三线程交替打印必背模板（Java）| Lock+Condition+4步口诀

以下是Java实现三个线程交替打印ABC的代码，添加了便于快速记忆的注释和口诀，并附赠标题：

代码实现（使用ReentrantLock+Condition）

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

public class AlternatePrint {
    private final ReentrantLock lock = new ReentrantLock();
    private final Condition conditionA = lock.newCondition();
    private final Condition conditionB = lock.newCondition();
    private final Condition conditionC = lock.newCondition();
    private int turn = 1; // 1:A 2:B 3:C

    public static void main(String[] args) {
        AlternatePrint printer = new AlternatePrint();
        
        Thread t1 = new Thread(() -> printer.print("A", 1, conditionA, conditionB), "Thread-A");
        Thread t2 = new Thread(() -> printer.print("B", 2, conditionB, conditionC), "Thread-B");
        Thread t3 = new Thread(() -> printer.print("C", 3, conditionC, conditionA), "Thread-C");
        
        t1.start();
        t2.start();
        t3.start();
    }

    public void print(String str, int currentTurn, Condition self, Condition next) {
        for (int i = 0; i < 10; i++) { // 打印10轮
            lock.lock();
            try {
                // 不是当前线程的轮次，等待
                while (turn != currentTurn) {
                    self.await();
                }
                
                System.out.print(str); // 打印字符
                
                // 更新轮次，唤醒下一个线程
                turn = (currentTurn % 3) + 1;
                next.signal();
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            } finally {
                lock.unlock();
            }
        }
    }
}

记忆口诀

三个线程交替打，ReentrantLock来管控
三个Condition各管各，turn变量轮流转
自己轮次就打印，不是轮次就等待
打印完后更新轮，唤醒下一个线程

关键点记忆：

1. 锁与条件变量 ：
   • ReentrantLock 保证线程互斥
   • 三个 Condition（A、B、C）分别控制三个线程的等待和唤醒
2. 轮次控制 ：
   • turn 变量（1:A 2:B 3:C）指示当前轮到哪个线程执行
3. 核心逻辑 ：
   • 非当前轮次的线程调用 self.await() 等待
   • 当前轮次的线程打印后，更新 turn 并调用 next.signal() 唤醒下一个