Proxy Pattern

Definition-Provide a surrogate or placeholder for another object to control access to it.

• Proxy   
• • 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 be substituted for for the real subject.
  • controls access to the real subject and may be responsible for creating and deleting it.
  • other responsibilites 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 images's extent.
    • protection proxies check that the caller has the access permissions required to perform a request.
• Subject
• • defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
• RealSubject
• • defines the real object that the proxy represents.

Types of Proxies

1.  Remote Proxy - Provides a reference to an object located in a different

address space on the same or different machine

2.  Virtual Proxy - Allows the creation of a memory intensive object on

demand. The object will not be created until it is really needed.

3.  Copy-On-Write Proxy - Defers copying (cloning) a target object until

required by client actions. Really a form of virtual proxy.

4.  Protection (Access) Proxy - Provides different clients with different levels

of access to a target object

5.  Cache Proxy - Provides temporary storage of the results of expensive target

operations so that multiple clients can share the results

6.  Firewall Proxy - Protects targets from bad clients (or vice versa)

7.  Synchronization Proxy - Provides multiple accesses to a target object

8.  Smart Reference Proxy - Provides additional actions whenever a target

object is referenced such as counting the number of references to the object

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