二次调用Thread类的start()方法会怎样
这个一般会出现在面试中,面试官会问的,那我门来看看Thread类的start(),看看它源码

/**
   * 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".
       */
      if (threadStatus != 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);

      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 */
          }
      }
  }

我们首先来翻译下它的注释
使该线程开始执行;Java虚拟机,调用该线程的run方法
然后再看看start()方法的执行流程是什么
先判断threadStatus这个变量是不是等于0,不等于0就抛出异常,然后加入线程组,再就是调用start0()方法,start0()是native层的方法,并没有在Java层实现,那么threadStatus变量是在什么时候进行赋值了呢?
在Thread类找了threadStatus所在用的地方都没有给threadStatus变量赋值,那就说明了JVM虚拟机在内层已经给他进行了赋值操作.
threadStatus的状态修改由底层的C++代码实现，而threadStatus的状态值变化对JVM是可见的，但是在Java里无法看到threadStatus改变的具体实现方式

总结

1.检查线程状态，只有NEW状态下的线程才能继续，否则会抛出IllegalThreadStateException（在运行中或者已结束的线程，都不能再次启动，详见CantStartTwice10类）
2.被加入线程组
3.调用start0()方法启动线程
注意点：
start方法是被synchronized修饰的方法，可以保证线程安全；
由JVM创建的main方法线程和system组线程，并不会通过start来启动