java-design-patterns/proxy/README.md

---
layout: pattern
title: Proxy
folder: proxy
permalink: /patterns/proxy/
categories: Structural
tags:
 - Gang Of Four
 - Decoupling
---

## Also known as
Surrogate

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

## Explanation
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 wizard tower interface and the ivory tower class

```java
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

```java
public class Wizard {

  private final String name;

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

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

Then we have the proxy to add access control to wizard tower

```java
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 tower entering scenario

```java
var proxy = new WizardTowerProxy(new IvoryTower());
proxy.enter(new Wizard("Red wizard")); // Red wizard enters the tower.
proxy.enter(new Wizard("White wizard")); // White wizard enters the tower.
proxy.enter(new Wizard("Black wizard")); // Black wizard enters the tower.
proxy.enter(new Wizard("Green wizard")); // Green wizard is not allowed to enter!
proxy.enter(new Wizard("Brown wizard")); // Brown wizard is not allowed to enter!
```

## Class diagram
![alt text](./etc/proxy.urm.png "Proxy pattern class diagram")

## Applicability
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.

## Typical Use Case

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

## Tutorials

* [Controlling Access With Proxy Pattern](http://java-design-patterns.com/blog/controlling-access-with-proxy-pattern/)

## Real world examples

* [java.lang.reflect.Proxy](http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Proxy.html)
* [Apache Commons Proxy](https://commons.apache.org/proper/commons-proxy/)
* Mocking frameworks Mockito, Powermock, EasyMock

## Credits

* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)