diff -r 2c3cc4b01880 -r c16ac7a2eba4 src/hotspot/share/jfr/recorder/checkpoint/types/jfrType.cpp
--- a/src/hotspot/share/jfr/recorder/checkpoint/types/jfrType.cpp	Wed Oct 30 16:14:56 2019 +0100
+++ b/src/hotspot/share/jfr/recorder/checkpoint/types/jfrType.cpp	Wed Oct 30 19:43:52 2019 +0100
@@ -31,13 +31,14 @@
 #include "gc/shared/gcTrace.hpp"
 #include "gc/shared/gcWhen.hpp"
 #include "jfr/leakprofiler/leakProfiler.hpp"
-#include "jfr/recorder/checkpoint/jfrCheckpointManager.hpp"
+#include "jfr/recorder/checkpoint/jfrCheckpointWriter.hpp"
 #include "jfr/recorder/checkpoint/types/jfrType.hpp"
 #include "jfr/recorder/jfrRecorder.hpp"
 #include "jfr/recorder/checkpoint/types/jfrThreadGroup.hpp"
 #include "jfr/recorder/checkpoint/types/jfrThreadState.hpp"
 #include "jfr/support/jfrThreadLocal.hpp"
 #include "jfr/writers/jfrJavaEventWriter.hpp"
+#include "jfr/utilities/jfrThreadIterator.hpp"
 #include "memory/metaspaceGCThresholdUpdater.hpp"
 #include "memory/referenceType.hpp"
 #include "memory/universe.hpp"
@@ -84,27 +85,18 @@
   void do_thread(Thread* t);
 };
 
-// Requires a ResourceMark for get_thread_name/as_utf8
 void JfrCheckpointThreadClosure::do_thread(Thread* t) {
   assert(t != NULL, "invariant");
-  assert_locked_or_safepoint(Threads_lock);
-  const JfrThreadLocal* const tl = t->jfr_thread_local();
-  assert(tl != NULL, "invariant");
-  if (tl->is_dead()) {
-    return;
-  }
   ++_count;
-  _writer.write_key(tl->thread_id());
-  _writer.write(t->name());
-  const OSThread* const os_thread = t->osthread();
-  _writer.write<traceid>(os_thread != NULL ? os_thread->thread_id() : 0);
+  _writer.write_key(JfrThreadId::jfr_id(t));
+  const char* const name = JfrThreadName::name(t);
+  assert(name != NULL, "invariant");
+  _writer.write(name);
+  _writer.write<traceid>(JfrThreadId::os_id(t));
   if (t->is_Java_thread()) {
-    JavaThread* const jt = (JavaThread*)t;
-    _writer.write(jt->name());
-    _writer.write(java_lang_Thread::thread_id(jt->threadObj()));
-    _writer.write(JfrThreadGroup::thread_group_id(jt, _curthread));
-    // since we are iterating threads during a safepoint, also issue notification
-    JfrJavaEventWriter::notify(jt);
+    _writer.write(name);
+    _writer.write(JfrThreadId::id(t));
+    _writer.write(JfrThreadGroup::thread_group_id((JavaThread*)t, _curthread));
     return;
   }
   _writer.write((const char*)NULL); // java name
@@ -113,13 +105,18 @@
 }
 
 void JfrThreadConstantSet::serialize(JfrCheckpointWriter& writer) {
-  assert(SafepointSynchronize::is_at_safepoint(), "invariant");
   JfrCheckpointThreadClosure tc(writer);
-  Threads::threads_do(&tc);
+  JfrJavaThreadIterator javathreads;
+  while (javathreads.has_next()) {
+    tc.do_thread(javathreads.next());
+  }
+  JfrNonJavaThreadIterator nonjavathreads;
+  while (nonjavathreads.has_next()) {
+    tc.do_thread(nonjavathreads.next());
+  }
 }
 
 void JfrThreadGroupConstant::serialize(JfrCheckpointWriter& writer) {
-  assert(SafepointSynchronize::is_at_safepoint(), "invariant");
   JfrThreadGroup::serialize(writer);
 }
 
@@ -278,19 +275,21 @@
 void JfrThreadConstant::serialize(JfrCheckpointWriter& writer) {
   assert(_thread != NULL, "invariant");
   assert(_thread == Thread::current(), "invariant");
-  assert(_thread->is_Java_thread(), "invariant");
-  ResourceMark rm(_thread);
-  const oop threadObj = _thread->threadObj();
-  assert(threadObj != NULL, "invariant");
-  const u8 java_lang_thread_id = java_lang_Thread::thread_id(threadObj);
-  const char* const thread_name = _thread->name();
-  const traceid thread_group_id = JfrThreadGroup::thread_group_id(_thread);
   writer.write_count(1);
-  writer.write_key(_thread->jfr_thread_local()->thread_id());
-  writer.write(thread_name);
-  writer.write((traceid)_thread->osthread()->thread_id());
-  writer.write(thread_name);
-  writer.write(java_lang_thread_id);
-  writer.write(thread_group_id);
-  JfrThreadGroup::serialize(&writer, thread_group_id);
+  writer.write_key(JfrThreadId::jfr_id(_thread));
+  const char* const name = JfrThreadName::name(_thread);
+  writer.write(name);
+  writer.write(JfrThreadId::os_id(_thread));
+  if (_thread->is_Java_thread()) {
+    writer.write(name);
+    writer.write(JfrThreadId::id(_thread));
+    JavaThread* const jt = (JavaThread*)_thread;
+    const traceid thread_group_id = JfrThreadGroup::thread_group_id(jt, jt);
+    writer.write(thread_group_id);
+    JfrThreadGroup::serialize(&writer, thread_group_id);
+    return;
+  }
+  writer.write((const char*)NULL); // java name
+  writer.write((traceid)0); // java thread id
+  writer.write((traceid)0); // java thread group
 }