Excellent java code read for learning purposes?

A common suggestion is to read the code of good programmers I am looking for Java code advice for learning I finished the code in Tomcat and found it very helpful I am now looking for other particularly good written / commented suggestions for open source code I'm interested in mainstream Java se applications (not looking for swing / JavaFX / UI code) I have about three years of working experience in Java, so I'm looking for additional Polish instead of language foundation

Solution

Google guava code has been improved, mainly bulletproof:

You can start with the base class: http://code.google.com/p/guava-libraries/source/browse/#git%2Fguava%2Fsrc%2Fcom%2Fgoogle%2Fcommon%2Fbase