Proxy Pattern

When use the Proxy Pattern

● A remote proxy provides a local representative for an object in
a different address space.
● A virtual proxy creates expensive objects on demand.
● A protection proxy controls access to the original object.
Protection proxies are useful when objects should have different
access rights.

Example - Virtual Proxy Example.

Consider an image viewer program that lists and displays high resolution photos. The program has to show a list of all photos however it does not need to display the actual photo until the user selects an image item from a list.

Step 1

Create an interface.

Image.java

public interface Image {
   void display();
}

Step 2

Create concrete classes implementing the same interface.

RealImage.java

public class RealImage implements Image {

   private String fileName;

   public RealImage(String fileName){
      this.fileName = fileName;
      loadFromDisk(fileName);
   }

   @Override
   public void display() {
      System.out.println("Displaying " + fileName);
   }

   private void loadFromDisk(String fileName){
      System.out.println("Loading " + fileName);
   }
}

ProxyImage.java

public class ProxyImage implements Image{

   private RealImage realImage;
   private String fileName;

   public ProxyImage(String fileName){
      this.fileName = fileName;
   }

   @Override
   public void display() {
      if(realImage == null){
         realImage = new RealImage(fileName);
      }
      realImage.display();
   }
}

Step 3

Use the ProxyImage to get object of RealImage class when required.

ProxyPatternDemo.java

public class ProxyPatternDemo {
 
   public static void main(String[] args) {
      Image image = new ProxyImage("test_10mb.jpg");

      //image will be loaded from disk
      //note that in this case all images have been loaded into memory
      //and not all have been actually displayed
      //this is a waste of memory resources

      image.display(); 
      System.out.println("");
      
      //image will not be loaded from disk
      image.display();  
   }
}

Step 4

Verify the output.

Loading test_10mb.jpg
Displaying test_10mb.jpg

Displaying test_10mb.jpg