同步器的代码示例

本文提供四个Java并发同步器的代码示例及使用方法，帮助您理解Java多线程编程中的同步机制。

1. CountDownLatch：一次性屏障，协调线程

CountDownLatch允许一个或多个线程等待，直到其他线程完成一组操作。

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {
    public static void main(String[] args) throws InterruptedException {
        int workerCount = 3;
        CountDownLatch latch = new CountDownLatch(workerCount);

        for (int i = 0; i < workerCount; i++) {
            new Thread(() -> {
                try {
                    // 模拟工作
                    Thread.sleep(1000);
                    System.out.println("Worker " + Thread.currentThread().getName() + " finished.");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    latch.countDown(); // 完成任务后计数器减一
                }
            }).start();
        }

        System.out.println("Main thread waiting for workers to finish...");
        latch.await(); // 主线程等待所有计数器变为0
        System.out.println("All workers finished. Main thread proceeding.");
    }
}

2. Semaphore：控制对共享资源的访问

Semaphore管理一组许可证，控制对有限资源的访问。

import java.util.concurrent.Semaphore;

public class SemaphoreExample {
    public static void main(String[] args) {
        int permits = 2; // 可用许可证数量
        Semaphore semaphore = new Semaphore(permits);

        for (int i = 1; i <= 5; i++) {
            new Thread(() -> {
                try {
                    semaphore.acquire(); // 获取许可证
                    System.out.println("Thread " + Thread.currentThread().getName() + " acquired permit.");
                    // 使用共享资源
                    Thread.sleep(1000);
                    System.out.println("Thread " + Thread.currentThread().getName() + " releasing permit.");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    semaphore.release(); // 释放许可证
                }
            }).start();
        }
    }
}

3. CyclicBarrier：可重用的屏障点同步

CyclicBarrier在公共点（屏障）同步多个线程，所有线程到达屏障点后可重复使用。

import java.util.concurrent.CyclicBarrier;

public class CyclicBarrierExample {
    public static void main(String[] args) {
        int threadCount = 3;
        CyclicBarrier barrier = new CyclicBarrier(threadCount, () -> {
            System.out.println("All threads have reached the barrier. Proceeding...");
        });

        for (int i = 0; i < threadCount; i++) {
            new Thread(() -> {
                try {
                    // 模拟工作
                    Thread.sleep(1000);
                    System.out.println("Thread " + Thread.currentThread().getName() + " reached the barrier.");
                    barrier.await(); // 等待其他线程到达屏障
                    System.out.println("Thread " + Thread.currentThread().getName() + " passed the barrier.");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }).start();
        }
    }
}

4. Phaser：高级动态线程同步

Phaser类似于CyclicBarrier，但支持动态加入和离开线程。

import java.util.concurrent.Phaser;

public class PhaserExample {
    public static void main(String[] args) {
        Phaser phaser = new Phaser(1); // 注册“主方”

        for (int i = 0; i < 3; i++) {
            new Thread(() -> {
                System.out.println("Thread " + Thread.currentThread().getName() + " registered.");
                phaser.register(); // 注册线程
                try {
                    Thread.sleep(1000);
                    System.out.println("Thread " + Thread.currentThread().getName() + " arrived at phase " + phaser.getPhase());
                    phaser.arriveAndAwaitAdvance(); // 到达并等待其他线程
                    System.out.println("Thread " + Thread.currentThread().getName() + " passed phase " + phaser.getPhase());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    phaser.arriveAndDeregister(); // 完成并注销
                }
            }).start();
        }
        phaser.arriveAndAwaitAdvance(); // 主线程等待所有线程完成第一阶段
        System.out.println("All threads finished.");
    }
}

这些示例演示了每个同步器的基本用法。您可以修改线程数量和等待时间来观察同步行为的变化。 记住处理潜在的InterruptedException。