前言
在《 ReferenceCountSet无锁实现 》中，详细介绍了如何在一个进程中实现一个无锁版本的ReferenceCountSet（或者说是在自己的代码里没有锁），但是最近遇到一个问题，如果是在分布式的环境中呢？如何实现这个引用计数？这个问题如果从头开始写，会是一个比较复杂的问题，在实际中，我们可以使用ZooKeeper设置时的version机制来实现，即CAS（Compare-And-Set）。这是一个本人在实际项目中遇到的一个问题，但是会更简单一些，因为在我们的项目中，我们使用GemFire，即已经有一个现成的分布式Map了（在Gemfire中叫做Region），所以问题简化成如果如何使用一个分布式Map实现引用计数？实现
如果对ConcurrentMap接口比较熟悉的话，这个其实是一个比较简单的问题。在ConcurrentMap中最主要的就是引入几个CAS相关的操作：
public
interface ConcurrentMap<K, V>
extends Map<K, V> {
V putIfAbsent(K key, V value);
boolean remove(Object key, Object value);
boolean replace(K key, V oldValue, V newValue);
V replace(K key, V value);
}
在《
ReferenceCountSet无锁实现
》中我们只需要使用putIfAbsent就可以了，剩下的实现可以交给AtomicInteger提供的CAS来实现，因为它是在同一个进程中，但是如果在分布式的环境中就不能使用这个AtomicInteger，这个时候应该怎么办呢？其实这个时候我们就可以求助于replace方法了。replace方法的注释中这样描述：
V putIfAbsent(K key, V value);
boolean remove(Object key, Object value);
boolean replace(K key, V oldValue, V newValue);
V replace(K key, V value);
}
/**
* Replaces the entry for a key only if currently mapped to a given value.
* This is equivalent to
* <pre>
* if (map.containsKey(key) && map.get(key).equals(oldValue)) {
* map.put(key, newValue);
* return true;
* } else return false;</pre>
* except that the action is performed atomically.
*
* @param key key with which the specified value is associated
* @param oldValue value expected to be associated with the specified key
* @param newValue value to be associated with the specified key
* @return <tt>true</tt> if the value was replaced
* @throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of a specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if a specified key or value is null,
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of a specified key
* or value prevents it from being stored in this map
*/
boolean replace(K key, V oldValue, V newValue);
* Replaces the entry for a key only if currently mapped to a given value.
* This is equivalent to
* <pre>
* if (map.containsKey(key) && map.get(key).equals(oldValue)) {
* map.put(key, newValue);
* return true;
* } else return false;</pre>
* except that the action is performed atomically.
*
* @param key key with which the specified value is associated
* @param oldValue value expected to be associated with the specified key
* @param newValue value to be associated with the specified key
* @return <tt>true</tt> if the value was replaced
* @throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of a specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if a specified key or value is null,
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of a specified key
* or value prevents it from being stored in this map
*/
boolean replace(K key, V oldValue, V newValue);
在ConcurrentMap的value中我们只需要给Integer，然后用replace去不断的尝试，即自己实现一个CAS：
private
int incrementRefCount(Object key) {
do {
Integer curCount = distributedMap.get(key);
if (curCount == null) {
curCount = distributedMap.putIfAbsent(key, new Integer(1));
if (curCount == null) {
return 1;
}
}
Integer newCount = new Integer(curCount.intValue() + 1);
if (distributedMap.replace(key, curCount, newCount)) {
return newCount;
}
} while ( true);
}
do {
Integer curCount = distributedMap.get(key);
if (curCount == null) {
curCount = distributedMap.putIfAbsent(key, new Integer(1));
if (curCount == null) {
return 1;
}
}
Integer newCount = new Integer(curCount.intValue() + 1);
if (distributedMap.replace(key, curCount, newCount)) {
return newCount;
}
} while ( true);
}
主要逻辑就是这样了，其实比较简单，只是之前没有遇到过这个问题，所以感觉可以记录下来。或许什么时候补充一下ZooKeeper版本的实现。
