ABC三个线程如何保证顺序执行

介绍

最近看看多线程练习题，练习下多线程，这道题：ABC三个线程如何保证顺序执行。三个线程同时启动，然后按照顺序执行，每个线程执行10次。

思路

首先想到了等待队列Condition唤醒部分线程，使用ReentrantLock进行加锁。

初始版实现

/**
 * @description A\B\C三个线程顺序执行10次
 * 
 * @author sunpy
 * @date 2018年11月28日  下午2:23:45
 */
public class MyTest {

    static class MyTask {
        private static ReentrantLock rl = new ReentrantLock();
        private static Condition conditionA = rl.newCondition();
        private static Condition conditionB = rl.newCondition();
        private static Condition conditionC = rl.newCondition();
        private static int number = 0;
    
        public void execute() {
            rl.lock();
            
            try {
                while (number < 30) {
                    if (number % 3 == 0) {
                        System.out.println(Thread.currentThread().getName() + " - " + number);
                        number++;
                        conditionB.signal();
                        conditionA.await();
                    }
                    
                    if (number % 3 == 1) {
                        System.out.println(Thread.currentThread().getName() + " - " + number);
                        number++;
                        conditionC.signal();
                        conditionB.await();
                    }
                    
                    if (number % 3 == 2) {
                        System.out.println(Thread.currentThread().getName() + " - " + number);
                        number++;
                        conditionA.signal();
                        conditionC.await();
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                rl.unlock();
            }
        }
    }
    
    
    public static void main(String[] args) {
        final MyTask myTask = new MyTask();
        new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.execute();
            }
        }, "A").start();
        
        new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.execute();
            }
        }, "B").start();
        
        new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.execute();
            }
        }, "C").start();
        
    }
}

说明：通过余数判断来执行，需要循环30次，有点不合理。想了下，执行10次，循环10次，但是让线程按照ABC执行就可以了。根据这个想法写出了改进版。

改进版实现

/**
 * @description A\B\C三个线程顺序执行10次
 * 
 * @author sunpy
 * @date 2018年11月28日  下午2:23:45
 */
public class MyTest {

    static class MyTask {
        private static ReentrantLock rl = new ReentrantLock();
        private static Condition conditionA = rl.newCondition();
        private static Condition conditionB = rl.newCondition();
        private static Condition conditionC = rl.newCondition();
    
        public void execute(String flag) {
            rl.lock();
            
            try {
                for (int i = 1 ; i <= 10 ; i++) {
                    if ("A".equals(flag)) {
                        System.out.println(Thread.currentThread().getName() + " - " + i);
                        conditionB.signal();
                        conditionA.await();
                    }
                    
                    if ("B".equals(flag)) {
                        System.out.println(Thread.currentThread().getName() + " - " + i);
                        conditionC.signal();
                        conditionB.await();
                    }
                    
                    if ("C".equals(flag)) {
                        System.out.println(Thread.currentThread().getName() + " - " + i);
                        conditionA.signal();
                        conditionC.await();
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                rl.unlock();
            }
        }
    }
    
    
    public static void main(String[] args) {
        final MyTask myTask = new MyTask();
        new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.execute("A");
            }
        }, "A").start();
        
        new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.execute("B");
            }
        }, "B").start();
        
        new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.execute("C");
            }
        }, "C").start();
    }
}