Proxy pattern

In computer programming, the proxy pattern is a software design pattern. A proxy, in its most general form, is a class functioning as an interface to something else. The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate. In short, 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. For the client, usage of a proxy object is similar to using the real object, because both implement the same interface.

Overview
The Proxy design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

===What problems can the Proxy design pattern solve? ===


 * The access to an object should be controlled.
 * Additional functionality should be provided when accessing an object.

When accessing sensitive objects, for example, it should be possible to check that clients have the needed access rights.

What solution does the Proxy design pattern describe?
Define a separate  object that
 * can be used as substitute for another object and
 * implements additional functionality to control the access to this subject.

This makes it possible to work through a  object to perform additional functionality when accessing a subject. For example, to check the access rights of clients accessing a sensitive object.

To act as substitute for a subject, a proxy must implement the  interface. Clients can't tell whether they work with a subject or its proxy. See also the UML class and sequence diagram below.

UML class and sequence diagram


In the above UML class diagram, the  class implements the   interface so that it can act as substitute for   objects. It maintains a reference to the substituted object so that it can forward requests to it.

The sequence diagram shows the run-time interactions: The  object works through a  object that controls the access to a  object. In this example, the  forwards the request to the , which performs the request.

Class diagram




Remote proxy
In distributed object communication, a local object represents a remote object (one that belongs to a different address space). The local object is a proxy for the remote object, and method invocation on the local object results in remote method invocation on the remote object. An example would be an ATM implementation, where the ATM might hold proxy objects for bank information that exists in the remote server.

Virtual proxy
In place of a complex or heavy object, a skeleton representation may be advantageous in some cases. When an underlying image is huge in size, it may be represented using a virtual proxy object, loading the real object on demand.

Protection proxy
A protection proxy might be used to control access to a resource based on access rights.