FindBugs [1] is a great code analysis tool for Java. It's free, open source, and supports plugins for writing your own checks. The FindBugs site reports an interesting story from a Google test day:
"Google held a global "fixit" day using UMD's FindBugs static analysis tool for finding coding mistakes in Java software. More than 700 engineers ran FindBugs from dozens of offices.
Engineers have already submitted changes that made more than 1,100 of the 3,800 issues go away. Engineers filed more than 1,700 bug reports, of which 600 have already been marked as fixed. Work continues on addressing the issues raised by the fixit, and on supporting the integration of FindBugs into the software development process at Google."