Question

我仍然是一个java新手，并试图玩学习线程。我的问题是它没有循环5次。它运行一次并退出。我使用a.class来锁定类对象，这样两个线程都锁定在同一个对象监视器上。

class a implements Runnable {
  Thread thr;
  int count;
  String time;

  a(String s) {
    thr = new Thread(this, s);
    thr.start();
  }

  public void run() {
    count++;

    if (Thread.currentThread().getName().compareTo("one") == 0) {
      synchronized (a.class) {

        try {
          for (int i = 0; i < 5; i++) {
            System.out.println("Now running thread " + Thread.currentThread().getName() + " with count " + count);

            time = "Tick";
            System.out.println(time);
            notify();

            while (time == "Tock") {
              wait();
            }
          }
        } catch (Exception e) {
        }

      }
    } else if (Thread.currentThread().getName().compareTo("two") == 0) {
      synchronized (a.class) {
        try {
          for (int j = 0; j < 5; j++) {
            System.out.println("Now running thread " + Thread.currentThread().getName() + " with count " + count);

            time = "Tock";
            System.out.println(time);
              notify();

            while (time == "Tick") {
              wait();
            }
          }
        } catch (Exception e) {
        }
      }
    }
  }
}

public class b {
  public static void main(String args[]) {

    a obj1 = new a("one");
    a obj2 = new a("two");
  }
}

Answer 1

在这里，您可以使用原始代码：

class a implements Runnable {
    Thread thr;
    int count;
    static String time = "Tock";

    a(String s) {
        thr = new Thread(this, s);
        thr.start();
    }

    public void run() {
        count++;

        if (Thread.currentThread().getName().compareTo("one") == 0) {
            synchronized (a.class) {

                try {
                    for (int i = 0; i < 5; i++) {
                        while (time.equals("Tock")) {
                            a.class.wait();
                        }

                        System.out.println("Now running thread "
                                + Thread.currentThread().getName()
                                + " with count " + count);

                        time = "Tock";
                        System.out.println(time);
                        a.class.notify();                       
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }
        } else if (Thread.currentThread().getName().compareTo("two") == 0) {
            synchronized (a.class) {
                try {
                    for (int j = 0; j < 5; j++) {
                        while (time.equals("Tick")) {
                            a.class.wait();
                        }

                        System.out.println("Now running thread "
                                + Thread.currentThread().getName()
                                + " with count " + count);

                        time = "Tick";
                        System.out.println(time);
                        a.class.notify();                       
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

public class Test {
    public static void main(String args[]) {

        a obj1 = new a("one");
        a obj2 = new a("two");
    }
}

问题是当你在wait对象上持有锁时，你在隐式notify对象上调用this和a.class，因此你必须调用wait/notify上的a.class。就是这样。

我还进行了一次小型重组，因为我假设您希望它们以交替的顺序打印Tick和Tock，对吗？

Answer 2

在比较字符串（和一般对象）时，应使用equals而不是==（通常为基元保留）：while(time.equals("Tock"))。对于字符串，==通常会导致false当你想要它（并认为它应该）返回true，因此你的循环将在预期之前退出。

Answer 3

为什么你只循环一次的答案就是你在一个未锁定的对象上调用notify()，因此抛出IllegalMonitorStateException并被空的catch语句捕获。

这是一种方法。不是说这是最好的。我试着让它贴近你的代码：

public class TickTock {
    static final int N = 4;

    Object lock = new Object();
    int token;

    class Worker extends Thread {
        int id;

        Worker(int id) {
            this.id = id;
        }

        @Override
        public void run() {
            try {
                synchronized (lock) {
                    for (int i = 0; i < 5; i++) {
                        while (id != token%N) lock.wait();

                        System.out.println(id + " " + i);

                        token++;
                        lock.notifyAll();
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    void start() {
        for (int i = 0; i < N; i++) {
            new Worker(i).start();
        }
    }

    public static void main(String[] args) {
        new TickTock().start();
    }
}

Java - 不在多线程程序中循环

3 个答案: