学习笔记：实现线程轮流打印的几种方式

一、引言

在多线程编程中，线程间的同步和通信是一个重要的主题。本文将探讨如何使用Java实现两个线程按照一定顺序交替打印字符A和B。

二、方法概述

1. 使用java.util.concurrent.locks.ReentrantLock与Condition
2. 使用synchronized关键字
3. 使用Semaphore

三、具体实现

1. 使用ReentrantLock与Condition

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

public class Data {
    private final Lock lock = new ReentrantLock();
    private final Condition conditionA = lock.newCondition();
    private final Condition conditionB = lock.newCondition();
    private int flag = 0;

    public void printA() throws InterruptedException {
        lock.lock();
        try {
            while (flag != 0) {
                conditionA.await();
            }
            System.out.println("当前线程：" + Thread.currentThread().getName() + ", 打印：A, 当前flag: " + flag);
            flag = 1;
            conditionB.signal();
        } finally {
            lock.unlock();
        }
    }

    public void printB() throws InterruptedException {
        lock.lock();
        try {
            while (flag != 1) {
                conditionB.await();
            }
            System.out.println("当前线程：" + Thread.currentThread().getName() + ", 打印：B, 当前flag: " + flag);
            flag = 0;
            conditionA.signal();
        } finally {
            lock.unlock();
        }
    }
}

2. 使用synchronized关键字

public class Data {
    private int flag = 0;

    public synchronized void printA() throws InterruptedException {
        while (flag != 0) {
            this.wait();
        }
        System.out.println("当前线程：" + Thread.currentThread().getName() + ", 打印：A, 当前flag: " + flag);
        flag = 1;
        this.notifyAll();
    }

    public synchronized void printB() throws InterruptedException {
        while (flag != 1) {
            this.wait();
        }
        System.out.println("当前线程：" + Thread.currentThread().getName() + ", 打印：B, 当前flag: " + flag);
        flag = 0;
        this.notifyAll();
    }
}

3. 使用Semaphore

import java.util.concurrent.Semaphore;

public class MethodThree {
    public static void main(String[] args) {
        Semaphore semaphoreA = new Semaphore(0); // A开始时等待
        Semaphore semaphoreB = new Semaphore(1); // B开始时允许执行

        Thread threadA = new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    semaphoreA.acquire();
                    System.out.println("当前线程：" + Thread.currentThread().getName() + ", 打印：A");
                    semaphoreB.release();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "Thread-A");

        Thread threadB = new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    semaphoreB.acquire();
                    System.out.println("当前线程：" + Thread.currentThread().getName() + ", 打印：B");
                    semaphoreA.release();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "Thread-B");

        threadA.start();
        threadB.start();
    }
}

四、总结

以上三种方法都可以实现线程之间的有序交互。选择哪种方法取决于实际需求以及对性能和可读性的考虑。使用synchronized和ReentrantLock提供了一种更直接的方式去控制线程的执行顺序，而Semaphore则提供了基于许可的控制机制，适用于更复杂的场景。