同一个线程多次调用start()会出现的问题

结果：

　　线程首先会运行一次，然后抛出java.lang.IllegalThreadStateException异常。

　　

根据控制台的异常信息，定位到Thread.java的第708行，也就start()方法内部，打个断点调试：

　　

调试发现，第一个次运行start()方法时，threadStatus是0，此时if条件不满足，继续执行，会将当前线程添加到线程组中去执行。

第二次运行start()方法时，threadStatus变成了2，if条件满足，于是抛出了java.lang.IllegalThreadStateException异常。

start()方法的源码：

/**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
　　　　  让这个线程开始执行。JVM会调用这个线程的run()方法。
     * <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.一个线程多次调用start()方法是非法的。
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *特别说明：一个线程执行完后，不太可能重新运行。
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
　　　　　　　　　　　　　　如果该线程已经启动，则再次调用start()方法，就会抛出IllegalThreadStateException异常。
     * @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".
         */
        if (threadStatus != 0)//新的线程threadState值是0
            throw new IllegalThreadStateException();

        /* 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);//通知线程组该线程将要开始运行，这样该线程就会被添加到线程列表中，此时列表的unstarted数将会减少。

        boolean started = false;
        try {
            start0();//调用原生方法态方法启动线程
            started = true;//已经运行的标记设置为true
        } finally {
            try {
                if (!started) {//若开始运行标记未设置成功，则通知线程组该线程尝试运行失败
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

group.add(this)  ---线程组添加线程源码分析：

/**
     * Adds the specified thread to this thread group.
     * 添加特定的线程到该线程组
     * <p> Note: This method is called from both library code
     * and the Virtual Machine. It is called from VM to add
     * certain system threads to the system thread group.
     *
     * @param  t
     *         the Thread to be added
     *
     * @throws  IllegalThreadStateException
     *          if the Thread group has been destroyed
     */
    void add(Thread t) {
        synchronized (this) {//线程同步
            if (destroyed) {//如果线程组的销毁标记(destroyed)是true，则抛出IllegalThreadStateException
                throw new IllegalThreadStateException();
            }
            if (threads == null) {//如果线程数组为空，则初始为4个新的线程
                threads = new Thread[4];
            } else if (nthreads == threads.length) {//如果当前线程组已满，则扩容至原来的2倍
                threads = Arrays.copyOf(threads, nthreads * 2);
            }
            threads[nthreads] = t;//将当前线程放入线程数组的末尾

            // This is done last so it doesn't matter in case the
            // thread is killed
            nthreads++;//线程数组元素个数+1

            // The thread is now a fully fledged member of the group, even
            // though it may, or may not, have been started yet. It will prevent
            // the group from being destroyed so the unstarted Threads count is
            // decremented.
            nUnstartedThreads--;//未启动线程数-1
        }
    }

threadStartFailed()源码分析：

/**
     * Notifies the group that the thread {@code t} has failed
     * an attempt to start.
     * 通知线程组该线程尝试运行失败
     * <p> The state of this thread group is rolled back as if the
     * attempt to start the thread has never occurred. The thread is again
     * considered an unstarted member of the thread group, and a subsequent
     * attempt to start the thread is permitted.
     *
     * @param  t
     *         the Thread whose start method was invoked
     */
    void threadStartFailed(Thread t) {
        synchronized(this) {//同步
            remove(t);//从线程组中移除该线程
            nUnstartedThreads++;//未启动线程数+1
        }
    }

结论：

同一个线程只能调用start()方法一次，多次调用会抛出java.lang.IllegalThreadStateException。启动一个线程，需要调用start()方法而不是run()方法。此时，当前线程会被添加到线程组中，进入就绪状态，等待线程调度器的调用，若获取到了资源，则能进入运行状态，run()方法只是线程体，即线程执行的内容，若没调用start()方法，run()方法只是一个普通的方法。