The idea is to have interfaces or some sort of contract (interfaces came up because those and abstract classes are the only thing we have in Java to do this) that defines some way of retrieving front end resources (JSF pages, HTML, etc) for a specific functionality like login. That may be an oversimplification, but that's the main idea.
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Jason Porter
http://lightguard-jp.blogspot.com
http://twitter.com/lightguardjp

Software Engineer
Open Source Advocate

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