A good tip with this view is to disable garbage collection, mark the current instant count and then run some load at your program. Sorting by change of instance count with show you which objects are being created in order to handle that load. Marking the instance count again and running garbage collection manually, will then allow you to see which of those objects are being discarded. Large performance gains can be obtained by targetting these object to reduce their creation and/or increate their reuse.
With early version of Jetty, this view was able to quickly show how many String instances where being created and discarded, all with the similar values "GET", "POST", etc. Avoid duplication of the strings associated with the HTTP protocol has greatly increased the performance of jetty.
The stacks may be view top-down (e.g. 75% of all String are allocated from calls originating in com.acme.MyThread.run()) or bottom-up (eg. 20% of String allocations where made in a call to java.io.BufferedReader.readLine().
Note that OptimizeIt can see allocations made inside libraries that you don't have to source for. It is surprising how inefficient in terms of object creation many common library methods are and Jetty has gained greatly by identifying these and replacing them with efficient versions. For example, the org.mortbay.util.StringUtils, ByteArrayISO8859Writer and StringMap classes have mostly been developed as efficient replacements of inefficient library APIs identified by OptimizeIt.
If you do have the source available, a simple double click will display the code at any level of the stack, with the allocation paths highlighted.
It is this view that allows "Object Leaks" to be traced by simply by identifying which objects are holding hard references to an instance to prevent it's garbage collection.
The view may be presented for the system, a thread group or and individual thread. Useful pop-ups give percentage summaries against the currently selected stack position.
This view allows hotspots to be quickly identified and analyzed.