使用 3 个线程按顺序打印数字

Question

I have a program where 3 Threads are trying to print numbers in sequence from 1 to 10. I am using a CountDownLatch to keep keep a count.

But the program stops just after printing 1.

Note: I am aware that using AtomicInteger instead of Integer can work. But I am looking to find out the issue in the current code.

public class Worker implements Runnable {
    private int id;
    private volatile Integer count;
    private CountDownLatch latch;

    public Worker(int id, Integer count, CountDownLatch latch) {
        this.id = id;
        this.count = count;
        this.latch = latch;
    }

    @Override
    public void run() {
        while (count <= 10) {
            synchronized (latch) {
                if (count % 3 == id) {
                    System.out.println("Thread: " + id + ":" + count);
                    count++;
                    latch.countDown();
                }
            }
        }
    }

}

Main program:

public class ThreadSequence {
    private static CountDownLatch latch = new CountDownLatch(10);
    private volatile static Integer count = 0;

    public static void main(String[] args) {
        Thread t1 = new Thread(new Worker(0, count, latch));
        Thread t2 = new Thread(new Worker(1, count, latch));
        Thread t3 = new Thread(new Worker(2, count, latch));

        t1.start();
        t2.start();
        t3.start();

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

Edited program with AtomicInteger :

public class ThreadSequence {
    private static AtomicInteger atomicInteger = new AtomicInteger(1);

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new WorkerThread(0, atomicInteger));
        Thread t2 = new Thread(new WorkerThread(1, atomicInteger));
        Thread t3 = new Thread(new WorkerThread(2, atomicInteger));
        t1.start();
        t2.start();
        t3.start();

        t1.join();
        t2.join();
        t3.join();

        System.out.println("Done with main");
    }
}


public class WorkerThread implements Runnable {
    private int id;
    private AtomicInteger atomicInteger;

    public WorkerThread(int id, AtomicInteger atomicInteger) {
        this.id = id;
        this.atomicInteger = atomicInteger;
    }

    @Override
    public void run() {
        while (atomicInteger.get() < 10) {
            synchronized (atomicInteger) {
                if (atomicInteger.get() % 3 == id) {
                    System.out.println("Thread:" + id + " = " + atomicInteger);
                    atomicInteger.incrementAndGet();
                }
            }

        }
    }
}

Answer 1

But the program stops just after printing 1.

No this is not what happens. None of the threads terminate.

You have a own count field in every worker. Other threads do not write to this field.

Therefore there is only one thread, where if (count % 3 == id) { yields true , which is the one with id = 0 . Also this is the only thread that ever modifies the count field and modifying it causes (count % 3 == id) to yield false in subsequent loop iterations, causing an infinite loop in all 3 threads.

Change count to static to fix this.

Edit

In contrast to Integer AtomicInteger is mutable. It is a class that holds a int value that can be modified. Using Integer every modification of the field replaces it's value, but using AtomicInteger you only modify the value inside the AtomicInteger object, but all 3 threads continue using the same AtomicInteger instance.

Answer 2

您的“计数”是每个线程的不同变量，因此在一个线程中更改它不会影响其余线程，因此他们都在等待它更改，而没有任何人可以做到。

Answer 3

将count保持为 Worker 类中的静态成员 - 对于类中的所有对象都是通用的。

Answer 4

You can use below code to print sequential numbers using multiple threads -

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

public class ThreadCall extends Thread {

    private BlockingQueue<Integer> bq = new ArrayBlockingQueue<Integer>(10);
    private ThreadCall next;

    public void setNext(ThreadCall t) {
        this.next = t;
    }

    public void addElBQ(int a) {
        this.bq.add(a);
    }

    public ThreadCall(String name) {
        this.setName(name);
    }

    @Override
    public void run() {
        int x = 0;
        while(true) {
            try {
                x = 0;
                x = bq.take();
                if (x!=0) {
                    System.out.println(Thread.currentThread().getName() + " =>" + x);
                    if (x >= 100) System.exit(0); // Need to stop all running threads
                    next.addElBQ(x+1);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        int THREAD_COUNT = 10;
        List<ThreadCall> listThread = new ArrayList<>();

        for (int i=1; i<=THREAD_COUNT; i++) {
            listThread.add(new ThreadCall("Thread " + i));
        }

        for (int i = 0; i < listThread.size(); i++) {
            if (i == listThread.size()-1) {
                listThread.get(i).setNext(listThread.get(0));
            }
            else listThread.get(i).setNext(listThread.get(i+1));
        }

        listThread.get(0).addElBQ(1);

        for (int i = 0; i < listThread.size(); i++) {
            listThread.get(i).start();
        }
    }
}

I hope this will resolve your problem