同步线程执行

您的代码中存在一些缺陷，这些缺陷有时会使它无法相应地工作：

1. 您打了电话

   thread_A.start()

   然后检查了

   thread_A.isAlive()

   。现在，如果

   thread_A.isAlive()

   条件检查之前thread_A已经完成，该怎么办？

   thread_B

   并且

   thread_C

   永远不会开始。您的应用程序失败。
2. 假设

   thread_A

   未完成且

   thread_A.isAlive()

   条件已通过，那么Java线程调度程序并不总是保证

   thread_B

   before的启动

   thread_C

   。同样，您的应用程序失败。
3. 假设检查之前

   thread_B

   开始，

   thread_C

   并且如果检查

   thread_B

   完成之前

   thread_B.isAlive()

   ，则

   if

   条件失败并且

   thread_D

   永远不会开始。同样，您的应用程序失败。

现在需要考虑的一点是：
在

join()

编辑
好，这是代码的修改版本。.我希望它可以让您了解线程的动态：

class RobotController implements Runnable{    private final Object lock = new Object();    private void notifyThread()    {        synchronized(lock)        { lock.notify();        }    }    public void run()     {        synchronized(lock)        { try {     System.out.println(Thread.currentThread().getName() + " started");     lock.wait();     System.out.println(Thread.currentThread().getName()+ " stopped"); } catch (InterruptedException ex) {     ex.printStackTrace(); }        }    }    public static void main(String args[]) throws InterruptedException    {        RobotController rca = new RobotController();        RobotController rcb = new RobotController();        RobotController rcc = new RobotController();        RobotController rcd = new RobotController();        Thread thread_A = new Thread(rca,"Thread A");        Thread thread_B = new Thread(rcb,"Thread B");        Thread thread_C = new Thread(rcc,"Thread C");        Thread thread_D = new Thread(rcd,"Thread D");        thread_A.start();        while (thread_A.getState() != Thread.State.WAITING)        { Thread.sleep(100);        }        thread_B.start();        thread_C.start();        while (thread_B.getState() != Thread.State.WAITING && thread_C.getState() != Thread.State.WAITING)        { Thread.sleep(100);        }        thread_D.start();        while (thread_D.getState() != Thread.State.WAITING)        { Thread.sleep(100);        }        rcd.notifyThread();        thread_D.join();        rcc.notifyThread();        thread_C.join();        rcb.notifyThread();        thread_B.join();        rca.notifyThread();    }}

这是输出：

Thread A startedThread B startedThread C startedThread D startedThread D stoppedThread C stoppedThread B stoppedThread A stopped