Applied Design Patterns with Java

Structural :: Proxy (207) {C ch 15}

Applicability

The 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 situations in which the Proxy pattern is applicable:

1. A remote Proxy provides a local representative for an object in a different address space;
2. A virtual Proxy creates expensive objects on demand. The ImageProxy described in the Motivation is an example of such a proxy;
3. A protection Proxy controls access to the original object. Protection proxies are useful when objects should have different access rights;
4. A smart reference is a replacement for a bare pointer that performs additional actions when an object is accessed. Typical uses include: counting the number of references to the real object so that it can be freed automatically when there are no more references (also called smart pointers); loading a persistent object into memory when it's first referenced; and checking that the real object is locked before it's accessed to ensure that no other object can change it.

Structure

The Proxy pattern has this general organization:

Here's a possible object diagram of a Proxy structure at run-time:


Participants

• Proxy (ImageProxy) maintains a reference that lets the Proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same; provides an interface identical to Subject's so that a Proxy can by substituted for the real subject; controls access to the real subject and may be responsible for creating and deleting it. Other responsibilities depend on the kind of Proxy:
  • remote proxies are responsible for encoding a request and its arguments and for sending the encoded request to the real subject in a different address space;
  • virtual proxies may cache additional information about the real subject so that they can postpone accessing it. For example, the ImageProxy from the Motivation caches the real image's extent;
  • protection proxies check that the caller has the access permissions required to perform a request.
• Subject (Graphic) defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
• RealSubject (Image) defines the real object that the Proxy represents.

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