Findbugs is an statical code analysis tool developed by University of Maryland. Check it out here, http://findbugs.sourceforge.net/
It has both ant and command line options to analyze the codes. However, most of commands won't work in windows. Windows users needs to get the Cygwin to get the commands working.
It offers the following,
1. You can write your own custom filters.
2. Filtering out results w.r.t class,jar,version etc
3. Suppressing the bugs which are false positives.
It is an alternative to the commercial tools which costs a bomb. Do check it out.