确保并发操作结果可观测性：理解ConcurrentHashMap与线程同步

import java.util.Collections;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class ConcurrentMapTest {

    public static void main(String[] args) throws InterruptedException {
        Map<Integer, Integer> map = new ConcurrentHashMap<>();

        Runnable runnable = () -> {
            for (int i = 0; i < 1000; i++) {
                map.put(i, i); // 键值对为(0,0)到(999,999)
            }
        };

        ExecutorService executorService = Executors.newFixedThreadPool(4);
        for (int i = 0; i < 4; i++) {
            executorService.submit(runnable);
        }

        // 尝试等待一段时间，但无法保证所有任务完成
        // TimeUnit.SECONDS.sleep(1); 

        System.out.println("Map size: " + map.size());
        executorService.shutdown(); // 关闭线程池
    }
}

import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class ConcurrentMapCorrectTest {

    public static void main(String[] args) throws InterruptedException {
        Map<Integer, Integer> map = new ConcurrentHashMap<>();

        Runnable runnable = () -> {
            for (int i = 0; i < 1000; i++) {
                // 每个线程都尝试插入键值对 (0,0) 到 (999,999)
                // 由于键是固定的，最终map中只会保留每个键的一个值
                // 预期结果是1000个不同的键
                map.put(i, i); 
            }
        };

        ExecutorService executorService = Executors.newFixedThreadPool(4);

        try {
            // 将Runnable转换为Callable，并创建4个相同的Callable任务
            List<Callable<Object>> tasks = Collections.nCopies(4, Executors.callable(runnable));

            // invokeAll会阻塞，直到所有任务完成
            List<Future<Object>> futures = executorService.invokeAll(tasks);

            // 此时所有任务已完成，可以安全地检查map的大小
            System.out.println("Map size: " + map.size()); // 预期输出 1000
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            System.err.println("任务执行被中断: " + e.getMessage());
        } finally {
            executorService.shutdown(); // 关闭线程池
            // 建议等待线程池终止，确保所有资源释放
            if (!executorService.awaitTermination(60, TimeUnit.SECONDS)) {
                System.err.println("线程池未在指定时间内终止。");
            }
        }
    }
}