Object counts in dynamic memory views and the heap walker

Wednesday, April 22, 2009 | Posted by

Often the question comes up why there are larger object counts in the dynamic memory views than in the heap walker. The simple explanation is that the dynamic memory views show all objects on the heap - even those that are unreferenced, while the heap walker only shows objects that are strongly referenced.

Here, for example, in the "All objects" view, a particular class has an object count of 6741:

In the heap walker, the object count is only 6282:

The difference comes from objects that are not referenced anymore, but that are still on the heap because the garbage collector has not collected them yet. Clicking on the "Run GC" button in JProfiler might collect some, but not all of them, since the garbage collector does not do full collections in modern JVMs. However, when you take a heap snapshot, a full collection is done internally, so you only look at objects that you can actually do something about.

Ideally, we would exclude unreferenced objects from the dynamic memory views too, but this information requires an expensive calculation that can only be performed when taking a heap snapshot.

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