控制三个线程按顺序交替输出数字123123…

synchronized（配合专用锁对象）

通过共享锁和 volatile 变量控制执行顺序，每个线程按指定顺序打印指定内容，确保输出序列如 “123123…”。使用 synchronized 和 wait/notifyAll 实现线程间协作。

/**
 * AlternateOutput类用于实现三个线程交替输出数字的功能
 * 通过使用synchronized关键字和wait/notify机制，确保三个线程按照1->2->3的顺序循环输出
 */
public class AlternateOutput {

    /**
     * 用于线程同步的锁对象
     */
    private static final Object lock = new Object();

    /**
     * 当前应该执行的线程编号
     */
    private static int currentThreadNum = 1;

    /**
     * 每个线程需要执行的循环次数
     */
    private static final int MAX_LOOP = 10;

    /**
     * 线程执行的任务方法，负责按照指定顺序输出内容
     * @param curThreadNum 当前线程的编号，用于判断是否轮到当前线程执行
     * @param nextThreadNum 下一个应该执行的线程编号，执行完当前任务后更新此值
     * @param output 当前线程需要输出的内容
     */
    private static void runTask(int curThreadNum, int nextThreadNum, String output) {
        // 循环执行指定次数的任务
        for (int i = 0; i < AlternateOutput.MAX_LOOP; i++) {
            // 使用synchronized块确保线程安全
            synchronized (lock) {
                // 等待直到轮到当前线程执行
                while (AlternateOutput.currentThreadNum != curThreadNum) {
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                        return;
                    }
                }
                // 输出当前线程的内容
                System.out.print(output);
                // 更新下一个应该执行的线程编号
                currentThreadNum = nextThreadNum;
                // 唤醒所有等待的线程
                lock.notifyAll();
            }
        }
    }

    /**
     * 程序入口点，创建并启动三个线程实现交替输出
     * @param args 命令行参数
     */
    public static void main(String[] args) {
        // 创建第一个线程，负责输出"1"
        Thread t1 = new Thread(() -> {
            runTask(1, 2, "1");
        });
        t1.start();
        
        // 创建第二个线程，负责输出"2"
        Thread t2 = new Thread(() -> {
            runTask(2, 3, "2");
        });
        t2.start();

        // 创建第三个线程，负责输出"3"
        Thread t3 = new Thread(() -> {
            runTask(3, 1, "3");
        });
        t3.start();

        // 等待所有线程执行完成
        try {
            t1.join();
            t2.join();
            t3.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

ReentrantLock和Condition

通过ReentrantLock和Condition控制执行顺序，确保按1→2→3循环打印10次。每个线程等待其编号轮次，执行打印后唤醒下一个线程。

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

public class PrintNumber {

    public static final int MAX_LOOP = 10;

    public static int currentNum = 1;

    public static final ReentrantLock lock = new ReentrantLock();

    public static final Condition c1 = lock.newCondition();
    public static final Condition c2 = lock.newCondition();
    public static final Condition c3 = lock.newCondition();

    public static void main(String[] args) {
        Thread t1 = new Thread(new PrintNumberTask(1));
        Thread t2 = new Thread(new PrintNumberTask(2));
        Thread t3 = new Thread(new PrintNumberTask(3));

        t1.start();
        t2.start();
        t3.start();
    }
}

import java.util.concurrent.locks.Condition;

public class PrintNumberTask implements Runnable {

    private int number;

    public PrintNumberTask(int num) {
        this.number = num;
    }

    private void printAndSwith(String output, int nextNum, Condition awaitCondition, Condition signalCondition)
            throws InterruptedException {
        while (PrintNumber.currentNum != this.number) {
            awaitCondition.await();
        }
        System.out.print(output);
        PrintNumber.currentNum = nextNum;
        signalCondition.signal();
    }

    @Override
    public void run() {
        for (int i = 0; i < PrintNumber.MAX_LOOP; i++) {
            PrintNumber.lock.lock();
            try {
                switch (number) {
                    case 1:
                        printAndSwith("1", 2, PrintNumber.c1, PrintNumber.c2);
                        break;
                    case 2:
                        printAndSwith("2", 3, PrintNumber.c2, PrintNumber.c3);
                        break;
                    case 3:
                        printAndSwith("3", 1, PrintNumber.c3, PrintNumber.c1);
                        break;
                    default:
                        break;
                }
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                PrintNumber.lock.unlock();
            }
        }
    }
}