代理模式

代理模式（英語：Proxy Pattern）是程式設計中的一種設計模式。

所謂的代理者是指一個類別可以作為其它東西的介面。代理者可以作任何東西的介面：網路連接、記憶體中的大物件、檔案或其它昂貴或無法複製的資源。

著名的代理模式例子為參照計數（英語：reference counting）指標物件。

當一個複雜物件的多份副本須存在時，代理模式可以結合享元模式以減少記憶體用量。典型作法是建立一個複雜物件及多個代理者，每個代理者會參照到原本的複雜物件。而作用在代理者的運算會轉送到原本物件。一旦所有的代理者都不存在時，複雜物件會被移除。

示例

Java

以下Java範例解釋"虛擬代理"模式。ProxyImage 類別用來存取遠端方法。

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
    }
}

程式的輸出為：

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();
        }
    }
}

程式的輸出為：

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