Proxy pattern

Proxy in UML

Proxy in LePUS3 (legend)

In computer programming, the proxy pattern is a software design pattern.

A proxy, in its most general form, is a class functioning as an interface to something else. The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate.

A well-known example of the proxy pattern is a reference counting pointer object.

In situations where multiple copies of a complex object must exist the proxy pattern can be adapted to incorporate the flyweight pattern in order to reduce the application's memory footprint. Typically 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.

Example

The following Java example illustrates the "virtual proxy" pattern. The ProxyImage class is used to delay the expensive operation of loading a file from disk until the result of that operation is actually needed. If the file is never needed, then the expensive load has been totally eliminated.

import java.util.*;
 
interface Image {
    public void displayImage();
}
 
class RealImage implements Image {
    private String filename;
    public RealImage(String filename) { 
        this.filename = filename;
        loadImageFromDisk();
    }

    private void loadImageFromDisk() {
        System.out.println("Loading   "+filename);
        // Potentially expensive operation
        // ...
    }

    public void displayImage() { 
        System.out.println("Displaying "+filename); 
    }
}
 
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); // load only on demand
        }
        image.displayImage();
    }
}
 
class ProxyExample {
    public static void main(String[] args) {
        Image image1 = new ProxyImage("HiRes_10MB_Photo1");
        Image image2 = new ProxyImage("HiRes_10MB_Photo2");     
        Image image3 = new ProxyImage("HiRes_10MB_Photo3");

        image1.displayImage(); // loading necessary
        image2.displayImage(); // loading necessary
        image1.displayImage(); // no loading necessary; already done
        // the third image will never be loaded - time saved!
    }
}

The program's output is:

Loading    HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading    HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1

See also

• Composite pattern
• Decorator pattern
• Lazy initialization

External links

• Proxy pattern in Java
• Proxy pattern in UML and in LePUS3 (a formal modelling language)
• Take control with the Proxy design pattern by David Geary, JavaWorld.com
• Jt J2EE Pattern Oriented Framework
• PerfectJPattern Open Source Project, Provides componentized implementation of the Proxy Pattern in Java