Interpretation of java hprof histogram

Question

The hprof, histogram shows the number of object by class.

is this the total number of object that have ever been allocated during the life of the jvm ?

or is this the number of object where are currently on the heap ?

or is this the number of currently reachable object on the heap ?

Answer 1

It depends. If you're generating dump with "live" (for example jmap -histo:live PID) option it's number of reachable objects. In other case it's number of objects currently allocated on heap.