java - How can I figure out what is holding on to unfreed objects?

Question

One of our programs is sometimes getting an OutOfMemory error on one user's machine, but of course not when I'm testing it. I just ran it with JProfiler (on a 10 day evaluation license because I've never used it before), and filtering on our code prefix, the biggest chunk both in total size and number of instances is 8000+ instances of a particular simple class.

I clicked the "Garbage Collect" button on JProfiler, and most instances of other classes of ours went away, but not these particular ones. I ran the test again, still in the same instance, and it created 4000+ more instances of the class, but when I clicked "Garbage Collect", those went away leaving the 8000+ original ones.

These instances do get stuck into various Collections at various stages. I assume that the fact that they're not garbage collected must mean that something is holding onto a reference to one of the collections so that's holding onto a reference to the objects.

Any suggestions how I can figure out what is holding onto the reference? I'm looking for suggestions of what to look for in the code, as well as ways to find this out in JProfiler if there are.

Answer 1

Dump the heap and inspect it.

I'm sure there's more than one way to do this, but here is a simple one. This description is for MS Windows, but similar steps can be taken on other operating systems.

1. Install the JDK if you don't already have it. It comes with a bunch of neat tools.
2. Start the application.
3. Open task manager and find the process id (PID) for java.exe (or whatever executable you are using). If the PID's aren't shown by default, use View > Select Columns... to add them.
4. Dump the heap using jmap.
5. Start the jhat server on the file you generated and open your browser to http://localhost:7000 (the default port is 7000). Now you can browse the type you're interested in and information like the number of instances, what has references to them, etcetera.

Here is an example:

C:dump>jmap -dump:format=b,file=heap.bin 3552

C:dump>jhat heap.bin
Reading from heap.bin...
Dump file created Tue Sep 30 19:46:23 BST 2008
Snapshot read, resolving...
Resolving 35484 objects...
Chasing references, expect 7 dots.......
Eliminating duplicate references.......
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.

To interpret this, it is useful to understand some of the array type nomenclature Java uses - like knowing that class [Ljava.lang.Object; really means an object of type Object[].