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 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 access a remote method.

interface Image {
    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 RealImage 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
        image1.displayImage(); // loading unnecessary
        image2.displayImage(); // loading necessary
        image2.displayImage(); // loading unnecessary
        image1.displayImage(); // loading unnecessary
    }
}

The program's output is:

Loading   HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading   HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1

References

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
• PerfectJPattern Open Source Project, Provides componentized implementation of the Proxy Pattern in Java
• Adapter vs. Proxy vs. Facade Pattern Comparison
• Proxy Design Pattern
• Proxy pattern description from the Portland Pattern Repository