多个线程正在尝试以毫秒为键进行写入，但是在 ConcurrentHashMap 中创建了许多键而不是一个键

Question

ConcurrentHashMap<Long, CopyOnWriteArrayList<Observation>
mostRecentObservationDB = new ConcurrentHashMap<Long,
    CopyOnWriteArrayList<Observation>(524288, 0.75f, 32);

这是我的地图。我正在尝试同时使用多个线程进行读写，但不知何故它总是创建多个键。

long timeInMilliseconds = System.currentTimeMillis();

if (/* the last key older than 10 seconds comparing to the new key*/) {
    CopyOnWriteArrayList<Observation> initializingObservation = new CopyOnWriteArrayList<>();
    initializingObservation.add(obs);

    mostRecentObservationDB.putIfAbsent(timeInMilliseconds, initializingObservation);
} else {
  // Update
}

Seperate Thread，通过删除早于 10 秒的键来过滤此哈希映射。

while (true) {
    try {
        Thread.sleep(4000);
        if(/*Time (key) older than 10 seconds*/) {
            mostRecentObservationDB.remove(key);
        }

    } catch (Exception e) {

    }
}

问题是删除密钥后，它会在初始化时创建多个密钥。这是我的日志。

key -> 1501779153776, value
key -> 1501779153826, value
key -> 1501779153876, value
key -> 1501779153896, value

我希望在删除操作时将它们存储为一个键。这是它应该如何存储。

key -> 1501779153776, value

但是，当我从中读取然后通过 remove() 方法删除所有条目时，我希望在读取地图内容然后删除的过程中没有其他线程写入地图他们。

这是行为异常的代码：

public static void main(String[] args) {
    ConcurrentHashMap<Long, String> tenSecondBucket =
        new ConcurrentHashMap<Long, String>();

    Thread writingThread = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                try {
                    Thread.sleep(1);

                    if(tenSecondBucket.size() > 0) {
                        // getting last key
                        long lastKey = 0;
                        for (long keyValue : tenSecondBucket.keySet()) {
                            lastKey = keyValue;
                        }

                        if(System.currentTimeMillis() - lastKey > 10000) {
                            tenSecondBucket.put(System.currentTimeMillis(), "secondEntry");
                        } else {
                            tenSecondBucket.put(lastKey, "updatedEntry");
                        }
                    } else {
                        tenSecondBucket.put(System.currentTimeMillis(), "newEntry");
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    });

    writingThread.start();

    Thread removingThread = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                try {
                    Thread.sleep(4000);

                    if(tenSecondBucket.size() > 0) {
                        tenSecondBucket.keySet().stream().forEach(key -> {
                            if(System.currentTimeMillis() - key > 10000) {
                                tenSecondBucket.remove(key);
                            }
                        });
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    });

    removingThread.start();

    Thread readingThread = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                try {
                    Thread.sleep(4000);

                    if(tenSecondBucket.size() > 0) {
                        tenSecondBucket.keySet().stream().forEach(key -> {
                            System.out.println("testing key which is timestamp " + key);
                        });
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    });

    readingThread.start();
}

Answer 1

问题在于您推导 "lastKey" 的方式。您似乎需要地图中存在的最高时间值，并且您假设它将是 tenSecondBucket.keySet() 的最后一个条目。但是，keySet() 方法 returns 一个 Set，它本质上是无序的（无论如何，映射不维护有序的键列表）

所以你需要替换这段代码-

long lastKey = 0;
for (long keyValue : tenSecondBucket.keySet()) {
     lastKey = keyValue;
}

使用此代码 -

long lastKey = 0;
for (long keyValue : tenSecondBucket.keySet()) {
    lastKey = keyValue > lastKey? keyValue : lastKey;
}

更改后，代码可以正常工作
请注意，尽管代码仍然有 improvement/refactoring

的空间

多个线程正在尝试以毫秒为键进行写入，但是在 ConcurrentHashMap 中创建了许多键而不是一个键

Mulitple threads are trying to write with milliseconds as key, but instead of one key many keys are created in ConcurrentHashMap

java

concurrency

concurrenthashmap