects are created, all of which contain a reference to the single original complex object. Any operations performed on the proxies are forwarded to the original object. Once all instances of the proxy are out of scope, the complex object's memory may be deallocated.
Types of Proxy Pattern:
Remote Proxy: Provides a reference to an object located in a different address space on the same or different machine.
Virtual Proxy: Allows the creation of a memory intensive object on demand. The object will not be created until it is really needed.
Copy-On-Write Proxy: Defers copying (cloning) a target object until required by client actions. Really a form of virtual proxy.
Protection (Access) Proxy: Provides different clients with different levels of access to a target object.
Cache Proxy: Provides temporary storage of the results of expensive target operations so that multiple clients can share the results.
Firewall Proxy: Protects targets from bad clients (or vice versa).
Synchronization Proxy: Provides multiple accesses to a target object.
Smart Reference Proxy: Provides additional actions whenever a target object is referenced, such as counting the number of references to the object.
Examples
[edit]
Java
The following Java example illustrates the "Virtual Proxy" type explained above. The output is:
Loading HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1
import java.util.*;
interface Image {
public void displayImage();
}
class RealImage implements Image {
private String filename;
public RealImage(String filename) {
this.filename = filename;
System.out.println("Loading "+filename);
}
public void displayImage() { System.out.println("Displaying "+filename); }
}
class ProxyImage implements Image {
private String filename;
private RealImage image;
public ProxyImage(String filename) { this.filename = filename; }
public void displayImage() {
if (image == null) {
image = new RealImage(filename); // load only on demand
}
image.displayImage();
}
}
class ProxyExample {
public static void main(String[] args) {
ArrayList<
> images = new ArrayList<>();
images.add( new ProxyImage("HiRes_10MB_Photo1") );
images.add( new ProxyImage("HiRes_10MB_Photo2") );
images.add( new ProxyImage("HiRes_10MB_Photo3") );
images.get(0).displayImage(); // loading necessary
images.get(1).displayImage(); // loading necessary
images.get(0).displayImage(); // no loading necessary; already done
// the third image will never be loaded - time saved!
}
}
C#
[edit]
Virtual Proxy Example
using System;
using System.Collections;
namespace ConsoleApplicationTest.FundamentalPatterns.VirtualProxyPattern
{
public interface IImage{
void DisplayImage();
}
public class RealImage : IImage{
private string filename;
public RealImage(string filename)
{
this.filename = filename;
Console.WriteLine("Loading "+filename);
}
public void DisplayImage() { Console.WriteLine("Displaying "+filename); }
}
public class ProxyImage : IImage {
private string filename;
private RealImage image;
public ProxyImage(string filename) { this.filename = filename; }
public void DisplayImage(){
if (image == null) image = new RealImage(filename); // load only on demand
image.DisplayImage();
}
}
class ProxyExample
{
[STAThread]
public static void Main(string[] args)
{
ArrayList images = new ArrayList();
images.Add( new ProxyImage("HiRes_10MB_Photo1") );
images.Add( new ProxyImage("HiRes_10MB_Photo2") );
images.Add( new ProxyImage("HiRes_10MB_Photo3") );
((IImage)images[0]).DisplayImage(); // loading necessary
((IImage)images[1]).DisplayImage(); // loading necessary
((IImage)images[0]).DisplayImage(); // no loading necessary; already done
// the third image will never be loaded - time saved!
Console.Read();
}
}
}
[edit]
Protection Proxy Example
The real client stores an Account Number. Only the users who know the valid password can access this Account Number.
Real client is protected by a proxy who knows the password.
If a user wants to get an Account Number, first the proxy asks him/here to authenticate itself. If the user
entered a correct password the proxy will call the real client and pass the Account Number to the user.
using System;
namespace ConsoleApplicationTest.FundamentalPatterns.ProtectionProxyPattern
{
public interface IClient{
string GetAccountNo();
}
public class RealClient : IClient{
private string accountNo="AB-111111";
public RealClient(){
Console.WriteLine("RealClient: Initialized");
}
public string GetAccountNo(){
Console.WriteLine("RealClient's AccountNo: " + accountNo);
return accountNo;
}
}
public class ProtectionProxy : IClient
{
private string password=null; //password to get secret
RealClient client=null;
public ProtectionProxy(string pwd)
{
Console.WriteLine("ProtectionProxy: Initialized");
password=pwd;
client=new RealClient();
}
///
/// Authenticate the user and return the Account Number
///
///
public String GetAccountNo()
{
Console.WriteLine("Password: ");
string tmpPwd= Console.ReadLine();
if(tmpPwd == password)
{
return client.GetAccountNo();
}
else
{
Console.WriteLine("ProtectionProxy: Illegal password!");
return "";
}
}
}
class ProtectionProxyExample
{
[STAThread]
public static void Main(string[] args){
IClient client =new ProtectionProxy("thePassword");
Console.WriteLine("main received: "+client.GetAccountNo());
Console.WriteLine("main received: "+client.GetAccountNo());
Console.Read();
}
}
}
