从Thread.start()方法看Thread源码，多次start一个线程会怎么样

这篇文章作为Thread类源码剖析的补充，从一个侧面来看Thread源码。也解答了面试高频问题：“多次start一个线程会怎么样？”

答案是：java.lang.IllegalThreadStateException   线程状态非法异常   继承关系是：--->extends IllegalArgumentException--->extends RuntimeException一个运行时异常，下面我们从源码来透彻分析一下start()时做了什么。

 /**
      * Causes this thread to begin execution; the Java Virtual Machine
      * calls the <code>run</code> method of this thread.
      * <p>线程被执行，JVM调用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.多次调用start方法启动一个线程是非法的
      * In particular, a thread may not be restarted once it has completed
      * execution.
      *
      * @exception  IllegalThreadStateException  if the thread was already已经启动的线程再次start，异常
      *               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)//状态校验  0：NEW 新建状态
             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);//添加进线程组

         boolean started = false;
         try {
             start0();//调用native方法执行线程run方法
             started = true;
         } finally {
             try {
                 if (!started) {
                     group.threadStartFailed(this);//启动失败，从线程组中移除当前前程。
                 }
             } 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)，把当前线程添加进线程组，源码如下：

 /**
      * 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) {//线程组状态校验
                 throw new IllegalThreadStateException();
             }
             if (threads == null) {
                 threads = new Thread[4];//初始化长度为4的Thread数组
             } else if (nthreads == threads.length) {//数组满了就扩容2倍
                 threads = Arrays.copyOf(threads, nthreads * 2);
             }
             threads[nthreads] = t;//新线程t添加进数组

             // This is done last so it doesn't matter in case the
             // thread is killed
             nthreads++;//线程数加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
         }
     }

启动失败后调用group.threadStartFailed(this)，都是加锁方法，从线程组中移除当前线程，源码如下

 void threadStartFailed(Thread t) {
         synchronized(this) {
             remove(t);//移除线程t
             nUnstartedThreads++;//未启动线程+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;
                 }
             }
         }
     }