java：并发集合(java: concurrent collections)

 我正在尝试找到一个或多个要使用的并发集合，我可以实现以下行为（这些名称是出于类比目的而设计的）： 
 
/**
 * Acts as a broker for a concurrent hash map that stores its keys in order 
 * of submission. At shipping time, the concurrent map is "sealed" 
 * (picture a truck with its cargo door being closed)
 * and its contents presented as an immutable map, and is replaced 
 * by a new concurrent map ready to accept values.
 *
 * Consumers of this class that submit information to it, are expected to 
 * know that this contains a concurrent collection, and should use the 
 * compareAndSet paradigm, e.g. the following:
 *
 * LoadingDock loadingDock = ...
 * boolean done = false;
 * while (!done)
 * {
 *    V oldValue = loadingDock.get();
 *    V newValue = computeNewValue(oldValue, otherInformation);
 *    if (oldValue == null)
 *       done = loadingDock.putIfAbsent(newValue) == null;
 *    else
 *       done = loadingDock.replace(oldValue, newValue) == oldValue;
 * }
 *    
 *
 * Keys and values must be non-null. Keys are not ordered.
 */
class LoadingDock<K,V>
{
    /**
     * analogous to ConcurrentMap's replace, putIfAbsent, and get methods
     */
    public boolean replace(K key, V oldValue, V newValue);
    public V putIfAbsent(K key, V value);
    public V get(K key)

    /* see above */
    public Map<K,V> ship();
}
 
 我有两个问题。 
 
 一个是Java和Guava都不包含ConcurrentLinkedHashMap。 这让我想知道为什么不 - 也许我错过了这样一个野兽的微妙之处。 看起来我可以通过使用一个类来装饰ConcurrentHashMap来自己创建一个，如果putIfAbsent()并且返回null，那么该类会向列表添加一个键 - 我不需要在ConcurrentHashMap中使用任何其他方法。上面，除了通过调用putIfAbsent()之外，没有办法向地图添加新密钥。 
 
 另一个更隐蔽的问题是，我似乎无法想到如何在不阻塞同步的情况下实现ship() - 当调用ship（）时，LoadingDock需要将所有新调用指向新映射，并且可以返回旧映射，直到确定所有并发写入完成为止。 （否则我只会使用AtomicReference来保存并发映射。） 
 
 有没有办法做到这一点无需同步？ 

I'm trying to find one or more concurrent collections to use that I can implement the following behavior (the names are contrived for analogy purposes):
 
/**
 * Acts as a broker for a concurrent hash map that stores its keys in order 
 * of submission. At shipping time, the concurrent map is "sealed" 
 * (picture a truck with its cargo door being closed)
 * and its contents presented as an immutable map, and is replaced 
 * by a new concurrent map ready to accept values.
 *
 * Consumers of this class that submit information to it, are expected to 
 * know that this contains a concurrent collection, and should use the 
 * compareAndSet paradigm, e.g. the following:
 *
 * LoadingDock loadingDock = ...
 * boolean done = false;
 * while (!done)
 * {
 *    V oldValue = loadingDock.get();
 *    V newValue = computeNewValue(oldValue, otherInformation);
 *    if (oldValue == null)
 *       done = loadingDock.putIfAbsent(newValue) == null;
 *    else
 *       done = loadingDock.replace(oldValue, newValue) == oldValue;
 * }
 *    
 *
 * Keys and values must be non-null. Keys are not ordered.
 */
class LoadingDock<K,V>
{
    /**
     * analogous to ConcurrentMap's replace, putIfAbsent, and get methods
     */
    public boolean replace(K key, V oldValue, V newValue);
    public V putIfAbsent(K key, V value);
    public V get(K key)

    /* see above */
    public Map<K,V> ship();
}
 
I'm having two issues with this. 
 
One is that neither Java nor Guava contain a ConcurrentLinkedHashMap. This makes me wonder why not -- maybe I'm missing the subtleties of such a beast. It looks like I could just make one myself by decorating a ConcurrentHashMap with a class that adds a key to a list if putIfAbsent() is ever called and returns null -- I don't need any of the other methods in ConcurrentHashMap beyond the ones above, so there's no way to add a new key to the map except through a call to putIfAbsent().
 
The other, more insidious issue, is that I can't seem to think of how to implement ship() without blocking synchronization -- when ship() is called, the LoadingDock needs to direct all new calls to the new map, and can't return the old map until it is certain all of the concurrent writes are done. (Otherwise I would just use AtomicReference to hold the concurrent map.)
 
Is there a way to do this w/o having to synchronize?

原文：https://stackoverflow.com/questions/9215418