BTrace Rocks!

At work we're using a maven based build process. On the build machine it takes a while to finish the complete build (about 2 hours).

One of the final parts of the build is to generate the mvn:site. This stage takes, in my opinion, longer than it would seem necessary. Looking at #top, the machine is almost idle and there are 14 processes listed in #ps, where the last one keeps changing (I guess it finishes a directory and a new is spawned for the next one).

Due to bad design, there's no logging whatsoever in this stage and the machine seems stuck. I wanted to debug this on the build machine but didn't want to distrupt the running build (so can't start in debug mode).

To the rescue comes BTrace. A DTrace clone for java, allowing you to write small java scripts that can monitor and profile running java code. BTrace allows you to attach to an existing JVM process, without requiring any special switches to be added a priori to its command line (although it does support an -agent scenario).

That was so cool! Just downloaded one of the examples on the BTrace site, found the process pid using #jps so I can attach to it, and voila. I get printouts of all file accesses done by maven.
The BTrace examples page is full of actuall useful scripts. For example there's a script to print stacktraces for all threads (similar to #jstack).