package org.hibernate.util;
import org.apache.commons.collections.ReferenceMap;
import org.apache.commons.collections.LRUMap;
import java.io.Serializable;
import java.io.IOException;
/**
* Cache following a "Most Recently Used" (MRY) algorithm for maintaining a
* bounded in-memory size; the "Least Recently Used" (LRU) entry is the first
* available for removal from the cache.
* <p/>
* This implementation uses a "soft limit" to the in-memory size of the cache,
* meaning that all cache entries are kept within a completely
* {@link java.lang.ref.SoftReference}-based map with the most recently utilized
* entries additionally kept in a hard-reference manner to prevent those cache
* entries soft references from becoming enqueued by the garbage collector.
* Thus the actual size of this cache impl can actually grow beyond the stated
* max size bound as long as GC is not actively seeking soft references for
* enqueuement.
*
* @author Steve Ebersole
*/
public class SoftLimitMRUCache implements Serializable {
public static final int DEFAULT_STRONG_REF_COUNT = 128;
private final int strongReferenceCount;
// actual cache of the entries. soft references are used for both the keys and the
// values here since the values pertaining to the MRU entries are kept in a
// seperate hard reference cache (to avoid their enqueuement/garbage-collection).
private transient ReferenceMap softReferenceCache = new ReferenceMap( ReferenceMap.SOFT, ReferenceMap.SOFT );
// the MRU cache used to keep hard references to the most recently used query plans;
// note : LRU here is a bit of a misnomer, it indicates that LRU entries are removed, the
// actual kept entries are the MRU entries
private transient LRUMap strongReferenceCache;
public SoftLimitMRUCache() {
this( DEFAULT_STRONG_REF_COUNT );
}
public SoftLimitMRUCache(int strongRefCount) {
this.strongReferenceCount = strongRefCount;
init();
}
public synchronized Object get(Object key) {
Object result = softReferenceCache.get( key );
if ( result != null ) {
strongReferenceCache.put( key, result );
}
return result;
}
public synchronized Object put(Object key, Object value) {
softReferenceCache.put( key, value );
return strongReferenceCache.put( key, value );
}
public synchronized int size() {
return strongReferenceCache.size();
}
public synchronized int softSize() {
return softReferenceCache.size();
}
private void init() {
strongReferenceCache = new LRUMap( strongReferenceCount );
}
private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
init();
}
public synchronized void clear() {
strongReferenceCache.clear();
softReferenceCache.clear();
}
}
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