Important notions are

• the current snapshot
  The heap walker operates on a static snapshot of the heap which is taken by
  • clicking on the corresponding toolbar button
  • using the "Take heap snapshot for selection" action in the memory views.

  If a snapshot has already been taken, it will be discarded after confirmation. If the current session is detached, it is not possible to take a new snapshot, Taking a snapshot may take from a few seconds to a few minutes depending on the heap size of the profiled application.

  A bookmark will be added when a heap snapshot is taken manually.

  Note that you can also use a trigger and the "Trigger heap dump" action to take a snapshot. This is especially useful for offline profiling.

• the initial object set
  After a snapshot has been fully prepared, you are taken to the the classes view and all objects in the snapshot are displayed. You can return to this view at any later point by
  • choosing View->Heap Walker Start View from JProfiler's main menu
  • clicking on the the corresponding toolbar button
• the current object set
  After each selection step a new object set is created which then becomes the current object set. Starting with the initial object set, you add selection steps and change the current object set to drill down toward your objective. The contents of the current object set (any number of instances of classes and arrays) are described in the title area of the heap walker.

  You can calculate the retained size and the deep size of the entire object set by clicking on the "Calculate retained and deep sizes" hyperlink in the title area. Once the calculation is finished, the hyperlink is replaced with the results. This action is not available for the initial set of all objects where the shallow size is equivalent to the retained and deep sizes.

  You can create a new object set with the objects that are retained by the current object set by clicking on the "Use retained objects" hyperlink in the title area. Retained objects are all objects that would be garbage collected if the entire current object set would be removed from memory. This always includes the current object set itself. This action is not available for the initial set of all objects where the set of retained objects and the current object set are the same.

  The history of your selection steps can be shown at the bottom by clicking on the the corresponding toolbar button

• the new objects
  

  The heap walker knows which objects have been allocated since the last heap dump or the last invocation of the  mark heap action.

  If there was a previous heap dump or a mark heap dump invocation, the title area of the heap walker shows the new instance count and two links titled "Use new" and "Use old" that allow you to select either the instances that have been allocated since that point in time, or the surviving instances that were allocated before. This information is available for each object set, so you can drill down first and select new or old instances later on.

• the view on the current object set
  All views share the same basic layout. There are 5 top-level views which show information on the current object set:
  • the classes view
  • the allocation view
  • the biggest objects view
  • the reference view
  • the time view

  In addition there are two more views:

  • the inspections view which shows inspection that operate on the current object set
  • the graph which does not show data from the current object set. You can add objects from the reference and biggest objects views that are not cleared when you add selection steps.

  The view is chosen either

  • using the view selector at the bottom of the heap walker.
  • or from the view helper dialog that is displayed each time a new object set is created. You can suppress this dialog in the heap walker view settings.

• the three types of size measurements
  

  The title area of the heap walker displays several sizes for single objects or object sets. All sizes include only the object data, they do not include internal JVM structures for classes, nor do they include class data or local variables.

  • shallow size
    The shallow size does not include the size of referenced arrays and instances but only the size of the corresponding pointers. Shallow sizes are trivially available for all objects and object sets and are displayed in all views.
  • retained size
    The retained size is calculated as the shallow size plus total size of all objects that would be garbage collected if the current object or object set were removed. This size tells you how much memory is really behind an object or object set. Retained size calculation is done for all objects when the heap dump is processed. Retained sizes are shown for single instances in several views.
  • deep size
    The deep size is calculated as the shallow size plus total size of all referenced objects. In extreme cases, this value may be a significant percentage of the entire heap. Deep size calculation is only available for the current object set.