Question

I’ve been experimenting with jmap -histo and jmap -dump today

When run in this sequence

jmap -dump:format=b,file=heap.1 [pid]
jmap -dump:live,format=b,file=heap.2 [pid]
jmap -dump:format=b,file=heap.3 [pid]

heap.3 resembles heap.2 more than heap.1. In particular, the “dead” objects that I’m interested in in heap.1 are absent from heap.3.

Seeing this, I started looking for documentation that would tell me what I should expect. The closest I managed to get was this discussion, where the comments from briand and alanb imply that in practice I can expect this GC to occur when I use the live option; but the answers are five years old, and posts to a forum seem a bit informal for a specification.

Where can I find the current behavior documented?

Answer 1

In order to determine liveness, Java has to run full GC, so yes, it does.

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To put the question to sleep… here is the answer, if anyone needs to dig deeper. Feel free.

part of /hotspot/agent/src/share/vm/services/attachListener.cpp taken from

openjdk http://download.java.net/openjdk/jdk7/
and you must accept http://www.gnu.org/licenses/gpl-2.0.html

// Implementation of "inspectheap" command
//
// Input arguments :-
//   arg0: "-live" or "-all"
static jint heap_inspection(AttachOperation* op, outputStream* out) {
  bool live_objects_only = true;   // default is true to retain the behavior before this change is made
  const char* arg0 = op->arg(0);
  if (arg0 != NULL && (strlen(arg0) > 0)) {
    if (strcmp(arg0, "-all") != 0 && strcmp(arg0, "-live") != 0) {
      out->print_cr("Invalid argument to inspectheap operation: %s", arg0);
      return JNI_ERR;
    }
    live_objects_only = strcmp(arg0, "-live") == 0;
  }
  VM_GC_HeapInspection heapop(out, live_objects_only /* request full gc */, true /* need_prologue */);
  VMThread::execute(&heapop);
  return JNI_OK;
}

in vmGCOperations.hpp it’s the definition

`VM_GC_HeapInspection(outputStream* out, bool request_full_gc,
                   bool need_prologue) :`