Proxy Design Pattern

Proxy in the Real World 

A Proxy can also be defined as a surrogate. In the real work a cheque or credit card is a proxy for what is in our bank account.  It can be used in place of cash, which is what is needed, and provides a means of accessing that cash when required. And that's exactly what the Proxy pattern does - controls and manage access to the object they are "protecting".

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
      image.display(); 
      System.out.println("");
      
      //image will not be loaded from DB/DISK, it can be loaded from Cache
      image.display();  
   }
}

Step 4

Verify the output.

Loading test_10mb.jpg
Displaying test_10mb.jpg

Displaying test_10mb.jpg