I was looking at the Proxy Pattern, and to me it seems an awful lot like the Decorator, Adapter, and Bridge patterns. Am I misunderstanding something? What's the difference? Why would I use the Proxy pattern versus the others? How have you used them in the past in real world projects?
13 Answers
Proxy, Decorator, Adapter, and Bridge are all variations on "wrapping" a class. But their uses are different.
Proxy could be used when you want to lazy-instantiate an object, or hide the fact that you're calling a remote service, or control access to the object.
Decorator is also called "Smart Proxy." This is used when you want to add functionality to an object, but not by extending that object's type. This allows you to do so at runtime.
Adapter is used when you have an abstract interface, and you want to map that interface to another object which has similar functional role, but a different interface.
Bridge is very similar to Adapter, but we call it Bridge when you define both the abstract interface and the underlying implementation. I.e. you're not adapting to some legacy or third-party code, you're the designer of all the code but you need to be able to swap out different implementations.
Facade is a higher-level (read: simpler) interface to a subsystem of one or more classes. Suppose you have a complex concept that requires multiple objects to represent. Making changes to that set of objects is confusing, because you don't always know which object has the method you need to call. That's the time to write a Facade that provides high-level methods for all the complex operations you can do to the collection of objects. Example: a Domain Model for a school section, with methods like
countStudents(),reportAttendance(),assignSubstituteTeacher(), and so on.
There's a great deal of overlap in many of the GoF patterns. They're all built on the power of polymorphism and sometimes only really differ in intent. (strategy vs. state)
My understanding of patterns increased 100 fold after reading Head First Design Patterns.
I highly recommend it!
They are quite similar, and the lines between them are quite gray. I suggest you read the Proxy Pattern and Decorator Pattern entries in the c2 wiki.
The entries and discussions there are quite extensive, and they also link to other relevant articles. By the way, the c2 wiki is excellent when wondering about the nuances between different patterns.
To sum the c2 entries up, I would say a decorator adds/changes behavior, but a proxy has more to do with access control (lazy instantiation, remote access, security etc). But like I said, the lines between them are gray, and I see references to proxies that could easily be viewed as decorators and vice versa.
I use it quite often when consuming web services. The Proxy Pattern should probably be renamed to something more pragmatic, like 'Wrapper Pattern". I also have a library that is a Proxy to MS Excel. It makes it very easy to automate Excel, without having to worry about background details such as what version is installed (if any).
Speaking detail implementation, I find a difference between Proxy and Decorator, Adapter, Facade ... In common implementation of these patterns there's a target object wrapped by a enclosing object. Client uses enclosing object instead of target object. And the target object actually play an important part inside some of methods of enclosing object.
However, in case of Proxy, enclosing object can play some methods by itself, it just initialize target object when client calls some methods that it needs target object take part in. This is lazy initialization. In case of other patterns, enclosing object is virtually based on target object. So target object is always initialized along with enclosing object in constructors/setters.
Another thing, a proxy does exactly what a target does whereas other patterns add more functionality to target.
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