Proxy - Proxy Design Pattern Source Design Patterns –...

Unformatted text preview: Proxy Design Pattern Source: Design Patterns – Elements of Reusable Object- Oriented Software; Gamma, et. al. Problem • You need to control access to an object Solution • Create a Proxy object that implements the same interface as the real object • The Proxy object (usually) contains a reference to the real object • Clients are given a reference to the Proxy, not the real object • All client operations on the object pass through the Proxy, allowing the Proxy to perform additional processing Client Proxy Real Object Solution +Request() «interface» Subject Client +Request() +realSubject Proxy +Request() Real Subject // do pre-processing here realSubject->Request(); // do post-processing here Consequences • Provides an additional level of indirection between client and object that may be used to insert arbitrary services • Proxies are invisible to the client, so introducing proxies does not affect client code Known Uses: Java Collections • Read-only Collections – Wrap collection object in a proxy that only allows read-only operations to be invoked on the collection – All other operations throw exceptions – List Collections.unmodifiableList(List list); • Returns read-only List proxy • Synchronized Collections – Wrap collection object in a proxy that ensures only one thread at a time is allowed to access the collection – Proxy acquires lock before calling a method, and releases lock after the method completes – List Collections.synchronizedList(List list); • Returns a synchronized List proxy Known Uses: Distributed Objects...
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