Object Definitions Blog

The project I’m working on currently has many server side java components running on linux. One of these in particular has been experiencing severe memory leaks when running in our production environment (but not UAT, surprisingly!)

It turns out there is a tool provided with sun’s jdk 1.5 + which will allow you to get a snapshot of the heap for a running process, without you having to have started the jvm with any particular options set. This tool is called jmap

running:
jmap -histo pid
where pid is the process id on a linux box

will give you a snapshot of the heap which looks like the below:

The most useful thing here is probably the object class count. There is a chance this could help you identify an area of your app which is producing an unexpected number of instances of a given class. Other jmap options give you a dump file in different formats – so this is worth investigating too.

There is an equivalent tool jstack (jstack pid), which dumps stack information for each of the running threads. Probably useful for diagnosing deadlocks etc.

The nice thing about both of these tools is that you don’t need to have started the jvm with any specific options. These tools just connect and work – so you don’t need to have planned your memory leak in advance

They only work on unix right now, unfortunately.
There is a good article on new tools with jdk1.6 here

nb. if jstack fails you may be able to get a thread dump on unix using kill -3 {pid}
Here is another useful link for suggestions on diagnosing performance problems

And here is a good link to help diagnose java app problems in general

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