hotspot/src/share/vm/classfile/javaClasses.cpp
bool java_lang_Thread::is_alive(oop java_thread) {
  {- -------------------------------------------
  (1) java_lang_Thread::thread() が NULL かどうかをリターン.
      ---------------------------------------- -}
      JavaThread* thr = java_lang_Thread::thread(java_thread);
      return (thr != NULL);
    }
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