说明

最近碰到一个问题，使用三个线程交替打印 0-100 。主要考察多线程并发同步，锁的使用。这里记录下我用 Lock 和 多个 Condition 的实现方式。

在刚开始实现时，发现在数字输出完毕后，主线程无法停止，最后只能在判断满足条件后直接退出 JVM。这个原因是有线程在调用 await 方法后，没有被唤醒，导致线程没有正常结束。

所以，在每个线程执行完毕退出释放锁前，都要进行一次顺序唤醒操作。

还有一点需要注意的是，尽量不要在循环中使用 try catch 语句。

正文

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class PrintNum {


    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Condition c1 = lock.newCondition();
        Condition c2 = lock.newCondition();
        Condition c3 = lock.newCondition();
        AtomicInteger count = new AtomicInteger(0);
        Runnable task1 = new Runnable() {
            @Override
            public void run() {
                try {
                    lock.lock();
                    while (true) {
                        if (count.get() > 100) {
                            return;
                        }
                        System.out.println(Thread.currentThread().getName() + "-" + count.getAndAdd(1));
                        c2.signal();

                        c1.await();
                    }
                } catch (Exception e) {
                } finally {
                    c2.signal();
                    lock.unlock();
                }

            }
        };

        Runnable task2 = new Runnable() {
            @Override
            public void run() {
                try {
                    lock.lock();
                    while (true) {
                        if (count.get() > 100)
                            return;
                        System.out.println(Thread.currentThread().getName() + "-" + count.getAndAdd(1));
                        c3.signal();
                        c2.await();
                    }
                } catch (Exception e) {
                } finally {
                    c3.signal();
                    lock.unlock();
                }

            }
        };

        Runnable task3 = new Runnable() {
            @Override
            public void run() {
                try {
                    lock.lock();
                    while (true) {
                        if (count.get() > 100)
                            return;
                        System.out.println(Thread.currentThread().getName() + "-" + count.getAndAdd(1));
                        c1.signal();
                        c3.await();
                    }
                } catch (Exception e) {
                } finally {
                    c1.signal();
                    lock.unlock();
                }

            }
        };
        Thread t1 = new Thread(task1, "A");
        t1.start();
        Thread t2 = new Thread(task2, "B");
        t2.start();
        Thread t3 = new Thread(task3, "C");
        t3.start();
        try {
            System.out.println("任务线程已启动");
            t1.join();
            t2.join();
            t3.join();
            System.out.println("线程执行完毕");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}