java - 使用并发 HashMap 时结果不一致

Question

I have a class Counter that has multiple endpoints. I want to retain the number of "visits" for each endpoint. The endpoints can be accessed from different threads and I decided to use ConcurrentHashMap.

这是我的代码，我创建了一个模拟此行为的类:

public class Counter {

    Map<String, Integer> endpoints = new ConcurrentHashMap<>();

    void load(String endpoint) {

        endpoints.put(endpoint, endpoints.get(endpoint) + 1);
    }

    void accessEndpoint(String endpoint, int times, int numberOfThreads) throws InterruptedException {

        ExecutorService executor = Executors.newFixedThreadPool(numberOfThreads);

        for (int i = 0; i < times; i++) {
            executor.submit(() -> load(endpoint));
        }

        executor.shutdown();
        executor.awaitTermination(1, TimeUnit.MINUTES);

    }

    public static void main(String[] args) throws InterruptedException {

        Counter counter = new Counter();
        counter.endpoints.put("www.google.com", 2);
        counter.accessEndpoint("www.google.com", 100, 10);
        System.out.println(counter.endpoints.get("www.google.com"));
    }
}

输出不一致。

预期:102

实际:95、100、102、66、100

Answer 1

endpoints.put(endpoint, endpoints.get(endpoint) + 1); 不是原子操作。因此，两个线程可以 get() 相同的数字(例如 100)，并放相同的数字(101)，从而使两次调用算作一次。

使用compute()代替，它是原子的:

endpoints.compute(endpoint, (k,v)-> v+1);

或者如果您想始终从零开始而不进行初始化。

endpoints.merge(endpoint, 1, Integer::sum);