Surrogate

Provide a surrogate or placeholder for another object to control access to it.

Real world example

Imagine a tower where the local wizards go to study their spells. The ivory tower can only be accessed through a proxy which ensures that only the first three wizards can enter. Here the proxy represents the functionality of the tower and adds access control to it.

In plain words

Using the proxy pattern, a class represents the functionality of another class.

Wikipedia says

A proxy, in its most general form, is a class functioning as an interface to something else. A proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked.

Programmatic Example

Taking our wizard tower example from above. Firstly we have the WizardTower interface and the IvoryTower class.

public interface WizardTower {

  void enter(Wizard wizard);
}

public class IvoryTower implements WizardTower {

  private static final Logger LOGGER = LoggerFactory.getLogger(IvoryTower.class);

  public void enter(Wizard wizard) {
    LOGGER.info("{} enters the tower.", wizard);
  }

}

Then a simple Wizard class.

public class Wizard {

  private final String name;

  public Wizard(String name) {
    this.name = name;
  }

  @Override
  public String toString() {
    return name;
  }
}

Then we have the WizardTowerProxy to add access control to WizardTower.

public class WizardTowerProxy implements WizardTower {

  private static final Logger LOGGER = LoggerFactory.getLogger(WizardTowerProxy.class);

  private static final int NUM_WIZARDS_ALLOWED = 3;

  private int numWizards;

  private final WizardTower tower;

  public WizardTowerProxy(WizardTower tower) {
    this.tower = tower;
  }

  @Override
  public void enter(Wizard wizard) {
    if (numWizards < NUM_WIZARDS_ALLOWED) {
      tower.enter(wizard);
      numWizards++;
    } else {
      LOGGER.info("{} is not allowed to enter!", wizard);
    }
  }
}

And here is the tower entering scenario.

var proxy = new WizardTowerProxy(new IvoryTower());
proxy.enter(new Wizard("Red wizard"));
proxy.enter(new Wizard("White wizard"));
proxy.enter(new Wizard("Black wizard"));
proxy.enter(new Wizard("Green wizard"));
proxy.enter(new Wizard("Brown wizard"));

Program output:

Red wizard enters the tower.
White wizard enters the tower.
Black wizard enters the tower.
Green wizard is not allowed to enter!
Brown wizard is not allowed to enter!

Proxy is applicable whenever there is a need for a more versatile or sophisticated reference to an object than a simple pointer. Here are several common situations in which the Proxy pattern is applicable.

• Remote proxy provides a local representative for an object in a different address space.
• Virtual proxy creates expensive objects on demand.
• Protection proxy controls access to the original object. Protection proxies are useful when objects should have different access rights.

• Control access to another object
• Lazy initialization
• Implement logging
• Facilitate network connection
• Count references to an object

• Controlling Access With Proxy Pattern

• java.lang.reflect.Proxy
• Apache Commons Proxy
• Mocking frameworks Mockito, Powermock, EasyMock

• Ambassador

• Design Patterns: Elements of Reusable Object-Oriented Software
• Head First Design Patterns: A Brain-Friendly Guide