方法一：使用synchronized和wait/notify

package com.demo;import java.util.concurrent.CompletableFuture;public class PrintABC {// 當前狀態private static String state = "A";// 共享鎖對象private static final Object lock = new Object();public static void main(String[] args) {Thread t1 = new Thread(() -> {synchronized (lock){System.out.println("current: A; state: " + state);// 等待while (!"A".equals(state)){try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("A: " + Thread.currentThread());// 修改B狀態state = "B";// 喚醒下一個lock.notifyAll();}});Thread t2 = new Thread(() -> {synchronized (lock){System.out.println("current: B; state: " + state);// 等待while (!"B".equals(state)){try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("B: " + Thread.currentThread());// 修改B狀態state = "C";// 喚醒下一個lock.notifyAll();}});Thread t3 = new Thread(() -> {synchronized (lock){System.out.println("current: C; state: " + state);// 等待while (!"C".equals(state)){try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("C: " + Thread.currentThread());// 修改B狀態state = "";// 喚醒下一個lock.notifyAll();}});t1.start();t2.start();t3.start();}
}

輸出

current: A; state: A
A: Thread[Thread-0,5,main]
current: C; state: B
current: B; state: B
B: Thread[Thread-1,5,main]
C: Thread[Thread-2,5,main]

方法二：使用CompletableFuture實現

package com.demo;import java.util.concurrent.CompletableFuture;public class PrintABC {public static void main(String[] args) {CompletableFuture<Void> t1 = CompletableFuture.runAsync(() -> {System.out.println("A: " + Thread.currentThread());});CompletableFuture<Void> t2 = t1.thenRunAsync(() -> {System.out.println("B: " + Thread.currentThread());});t2.thenRunAsync(()->{System.out.println("C: " + Thread.currentThread());});}
}

輸出

A: Thread[ForkJoinPool.commonPool-worker-1,5,main]
B: Thread[ForkJoinPool.commonPool-worker-1,5,main]
C: Thread[ForkJoinPool.commonPool-worker-2,5,main]

參考

華為OD面試：三個線程交替打印ABC如何實現？
面試題：三個線程如何交替打印ABC100次