Ayende @ Rahien

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Design patterns in the test of time: Adapter

In computer programming, the adapter pattern (often referred to as the wrapper pattern or simply a wrapper) is a design pattern that translates one interface for a class into a compatible interface.

More about this pattern.

This pattern is the first of the first of the Structural Patterns in the G04 book, and for the most part, it is meant to be used solely when you are integrating two or more external systems / libraries. When I saw it used inside a system, it almost always a Bad Thing, mostly because inside a single system, you want to use one of the other structural patterns.

HttpContextWrapper is a good example of using this pattern, and it links back nicely to the previous discussion on Singletons. HttpContext.Current was an issue, because it didn’t allow easy overriding / mocking / testing. The new HttpContextBase class was introduced, but due to backward compatibility concerns, HttpContext’s base class could not be changed. The solution, introduce HttpContextWrapper implementation that adapts between the two.

Useful pattern, but all too often people try to use it to abstract things, and that is most certainly not what it is meant for.

Recommendation: Use when you need to integrate with code that you can’t change (3rd party, legacy, compatibility concerns) using a common interface, but avoid otherwise.

Comments

Rafal
11/01/2012 10:30 AM by
Rafal

I don't quite get your recommendations. You say some people use wrappers to abstract things and it's not what wrappers are meant for. But at the same time you give an example (HttpContextBase) of abstracting things through a wrapper class.

Ayende Rahien
11/01/2012 10:32 AM by
Ayende Rahien

Rafal, It depends on whatever you control the code or not.

Jeff
11/01/2012 04:27 PM by
Jeff

Spot on...very useful patter if used in the correct context and not as a means of abstraction.

Mike McG
11/02/2012 08:08 AM by
Mike McG

I'm not sure that I follow your argument here. You say that it should be used only to "integrate two or more external systems/libraries" but then say that HttpContextWrapper is a good example of it. HttpContextWrapper involves only a single external library (System.Web being external to System.Web.Mvc). These two statements contradict.

I've found this pattern critically useful whenever I have a high-level interface-based design, and want to leverage third-party libraries to implement the interfaces at a later time. Those external libraries by definition do not implement my internal interfaces, requiring me to write an adapter. Thinner adapters are better, but sometimes some non-trivial shimming must be implemented to make the adaptation workable. Either way, the external dependency is fully abstracted by the interface, and the adapter is what makes this all work. It's perhaps a matter of semantics, then, that the adapter does or does not provide abstraction.

In my opinion, especially in an era of FLOSS and web service explosion, leveraging external resources without sacrificing design is critical. The adapter pattern does this and therefore is as important as ever.

Rafal
11/02/2012 08:29 AM by
Rafal

'Adapter' in the context of this post is just another name for wrapper. But imho to be called an 'adapter' the module should include a nontrivial mapping/translation logic - otherwise it's just a wrapper. And I disagree that wrappers should be used only when you don't control the code - sometimes you introduce them deliberately to reduce dependencies on external libraries by abstracting them. Example: logging library wrappers.

Ayende Rahien
11/02/2012 01:09 PM by
Ayende Rahien

Mike, There are actually two of them here, System.Web, System.Web.Abstractions and System.Web.MVC. They couldn't change System.Web, and require the extra abstraction to allow to swap things.

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