组合putIfAbsent并替换为ConcurrentMap

问题描述

我有一个usecase，我必须

I have a usecase where I have to

• 如果ConcurrentHashMap中不存在该键，则插入一个新值


• 如果
  中的键已经存在，则使用新值替换旧值ConcurrentHashMap，其中新值从旧值派生（不是昂贵的操作）

我有以下代码：

public void insertOrReplace(String key, String value) {
        boolean updated = false;
        do {
            String oldValue = concurrentMap.get(key);
            if (oldValue == null) {
                oldValue = concurrentMap.putIfAbsent(key, value);
                if (oldValue == null) {
                    updated = true;
                }
            }
            if (oldValue != null) {
                final String newValue = recalculateNewValue(oldValue, value);
                updated = concurrentMap.replace(key, oldValue, newValue);
            }
        } while (!updated);
    }

你认为这是正确的和线程安全的吗？

Do you think it's correct and thread-safe?

有更简单的方法吗？

推荐答案

下面是相当于你的。我已经压力测试了一些与成千上万的线程并发访问它：它的工作原理，与一些重试（循环）执行（显然，你永远不能证明正确性在并发世界中的测试）。

You could make it a little shorter with the code below which is equivalent to yours. I have stress tested it a little with thousands of threads accessing it concurrently: it works as expected, with a number of retries (loops) being performed (obviously, you can never prove correctness with testing in the concurrent world).

public void insertOrReplace(String key, String value) {
    for (;;) {
        String oldValue = concurrentMap.putIfAbsent(key, value);
        if (oldValue == null)
            return;

        final String newValue = recalculateNewValue(oldValue, value);
        if (concurrentMap.replace(key, oldValue, newValue))
            return;
    }
}

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