Interesting open-source APIs (Java) for self-education? [closed]

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Which is your top 3 open-source APIs (in Java) you recommend as an example of well-designed piece of art? That would be code that you had pleasure to browse through and got some insights from it. Any problem domain acceptable.

Emphasis here is on educational/study quality of code, complexity level - intermediate to top.

Thanks a lot for responses.

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I think that google collections is a great place to start. Josh Bloch advised the development of a lot of it, and it's a very well done API. While Spring is great, it's a little hard to know where to start. A good introduction to google collections is "coding in the small with google collections" (I can't post the actual link because of stackoverflow spam filter).

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1.Spring 2.Hibernate

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• Spring - it's a very well written and designed framework. It's a hell of a big bit of software but if want an example of how to build in a modular manner you can't go to fair wrong looking at the spring code base

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The sources of the java libraries are well documented.

In my experience the most valuable works include the documentation of desgin decisions, if you see a nice API it would be very interessting what could be the alternatives to that. Unfortunatly this is mostly burried mailing-lists of a project.

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Not an external library - but the java.util.concurrent package is very nicely written. The code isn't simple, but it's very well thought out and, in my opinion, has been written brilliantly.