ConcurrentHashMap等待密钥可能吗？ [英] ConcurrentHashMap wait for key possible?

问题描述

我有多线程通信。
1线程正在将数据分发到其他线程。

i have multithread communication. 1 Thread is dispatching datas to other threads.

主线程正在推送数据：

Main线程：
ConcurrentHashMap map = Global.getInstance（）。getMap（）;
//将数据推送到其他线程
map.put（1， Test）;

Main Thread: ConcurrentHashMap map = Global.getInstance().getMap(); //push data to some other thread map.put(1,"Test");

线程1：
字符串数据= map.get（1）;
//直接返回null，但是我要等到推送数据

Thread 1: String data = map.get(1); //returns null directly , but i want to wait until data pushed

如果主线程不推送任何数据，线程1返回null。
但是我要等到获得数据后，才能等待
呢？

Thread 1 returns null if main thread doesn't push any data. But i want to wait until i got data , how can i wait ?

TransferQueue对于我当前的实现不是一个好的解决方案。
我和ConcurrentHashMap有关。

TransferQueue is not good solution to my current implementation. I have to do with ConcurrentHashMap.

有人知道任何解决方案吗？

Does someone know any solution?

推荐答案

您可以创建一个BlockingMap，如下所示：根据使用情况，还应该使用一种机制来删除未使用的键和与其关联的队列，以避免内存泄漏。

You can create a BlockingMap, something like this; depending on usage, you should also device a mechanism to remove unused keys and queues associated to them in order to avoid a memory leak.

public class BlockingMap<K, V> {
    private final Map<K, BlockingQueue<V>> map = new ConcurrentHashMap<>();

    private synchronized BlockingQueue<V> ensureQueueExists(K key) {
        //concurrentMap.putIfAbsent would require creating a new
        //blocking queue each time put or get is called
        if (map.containsKey(key)) {
            return map.get(key);
        } else {
            BlockingQueue<V> queue = new ArrayBlockingQueue<>(1);
            map.put(key, queue);
            return queue;
        }
    }

    public boolean put(K key, V value, long timeout, TimeUnit timeUnit) {
        BlockingQueue<V> queue = ensureQueueExists(key);
        try {
            return queue.offer(value, timeout, timeUnit);
        } catch (InterruptedException e) {
            throw new IllegalStateException(e);
        }
    }

    public V get(K key, long timeout, TimeUnit timeUnit) {
        BlockingQueue<V> queue = ensureQueueExists(key);
        try {
            return queue.poll(timeout, timeUnit);
        } catch (InterruptedException e) {
            throw new IllegalStateException(e);
        }
    }
}

自Java 8起，<$可以将c $ c> ensureQueueExists 编写为：

Since Java 8, ensureQueueExists can be written:

private synchronized BlockingQueue<V> ensureQueueExists(K key) {
    return map.computeIfAbsent(key, k -> new ArrayBlockingQueue<>(1));
}

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