Ask HN: Recommended resources for learning Java?

Next month I'll be switching to a job that involves working mainly on a Java backend with Google App Engine. I'm guessing I'll be working on a REST API for mobile apps. Trouble is most of my experience is in frontend development with a bit of Rails.

What are some recommended articles/books to get up to speed with modern Java development? What are the most common tools/packages/build systems used in this area? Would it help to contribute to any particular open source projects to gain experience?