这是一道常见的面试题，如果遇到需要手写代码这种题，首先是要理清思路，再就是尽量体现出代码风格，关键次是顺序，
一、基于Semaphore
二、基于Synchronized
三、基于ReentrantLock

下面代码是用Semaphore的实现方式。

public static void main(String[] args) throws InterruptedException {
        Semaphore a = new Semaphore(1);
        Semaphore b  = new Semaphore(0);
        Semaphore c = new Semaphore(0);

        ExecutorService poolService = Executors.newFixedThreadPool(3);
        Integer count = 10;
        poolService.execute(new Worker(a, b, "A", count));
        poolService.execute(new Worker(b, c, "B", count));
        poolService.execute(new Worker(c, a, "C", count));

        Thread.sleep(1000);
        poolService.shutdownNow();
    }

    public static class Worker implements Runnable {
        private String key;
        private Semaphore current;
        private Semaphore next;
        private Integer count;
        public Worker(Semaphore current, Semaphore next, String key, Integer count) {
            this.current = current;
            this.next = next;
            this.key = key;
            this.count = count;
        }
        public void run() {
            for(int i = 0; i < count; i++) {
                try {
                    //获取当前的锁
                    current.acquire(); //current - 1
                    System.out.println(i+","+key);
                    //释放next的锁
                    next.release();    //next + 1
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

下面是Synchronized的实现

private static volatile Integer state = new Integer(1);

    public static void main(String[] args) throws InterruptedException {

        ExecutorService poolService = Executors.newFixedThreadPool(3);
        Integer count = 10;
        Object lock = new Object();
        poolService.execute(new Worker("A", count, lock));
        poolService.execute(new Worker("B", count, lock));
        poolService.execute(new Worker("C", count, lock));
        Thread.sleep(1000);
        poolService.shutdownNow();
    }

    public static class Worker implements Runnable {
        private String key;
        private Integer count;
        private Object lock;
        public Worker(String key, Integer count, Object lock) {
            this.key = key;
            this.count = count;
            this.lock  = lock;
        }
        public void run() {
            for(int i = 0; i < count; i++) {
                if(key == "A") {
                    synchronized (lock) {
                        while (state != 1) {
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        System.out.println(i+","+key);
                        state = 2;
                        lock.notifyAll();
                    }
                } else if(key == "B") {
                    synchronized (lock) {
                        while (state != 2) {
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        System.out.println(i+","+key);
                        state = 3;
                        lock.notifyAll();
                    }
                } else if (key == "C") {
                    synchronized (lock) {
                        while (state != 3) {
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        System.out.println(i+","+key);
                        state = 1;
                        lock.notifyAll();
                    }
                }
            }
        }
    }

下面是基于ReentrantLock实现

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class PrintABC3 {

    private static ReentrantLock lock = new ReentrantLock();
    private static Condition conditionA = lock.newCondition();
    private static Condition conditionB = lock.newCondition();
    private static Condition conditionC = lock.newCondition();
    private static volatile int state = 1;

    public static void main(String[] args) throws InterruptedException {

        ExecutorService poolService = Executors.newFixedThreadPool(3);
        Integer count = 10;
        poolService.execute(new Worker("A", count, 1, lock, conditionA, conditionB));
        poolService.execute(new Worker("B", count, 2, lock, conditionB, conditionC));
        poolService.execute(new Worker("C", count, 3, lock, conditionC, conditionA));
        Thread.sleep(5000);
        poolService.shutdownNow();
    }

    public static class Worker implements Runnable {
        private String key;
        private Integer count;
        private Lock lock;
        private Condition current;
        private Condition next;
        private int targetState;

        public Worker(String key, Integer count, int targetState, Lock lock, Condition cur, Condition next) {
            this.key = key;
            this.count = count;
            this.lock = lock;
            this.current = cur;
            this.next = next;
            this.targetState = targetState;
        }

        public void run() {
            this.lock.lock();
            try {
                for (int i = 0; i < count; i++) {
                    while (state != targetState) {
                        current.await();
                    }
                    System.out.println(i + "," + key);
                    state++;
                    if (state > 3) {
                        state = 1;
                    }
                    next.signal();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                this.lock.unlock();
            }
        }
    }
}