Computer Science 209

The Proxy Pattern

Delayed Instantiation if (obj == null) obj = <get the object from whatever source>

<use obj>

It’s expensive to load an image

If the user never looks at an image, don’t load it

Load the image when the user needs to look at it

Especially effective when there are lots of images

Use a Proxy Object

// Load the image at instantiation, which can be expensive

JLabel label = new JLabel(new ImageIcon(imageName));

// Or delay loading until painting, which is cheaper

JLabel label = new JLabel(new ImageProxy(imageName));

ImageProxy implements the Icon interface just like ImageIcon, but delays loading until paintIcon is called

Implementing the Proxy Class public class ImageProxy implements Icon{

private String name; private ImageIcon image;

public ImageProxy(String n){ name = n; }

public void paintIcon(Component c, Graphics g, int x, int y){ if (image == null) image = new ImageIcon(name); image.paintIcon(c, g, x, y); }

// Other Icon methods}

The Context of the Proxy Pattern

• A class (the real subject) provides a service specified by an interface type (the subject)

• The service must be modified to make it more versatile

• Neither the client nor the subject should be affected by the modification

Solution of the Proxy Pattern

• Define a proxy class that implements the subject interface

• The proxy holds a reference to the real subject or knows how to locate it

• The client uses a proxy object

• Each proxy method invokes the same method on the real subject and provides the necessary modifications

The Proxy Setup

<<Interface>>Subject

Proxy

Client

request()

request()

RealSubject

request()

Another Example: A Sorted List

• A sorted list includes all of the methods in the List interface

• Some methods, such as add(i) and set(i), are not supported

• Other methods, such as add(obj) and contains(obj) are modified to enforce the natural ordering of elements or to improve the efficiency of searches

• The sorted list holds a reference to a list and calls the corresponding methods for the most part