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Proxy
Structural
en
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 WizardTower interface and the IvoryTower class.

interface WizardTower {
  fun enter(wizard: Wizard)
}

class IvoryTower : WizardTower {
  override fun enter(wizard: Wizard) =
      logger.info("$wizard enters the tower.")
}

Then a simple Wizard class.

class Wizard(private val name: String) {
  override fun toString() = name
}

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

class WizardTowerProxy(private val tower: WizardTower) : WizardTower {
  private var numWizards = 0

  override fun enter(wizard: Wizard) {
    if (numWizards < NUM_WIZARDS_ALLOWED) {
      tower.enter(wizard)
      numWizards++
    } else {
      logger.info("$wizard is not allowed to enter!")
    }
  }

  companion object {
    private const val NUM_WIZARDS_ALLOWED = 3
  }
}

And here is the tower entering scenario.

val proxy = WizardTowerProxy(IvoryTower())
proxy.enter(Wizard("Red wizard"))
proxy.enter(Wizard("White wizard"))
proxy.enter(Wizard("Black wizard"))
proxy.enter(Wizard("Green wizard"))
proxy.enter(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!

Class diagram

alt text

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.

Typically, the proxy pattern is used to

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

Tutorials

Related patterns

Credits