Java线程忙着等待

Question

嗨，我正在做一个项目，我已经达到了我非常困的部分。我试图寻找方法来学习如何在繁忙的等待中编写 while 循环，但我没有找到任何东西，我的代码只是作为无限循环运行。有人可以帮助我解释一个繁忙的等待循环应该如何工作，并帮助我打破这个无限循环吗？

该项目希望做到以下几点：早上，学生醒来后（这需要一段随机的时间），他会去洗手间为新的上学日做准备。如果浴室已经客满，学生需要Rest一下（使用yield（）），然后他会等待（使用busy waiting），等待浴室可用。学生将以先到先得的方式使用浴室（您可以使用布尔数组/向量将其按顺序释放）。

 public class Student implements Runnable 
    {
        private Random rn = new Random();
        private String threadNum;
        private volatile boolean bathroomFull = false;
        private static long time = System.currentTimeMillis();
        private Thread t;


    public Student(String studentID) 
    {
      threadNum = studentID;

      t = new Thread(this, "Student Thread #"+threadNum);
      System.out.println("thread created = " + t);
      // this will call run() function
      t.start();
   }

   public void run() 
   {
       int waitTime = rn.nextInt(4000 - 2000 + 1)+2000;

        System.out.println( "the current time is " + (System.currentTimeMillis() - time) + "and the wait time is: " +waitTime );

         //Student wakes up after random time
        while((System.currentTimeMillis()-time) < waitTime)
       {
          // System.out.println("the remaining sleep time is " + (System.currentTimeMillis()-time));
            ;
       }

      int a = rn.nextInt(4000 - 2000 + 1)+2000;
      try 
      {
          //System.out.println("I'm going to sleep for " +a + " milliseconds");
        Thread.sleep(a);
      } 
      catch (InterruptedException e) 
      {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }

      //this is the busy wait loop where is the bathroom is full then a thread will yield until it is available
    int l = rn.nextInt(10 - 1)+1;
  bathroomFull = true;

      while(bathroomFull)
        {
          for(int j = 0; j < l; j++)
          {
              System.out.println("i am in the bathroom for " + l + "minutes " + Thread.currentThread());
          }
          Thread.yield();
          bathroomFull = false;
          //exitBathroom();

        }
    bathroomFull = true;

这是我的主要方法，它允许用户指定他们想要多少学生线程。是的，我不明白如何实现值的改变，以便繁忙的wait while循环可以被打破。

 public static void main(String args[]) 
   {
       int numberOfStudents;
       numberOfStudents = Integer.parseInt(JOptionPane.showInputDialog("How many students are there in the university? "));
      // System.out.println("there are " + numberOfStudents);

       for(int i = 0; i < numberOfStudents; i++)
       {   
           new Student(String.valueOf(i+1));
       }
          new Teacher();
   }

Answer 1

这是一个繁忙等待的工作示例。它使用原子布尔值来指示浴室是否被占用。原子操作一步执行，这对于保证线程安全很重要。我们也可以使用普通布尔值并自己编写compareAndSet：

private static synchronized boolean compareAndSet(boolean expected, boolean value) {
    if (occupied == expected) { // (1)
        occupied = value; // (2)
        return true;
    } else {
        return false;
    }
}

需要同步，否则在执行（2）之前，两个线程可能会在（1）成功测试（因为这两个操作不是原子的），然后两个人会一起去浴室……

import java.util.concurrent.atomic.AtomicBoolean;

public class Student extends Thread {

    // note the static: there is only one bathroom for all students
    private static AtomicBoolean occupied = new AtomicBoolean(false);

    private String name;

    public Student(String name) {
        this.name = name;
    }

    private void sleep(int millis) {
        try {
            Thread.sleep(millis);
        } catch (InterruptedException e) {
            System.out.println(name + " wet his/her pants");
        }
    }

    @Override
    public void run() {
        int r = (int)(Math.random() * 5000);
        System.out.println(name + " sleeps for " + r + " ms");
        sleep(r);
        System.out.println(name + " goes to bathroom");
        // ***** busy wait *****
        while (!occupied.compareAndSet(false, true)) {
            System.out.println(name + " takes a break");
            Thread.yield();
            sleep(1000);
        }
        // ***** end (in bathroom) *****
        System.out.println(name + " is in the bathroom");
        sleep(1000);
        occupied.set(false);
        System.out.println(name + " goes to university");
    }

    public static void main(String[] args) {
        new Student("Bob").start();
        new Student("Alice").start();
        new Student("Peter").start();
        new Student("Marcia").start();
        new Student("Desmond").start();
        new Student("Sophia").start();
    }

}

可能的输出：

鲍勃睡了2128位女士< br >玛西娅睡了3357位女士< br >爱丽丝睡了1289位女士< br >彼得睡了820位女士< br >德斯蒙德睡了1878位女士< br >索菲亚睡了2274位女士< br >彼得去了洗手间< br >彼得去了洗手间< br >爱丽丝去了洗手间< br >爱丽丝Rest了一会儿< br >彼得去了大学< br >德斯蒙德去了洗手间< br >德斯蒙德去了洗手间< br >鲍勃去了洗手间< br 去上大学鲍勃在洗手间索菲亚Rest爱丽丝去上大学玛西娅在洗手间玛西娅去上大学