JVisualVM makes a second appearance in this chapter. In the first section we ruled JVisualVM out from the list of tools monitoring your JVM for GC behavior, but in this section we will demonstrate its strengths in allocation profiling.

Attaching JVisualVM to your JVM is done via GUI where you can attach the profiler to a running JVM. After attaching the profiler:

1. Open the “Profiler” tab and make sure that under the “Settings” checkbox you have enabled “Record allocations stack traces”.
2. Click the “Memory” button to start memory profiling.
3. Let your application run for some time to gather enough information about object allocations.
4. Click the “Snapshot” button. This will give you a snapshot of the profiling information gathered.

After completing the steps above, you are exposed to information similar to the following:

From the above, we can see the allocations ranked by the number of objects created per class. In the very first line we can see that the vast majority of all objects allocated were int[] arrays. By right-clicking the row you can access all stack traces where those objects were allocated:

Compared to hprof, JVisualVM makes the information a bit easier to process – for example in the screenshot above you can see that all allocation traces for the int[] are exposed in single view, so you can escape the potentially repetitive process of matching different allocation traces.