How to find a Java Memory Leak

Question

How do you find a memory leak in Java (using, for example, JHat)? I have tried to load the heap dump up in JHat to take a basic look. However, I do not understand how I am s

Answer 1

I use following approach to finding memory leaks in Java. I've used jProfiler with great success, but I believe that any specialized tool with graphing capabilities (diffs are easier to analyze in graphical form) will work.

1. Start the application and wait until it get to "stable" state, when all the initialization is complete and the application is idle.
2. Run the operation suspected of producing a memory leak several times to allow any cache, DB-related initialization to take place.
3. Run GC and take memory snapshot.
4. Run the operation again. Depending on the complexity of operation and sizes of data that is processed operation may need to be run several to many times.
5. Run GC and take memory snapshot.
6. Run a diff for 2 snapshots and analyze it.

Basically analysis should start from greatest positive diff by, say, object types and find what causes those extra objects to stick in memory.

For web applications that process requests in several threads analysis gets more complicated, but nevertheless general approach still applies.

I did quite a number of projects specifically aimed at reducing memory footprint of the applications and this general approach with some application specific tweaks and trick always worked well.