It's been a while (so busy...), but here are useful "hardcore Java" tools I've used in the past couple of months (all free-open-source):
"Soft" real-time Java library, with incredibly fast implementations of StringBuffer, HashMap and XML parsers. Not essential, but may be a good first step at optimizing Java code performance if it tends to do a lot of data processing (creating and deleting objects). And simply using their classes can increase performance relative to Java's built-in classes in java.lang and java.util.
This tool lists all dependencies between JAR files in a nice text, XML of HTML report. This is when you're not sure you were given all JAR files needed to run some package, or worse, not enough.
Java bytecode decompiler. Make sure you build it yourself from the code in Subversion, since only that version properly supports Java 1.4. Very useful. I haven't looked at Jdec yet.
Transforms Java 1.5 bytecode to something compatible with Java 1.4. Combine this with Jode above to decompile any Java 1.5 code.
Published on October 9, 2006 at 17:25 EDT
Older post: Building NetBeans projects without NetBeans
Newer post: Living with a RAZR