如何从单独的线程暂停主线程？

Question

我正在开发一个应用程序，需要能够在处理过程中暂停主线程。我的想法是拥有三个线程：主线程，一个监听输入（STOP 或 GO 命令）的线程，然后另一个线程使主线程停止：

public static boolean stopped = false;
public static void main (String[] args){

    Thread main = Thread.currentThread();

    Runnable t1 = new Runnable() {
        public void run() {
            while(true){
                Scanner s = new Scanner(System.in);
                // wait for command
                String in = s.nextLine();
                if(in.equals("STOP")){
                    synchronized(main){
                        stopped = true;
                        //create new thread to make main wait
                        Runnable t2 = new Runnable() {
                            public void run(){
                                synchronized(main){
                                    while(stopped){
                                        main.wait();
                                    }
                                }   
                            }
                        };
                        Thread th2 = new Thread(t2);
                        th2.start();
                    }
                } 
                if (in.equals("GO")){
                    synchronized(main){
                        stopped = false;
                        main.notifyAll();
                    }
                }
            }
        }
    };

    Thread th1 = new Thread(t1);
    th1.start();

    while(true){
        synchronized(main){
            // Do lots of stuff that needs to pause when user enters STOP
            System.out.println("Main is running");
            Thread.sleep(5000);
        }

    }       
}
}

即使在发出 STOP 命令后，目前 main 仍在继续其循环。有没有办法锁定 main 直到另一个线程释放它而不必在 main 中执行 wait()?

Answer 1

您不想按照自己的方式构建它。您不希望线程在任意点停止；尝试恢复时可能会导致问题。相反，给线程明确的停止点。

例如，您可以创建一个工作线程，让程序的主线程管理命令并将指令委托给您的工作线程。 process 方法是一种增量工作量，例如读取大文件的一行。例如，假设这是您的辅助任务：

public abstract class Worker implements Runnable {
  private final Object lock = new Object();
  private final AtomicBoolean shouldWait = new AtomicBoolean();

  protected abstract boolean processingIsComplete();
  protected abstract void process();
  protected abstract void cleanUpResources();

  public Object getLock() {
    return lock;
  }

  public void disable() {
    shouldWait.set(false);
  }

  public void enable() {
    shouldWait.set(true);
  }

  @Override
  public void run() {
    try {
      while(!processingIsComplete()) {
        while(!shouldWait.get()) {
          synchronized(lock);
            lock.wait();
          }
        }
        process();
      }
    } catch(InterruptedException e) {
      System.out.println("Worker thread stopped");
    } finally {
      cleanUpResources();
    }
  }
}

然后，修改/subclass worker 来真正做处理工作。然后，在您的主 class 中，您可以根据需要打开和关闭您的工作人员。这只是一个骨架，显然您可以将行为重构为多个方法，添加其他控件，例如读取工作线程的状态等：

public static void main (String[] args) throws Exception {
  Worker worker = new WorkerImpl(/* whatever args are needed */);
  Thread workerThread = new Thread(worker);
  System.out.println("Starting process...");

  worker.start();
  Scanner sc = new Scanner(System.in);

  while(true) {
    System.out.printn("Please enter command: ");
    String command = sc.nextLine();
    if("END".equals(command)) {
      System.out.println("Terminating program... ");
      workerThread.interrupt();
      break;
    } else if ("GO".equals(command)) {
      synchronized(worker.getLock()) {
        System.out.println("Continuing worker thread... ");
        worker.enable();
        worker.getLock().notifyAll();
      }
    } else if ("STOP".equals(command)) {
      synchronized(worker.getLock()) {
        System.out.println("Stopping worker thread ");
        worker.disable();
        worker.getLock().notifyAll();
      }
    } else {
      printCommandHelp();
    }
  }
};

进一步阅读：How to use wait and notify in Java?

如何从单独的线程暂停主线程？

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