I recently wrote an article about jmap some time ago, but since I had a real-world incident recently, I want to present jmap again, since it’s so incredibly helpful:

Recently, we encountered a strange behaviour with a Java service, which was running for several weeks without interruption. Every twenty minutes, it showed really weird behaviour, compareable with a short-time amnesia.

Logfiles, network stats, manual tests, everything was OK besides from the actual amnesia effects. No trace of a real problem.

That’s the time, when I brought jmap in the game. It’s a really, really useful tool to inspect your running java process, and there most importantly it’s heap. A simple command

jmap -histo 1234

for the java process 1234 (find it out with ps -ef | grep java) showed me, that we had three million objects with “connection” in its name on the heap. Since I knew, that we don’t even need thirty thousand connections at the same, it was a clear indication, that we have a kind of leak here.

Since it was a third-party application, we decided, that instead of time-consuming debugging, we just do a restart of the service and voilà, all problems were solved.

So, if you encounter weird behaviour, it’s not the worst idea, to take a look onto the heap stack of your process to check, if you have a memory and/or an object leak somewhere.