最近和人聊天得知这个经常在面试中被问到，而且说有一定难度，所以自己回来尝试写了一下。
基本思路如同阻塞队列，同一个锁，有两个Condition，依次打印，唤醒对方，阻塞自己释放锁。这里有一点要注意的就是 - 先唤醒对方，再阻塞自己，否则没有机会唤醒了。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class PrintInTurn {
    ReentrantLock lock = new ReentrantLock();
    Condition thisPrintcondition = lock.newCondition();
    Condition otherPrintcondition = lock.newCondition();
    

    public static void main(String[] args) {
        (new PrintInTurn()).start();
    }
    
    public void start() {
        PrintThread p1 = new PrintThread(lock, thisPrintcondition, otherPrintcondition);
        PrintThread p2 = new PrintThread(lock, otherPrintcondition, thisPrintcondition);
        (new Thread(p1)).start();
        (new Thread(p2)).start();
    }
    
    class PrintThread implements Runnable {
        ReentrantLock lock;
        Condition thisPrintcondition;
        Condition otherPrintcondition;
        
        public PrintThread(ReentrantLock lock, Condition thisPrintcondition, Condition otherPrintcondition) {
            this.lock = lock;
            this.thisPrintcondition = thisPrintcondition;
            this.otherPrintcondition = otherPrintcondition;
        }

        @Override
        public void run() {
            try {
                lock.lock();
                while(true) {           
                    System.out.println(Thread.currentThread().getName());
                    otherPrintcondition.signal();
                    thisPrintcondition.await();                 
                }
            }catch(Exception e) {
                e.printStackTrace();
            }finally {
                lock.unlock();
            }
        }
    }
}