Proxy - Proxy Design Pattern Source Design Patterns...

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