jmap

源码位于 JMap.java at jdk8-b120，可以看到对参数的解析以及对实际命令的调用。如果是堆转储会调用至 attachListener.cpp at jdk8-b120:

// Implementation of "dumpheap" command.
// See also: HeapDumpDCmd class
//
// Input arguments :-
//   arg0: Name of the dump file
//   arg1: "-live" or "-all"
jint dump_heap(AttachOperation* op, outputStream* out) {
  const char* path = op->arg(0);
  if (path == NULL || path[0] == '\0') {
    out->print_cr("No dump file specified");
  } else {
    bool live_objects_only = true;   // default is true to retain the behavior before this change is made
    const char* arg1 = op->arg(1);
    if (arg1 != NULL && (strlen(arg1) > 0)) {
      if (strcmp(arg1, "-all") != 0 && strcmp(arg1, "-live") != 0) {
        out->print_cr("Invalid argument to dumpheap operation: %s", arg1);
        return JNI_ERR;
      }
      live_objects_only = strcmp(arg1, "-live") == 0;
    }

    // Request a full GC before heap dump if live_objects_only = true
    // This helps reduces the amount of unreachable objects in the dump
    // and makes it easier to browse.
    HeapDumper dumper(live_objects_only /* request GC */);
    int res = dumper.dump(op->arg(0));
    if (res == 0) {
      out->print_cr("Heap dump file created");
    } else {
      // heap dump failed
      ResourceMark rm;
      char* error = dumper.error_as_C_string();
      if (error == NULL) {
        out->print_cr("Dump failed - reason unknown");
      } else {
        out->print_cr("%s", error);
      }
    }
  }
  return JNI_OK;
}

可以看出如果指定了 live 选项，则会在堆转储前执行一次 Full GC 以降低堆转储的大小，便于分析。在 jmap 中也有提到：

The live suboption is optional, but when specified, only the active objects in the heap are dumped.

值得注意的是在堆转储文件中的 int[] 部分来源于 GC、TLAB 等填充的内存空间，如 collectedHeap.cpp at jdk8-b120 中的 fill_with_object 方法：

void CollectedHeap::fill_with_object(HeapWord* start, size_t words, bool zap)
{
  DEBUG_ONLY(fill_args_check(start, words);)
  HandleMark hm;  // Free handles before leaving.
  fill_with_object_impl(start, words, zap);
}

void
CollectedHeap::fill_with_object_impl(HeapWord* start, size_t words, bool zap)
{
  assert(words <= filler_array_max_size(), "too big for a single object");

  if (words >= filler_array_min_size()) {
    fill_with_array(start, words, zap);
  } else if (words > 0) {
    assert(words == min_fill_size(), "unaligned size");
    post_allocation_setup_common(SystemDictionary::Object_klass(), start);
  }
}

void
CollectedHeap::fill_with_array(HeapWord* start, size_t words, bool zap)
{
  assert(words >= filler_array_min_size(), "too small for an array");
  assert(words <= filler_array_max_size(), "too big for a single object");

  const size_t payload_size = words - filler_array_hdr_size();
  const size_t len = payload_size * HeapWordSize / sizeof(jint);
  assert((int)len >= 0, err_msg("size too large " SIZE_FORMAT " becomes %d", words, (int)len));

  // Set the length first for concurrent GC.
  ((arrayOop)start)->set_length((int)len);
  post_allocation_setup_common(Universe::intArrayKlassObj(), start);
  DEBUG_ONLY(zap_filler_array(start, words, zap);)
}

fill_with_object 方法被不少 GC 收集器调用，也被 ThreadLocalAllocBuffer 调用，如 threadLocalAllocBuffer.cpp at jdk8-b120:

// Fills the current tlab with a dummy filler array to create
// an illusion of a contiguous Eden and optionally retires the tlab.
// Waste accounting should be done in caller as appropriate; see,
// for example, clear_before_allocation().
void ThreadLocalAllocBuffer::make_parsable(bool retire) {
  if (end() != NULL) {
    invariants();

    if (retire) {
      myThread()->incr_allocated_bytes(used_bytes());
    }

    CollectedHeap::fill_with_object(top(), hard_end(), retire);

    if (retire || ZeroTLAB) {  // "Reset" the TLAB
      set_start(NULL);
      set_top(NULL);
      set_pf_top(NULL);
      set_end(NULL);
    }
  }
  assert(!(retire || ZeroTLAB)  ||
         (start() == NULL && end() == NULL && top() == NULL),
         "TLAB must be reset");
}

Reference

jdk/HeapHprofBinWriter.java at jdk8-b120 · openjdk/jdk · GitHub
Can jmap -histo trigger full garbage collection? - Stack Overflow