從Thread.start()方法看Thread源碼，多次start一個線程會怎么樣

這篇文章作為Thread類源碼剖析的補充，從一個側面來看Thread源碼。也解答了面試高頻問題：“多次start一個線程會怎么樣？”

答案是：java.lang.IllegalThreadStateException   線程狀態非法異常   繼承關系是：--->extends IllegalArgumentException--->extends RuntimeException一個運行時異常，下面我們從源碼來透徹分析一下start()時做了什么。

/**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>線程被執行，JVM調用run方法
     * 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.多次調用start方法啟動一個線程是非法的
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already已經啟動的線程再次start，異常
     *               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".
         */
        if (threadStatus != 0)//狀態校驗 0：NEW 新建狀態 28             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);//添加進線程組 34 
        boolean started = false;
        try {
            start0();//調用native方法執行線程run方法
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);//啟動失敗，從線程組中移除當前前程。 43                 }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

    private native void start0();

greop.add(this)，把當前線程添加進線程組，源碼如下：

/**
     * Adds the specified thread to this thread group.
     *
     * <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) {
            if (destroyed) {//線程組狀態校驗
                throw new IllegalThreadStateException();
            }
            if (threads == null) {
                threads = new Thread[4];//初始化長度為4的Thread數組
            } else if (nthreads == threads.length) {//數組滿了就擴容2倍 22                 threads = Arrays.copyOf(threads, nthreads * 2);
            }
            threads[nthreads] = t;//新線程t添加進數組 25 
            // This is done last so it doesn't matter in case the
            // thread is killed
            nthreads++;//線程數加1 29 
            // 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--;//未啟動線程數-1 35         }
    }

啟動失敗后調用group.threadStartFailed(this)，都是加鎖方法，從線程組中移除當前線程，源碼如下

void threadStartFailed(Thread t) {
        synchronized(this) {
            remove(t);//移除線程t
            nUnstartedThreads++;//未啟動線程+1
        }
    }

private void remove(Thread t) {
        synchronized (this) {
            if (destroyed) {
                return;
            }
            for (int i = 0 ; i < nthreads ; i++) {
                if (threads[i] == t) {
                    System.arraycopy(threads, i + 1, threads, i, --nthreads - i);
                    // Zap dangling reference to the dead thread so that
                    // the garbage collector will collect it.
                    threads[nthreads] = null;
                    break;
                }
            }
        }
    }