代理模式

代理模式（英语：）是编程中的一种设计模式。

代理模式的统一建模语言(UML)图
代理模式的LePUS3建模语言 (图例)

所谓的代理者是指一个类别可以作为其它东西的接口。代理者可以作任何东西的接口：网络连接、内存中的大对象、文件或其它昂贵或无法拷贝的资源。

著名的代理模式例子为参照计数（英语：）指针对象。

当一个复杂对象的多份副本须存在时，代理模式可以结合享元模式以减少内存用量。典型作法是创建一个复杂对象及多个代理者，每个代理者会参照到原本的复杂对象。而作用在代理者的运算会转送到原本对象。一旦所有的代理者都不存在时，复杂对象会被移除。

示例

Java

以下Java范例解释"虚拟代理"模式。ProxyImage 类别用来访问远程方法。

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
    }
}

程序的输出为：

Loading    HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading    HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2

C#

 
using System;

namespace Proxy
{
    class Program
    {
        interface IImage
        {
            void Display();
        }

        class RealImage : IImage
        {
            public RealImage(string fileName)
            {
                FileName = fileName;
                LoadFromFile();
            }

            private void LoadFromFile()
            {
                Console.WriteLine("Loading " + FileName);
            }

            public String FileName { get; private set; }

            public void Display()
            {
                Console.WriteLine("Displaying " + FileName);
            }
        }

        class ProxyImage : IImage
        {
            public ProxyImage(string fileName)
            {
                FileName = fileName;
            }

            public String FileName { get; private set; }

            private IImage image;

            public void Display()
            {
                if (image == null)
                    image = new RealImage(FileName);
                image.Display();
            }
        }

        static void Main(string[] args)
        {
            IImage image = new ProxyImage("HiRes_Image");
            for (int i = 0; i < 10; i++)
                image.Display();
        }
    }
}

程序的输出为：

Loading HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image

另见

外部链接


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