Openfire Cache
Openfire作为一个聊天服务器，已经把调用频繁的接口进行了缓存。例如userManager中的userCache，rosterManager 中的rosterCache，曾经让我很郁闷的vcardCache等。
想吃掉这个总是捣乱的Openfire，唯一的办法就是搞懂他，迭代空挡，整理一下Openfire的缓存机制。
interface Cache<K,V> extends java.util.Map<K,V>
提供了基本的缓存接口，默认是通过DefaultCache类实现的。该类提供了cache操作的基本方法：put、get、remove、getCacheSize等。
Openfire通过CacheFactory管理cache创建，提供了一个统一的创建和使用Cache的工厂。
/**
* Storage for all caches that get created.
*/
private static Map<String, Cache> caches = new ConcurrentHashMap<String, Cache>();
/**
* This map contains property names which were used to store cache configuration data
* in local xml properties in previous versions.
*/
private static final Map<String, String> cacheNames = new HashMap<String, String>();
/**
* Default properties to use for local caches. Default properties can be overridden
* by setting the corresponding system properties.
*/
private static final Map<String, Long> cacheProps = new HashMap<String, Long>();
以上三个变量分别存储所有已创建的cache，已创建的cache的名称，已创建cache的属性。需要注意的是对Cache的操作需要考虑线程的同步和互斥，Openfire采用了ConcurrentHashMap来保证线程安全。
放入Cache的对象要实现Cacheable接口，可以看出，这是一个序列化的接口。并实现getCachedSize()方法，返回放入对象的size。
public interface Cacheable extends java.io.Serializable {
/**
* Returns the approximate size of the Object in bytes. The size should be
* considered to be a best estimate of how much memory the Object occupies
* and may be based on empirical trials or dynamic calculations.<p>
*
* @return the size of the Object in bytes.
*/
public int getCachedSize() throws CannotCalculateSizeException;
}
例如rosterCache中的缓存对象Roster，其计算cachedSize方法如下：
public int getCachedSize() throws CannotCalculateSizeException {
// Approximate the size of the object in bytes by calculating the size
// of the content of each field, if that content is likely to be eligable for
// garbage collection if the Roster instance is dereferenced.
int size = 0;
size += CacheSizes.sizeOfObject(); // overhead of object
size += CacheSizes.sizeOfCollection(rosterItems.values()); // roster item cache
size += CacheSizes.sizeOfString(username); // username
// implicitFrom
for (Map.Entry<String, Set<String>> entry : implicitFrom.entrySet()) {
size += CacheSizes.sizeOfString(entry.getKey());
size += CacheSizes.sizeOfCollection(entry.getValue());
}
return size;
}
Roster对象的size用CacheSizes.sizeOfObject()、CacheSizes.sizeOfCollection(rosterItems.values())、CacheSizes.sizeOfString(username)等来计算。
CacheFactoryStrategy是Openfire缓存策略的接口，默认实现是DefaultLocalCacheStrategy，该实现不支持集群。另外一个实现类：ClusteredCacheFactory则是缓存集群的解决方案。
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