Clojure and JMX

There are some folks out there that are using Clojure and only know what they have to about Java. This post is for you.

Last night I was working on a little fun Clojure project that involved reading very large files. While reading and processing a 700MB file, my application crashed complaining that it had run out of memory. I had given it 500MB of RAM. What else does it need? Have you ever been in this situation where you are having Java memory issues and need to know exactly what’s going on? When this happens, your best friend is JMX and jconsole.

jconsole comes with the JDK so you already have it. To use these two together, you first need to start Clojure with the Java option -Dcom.sun.management.jmxremote. Once the app is running you can run jconsole which can be found in the bin directory of the JDK. This will open a window which will allow you to select the Java process to monitor. Once you select a process, you will see something like this:



This allows you to see the internals of the JVM as it is running. Armed with this information you should be able to figure out what is going on. After that, I'm sure you will go back to being mad at Java.

This is easy but I needed to make it a little easier. I updated my clj script to add the option above to Java when I pass it the -Xjmx option.

You can do a lot more with JMX and Clojure. If you wish to get into it in more detail, check out the jmx library in clojure.contrib by Stuart Halloway.