Proxy Pattern

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Proxy Pattern

Intent

• The intent of this pattern is to provide a �Placeholder� for an object to control references to it.

Implementation

The figure below shows a UML class diagram for the Proxy Pattern:

The participants classes in the proxy pattern are:

• Subject - Interface implemented by the RealSubject and representing its services. The interface must be implemented by the proxy as well so that the proxy can be used in any location where the RealSubject can be used.
• Proxy
• Maintains a reference that allows the Proxy to access the RealSubject.
• Implements the same interface implemented by the RealSubject so that the Proxy can be substituted for the RealSubject.
• Controls access to the RealSubject and may be responsible for its creation and deletion.
• Other responsibilities depend on the kind of proxy.
• RealSubject - the real object that the proxy represents

Description

A client obtains a reference to a Proxy, the client then handles the proxy in the same way it handles RealSubject and thus invoking the method doSomething(). At that point the proxy can do different things prior to invoking RealSubject�s doSomething() method. The client might create a RealSubject object at that point, perform initialization, check permissions of the client to invoke the method, and then invoke the method on the object. The client can also do additional tasks after invoking the doSomething() method, such as incrementing the number of references to the object.

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