如何以线程安全的方式清理 ConcurrentHashMap

Question

我有一个 ConcurrentHashMap oricginalCahce，其中多个线程从代码的许多地方同时写入。 我必须定期刷新 map，所以我在下面创建了 Map 的临时副本并清理了原始 Map

void flushcopyOfOriginalCache(String queueName) {
        Map<Integer, Date> copyOfOriginalCache = null;
        Object lock = getLastscheduledTimeUpdateLock(queueName);
        //this lock is not aquired at the time of writing as the writing in taking place in many places of the code
        synchronized (lock) {
        //rechecking for whether expired or not
            if (isOrigCacheExpired(queueName)) {
                log.info("origCache Expired for queue {}", queueName);
                if (orignalCache.containsKey(queueName)) {
                    copyOfOriginalCache = new ConcurrentHashMap<>(
                            orignalCache.get(queueName));
                    //How safe is below clean ??? as parallel writes might be going on orignalCache!
                    orignalCache.get(queueName).clear();
                }

            }
        }

        if (copyOfOriginalCache == null || copyOfOriginalCache.isEmpty()) {
            return;
        }
        //Below is expenssive DB operations on with data in copyOfOriginalCache  

        }

如何确保这个干净的方法不应该删除正在写入的条目。 请指导。

Answer 1

对于 ConcurrentHashMap，您可以使用 computeIfPresent 和谓词canDeleted(Value v)

concurrentHashmap.forEach((key, value) -> 
    concurrentHashmap.computeIfPresent(key, (t, s) -> (canDeleted(s) ? null : s)))

这是线程安全的并且易于实现。