void ThreadSnapshot::dump_stack_at_safepoint(int max_depth, bool with_locked_monitors) {

  {- -------------------------------------------
  (1) 新しい ThreadStackTrace オブジェクトを作り, 
      ThreadStackTrace::dump_stack_at_safepoint() でスタックトレースを取得する.
      ---------------------------------------- -}

      _stack_trace = new ThreadStackTrace(_thread, with_locked_monitors);
      _stack_trace->dump_stack_at_safepoint(max_depth);
    }