一道面试题：多个线程按顺序输出

约定不等于承诺〃 2022-07-12 04:46 152阅读 0赞

题目：编写一个程序，开启 3 个线程，这三个线程的 ID 分别为 A、B、C，每个线程将自己的 ID 在屏幕上打印 10 遍，要求输出的结果必须按顺序显示。如：ABCABCABC……  
　　  
　　解题源码：

package concurrent;
    
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReentrantLock;
    
    class AlternateDemo {
        //标志当前由哪一个线程输出，1代表A，2代表B，3代表C
        private int number = 1;
    
        Lock lock = new ReentrantLock();
    
        //Condition的强大之处在于它可以为多个线程间建立不同的Condition
        Condition condition1 = lock.newCondition();
        Condition condition2 = lock.newCondition();
        Condition condition3 = lock.newCondition();
    
        // loopNum:当前循环轮数
        public void loopA(int loopNum) {
            //上锁
            lock.lock();
            try {
                while (number != 1) {
                    //等待
                    condition1.await();
                }
    
                System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
                number = 2;
                //唤醒
                condition2.signal();
    
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                //确保释放锁
                lock.unlock();
            }
        }
    
        public void loopB(int loopNum) {
            lock.lock();
            try {
                while (number != 2) {
                    condition2.await();
                }
    
                System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
                number = 3;
                condition3.signal();
    
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    
        public void loopC(int loopNum) {
            lock.lock();
            try {
                while (number != 3) {
                    condition3.await();
                }
    
                System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
                number = 1;
                condition1.signal();
    
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
    
    public class TestABCAlternate { 
    
        public static void main(String[] args) {
            AlternateDemo ad = new AlternateDemo();
    
            new Thread(new Runnable() {
    
                @Override
                public void run() {
                    for (int i = 0; i < 10; i++)
                        ad.loopA(i);
                }
            }, "A").start();
    
            new Thread(new Runnable() {
    
                @Override
                public void run() {
                    for (int i = 0; i < 10; i++)
                        ad.loopB(i);
                }
            }, "B").start();
    
            new Thread(new Runnable() {
    
                @Override
                public void run() {
                    for (int i = 0; i < 10; i++)
                        ad.loopC(i);
                }
            }, "C").start();
        }
    }

这道题充分体现了Condition的强大之处：它可以为多个线程间建立不同的Condition，并指定唤醒哪一个线程。