/**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>
     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * <code>start</code> method) and the other thread (which executes its
     * <code>run</code> method).
     * <p>
     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */

    public synchronized void start() {

            /**
             * This method is not invoked for the main method thread or "system"
             * group threads created/set up by the VM. Any new functionality added
             * to this method in the future may have to also be added to the VM.
             *
             * A zero status value corresponds to state "NEW".
             */

  {- -------------------------------------------
  (1) (生成直後のスレッドでは, threadStatus フィールドは 0 になっているはず)
      既に start 済みのスレッドにもう一度 start() が呼ばれた場合, IllegalThreadStateException.
      ---------------------------------------- -}

            if (threadStatus != 0)
                throw new IllegalThreadStateException();

  {- -------------------------------------------
  (1) java.lang.ThreadGroup.add() で, ThreadGroup にこのスレッドを追加する.
      ---------------------------------------- -}

            /* Notify the group that this thread is about to be started
             * so that it can be added to the group's list of threads
             * and the group's unstarted count can be decremented. */
            group.add(this);

  {- -------------------------------------------
  (1) java.lang.ThreadGroup.start0() で, このスレッドを開始させる.

      (なお, もし java.lang.ThreadGroup.start0() が失敗した(例外が出た)場合には, 
      java.lang.ThreadGroup.threadStartFailed() で ThreadGroup からこのスレッドを除去している)
      ---------------------------------------- -}

            boolean started = false;
            try {
                start0();
                started = true;
            } finally {
                try {
                    if (!started) {
                        group.threadStartFailed(this);
                    }
                } catch (Throwable ignore) {
                    /* do nothing. If start0 threw a Throwable then
                      it will be passed up the call stack */
                }
            }
        }