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package oracle.toplink.essentials.internal.identitymaps;
import java.util.*;
import oracle.toplink.essentials.exceptions.*;
/**
* <p><b>Purpose</b>: A fixed size LRU cache<p>
* Using a linked list as well as the hashtable from the superclass a LRU cache is maintained.
* When a get is executed the LRU list is updated and when a new object is inserted the object
* at the start of the list is deleted (provided the maxSize has been reached).
* <p><b>Responsibilities</b>:<ul>
* <li> Guarantees identity through primary key values
* <li> Keeps the LRU linked list updated.
* </ul>
* @since TOPLink/Java 1.0
*/
public class CacheIdentityMap extends FullIdentityMap {
/** Provide handles on the linked list */
protected LinkedCacheKey first;
/** Provide handles on the linked list */
protected LinkedCacheKey last;
/**
* Initialize newly instantiated CacheIdentityMap.
* @param size int The size of the Cache
*/
public CacheIdentityMap(int size) {
super(size);
this.first = new LinkedCacheKey(new Vector(2), null, null, 0);
this.last = new LinkedCacheKey(new Vector(2), null, null, 0);
this.first.setNext(this.last);
this.last.setPrevious(this.first);
}
public CacheKey createCacheKey(Vector primaryKey, Object object, Object writeLockValue, long readTime) {
return new LinkedCacheKey(primaryKey, object, writeLockValue, readTime);
}
/**
* Reduces the size of the receiver down to the maxSize removing objects from the
* start of the linked list.
*/
protected void ensureFixedSize() {
// protect the case where someone attempts to break the cache by
// setting max size to 0.
synchronized(this.first) {
while (getMaxSize() > 0 && getSize() > getMaxSize()) {
remove(last.getPrevious());
}
}
}
/**
* Access the object within the table for the given primaryKey.
* Move the accessed key to the top of the order keys linked list to maintain LRU.
* @param aVector is the primary key for the object to search for.
* @return The LinkedCacheKey or null if none found for primaryKey
*/
protected CacheKey getCacheKey(Vector primaryKeys) {
LinkedCacheKey cacheKey = (LinkedCacheKey)super.getCacheKey(primaryKeys);
if (cacheKey != null) {
synchronized (this.first) {
removeLink(cacheKey);
insertLink(cacheKey);
}
}
return cacheKey;
}
/**
* Insert a new element into the linked list of LinkedCacheKeys.
* New elements (Recently Used) are added at the end (last).
* @return The added LinkedCacheKey
*/
protected LinkedCacheKey insertLink(LinkedCacheKey key) {
if (key == null){
return key;
}
// no sence on locking the entire cache, just lock on the list
synchronized (this.first){
this.first.getNext().setPrevious(key);
key.setNext(this.first.getNext());
key.setPrevious(this.first);
this.first.setNext(key);
}
return key;
}
/**
* Store the object in the identity map with the linked cache key
*/
protected void put(CacheKey cacheKey) {
super.put(cacheKey);
insertLink((LinkedCacheKey)cacheKey);
ensureFixedSize();
}
/**
* Remove the LinkedCacheKey from the cache as well as from the linked list.
* @return The LinkedCacheKey to be removed
*/
public Object remove(CacheKey key) {
super.remove(key);
// CR2408
if (key == null) {
Class cacheItemClass = null;
// Get the class of the CacheKey which we could not remove
// (if possible) for client debugging purposes.
// We can't get the descriptor, because we don't know the session.
if (!getCacheKeys().isEmpty()) {
CacheKey aKey = (CacheKey)getCacheKeys().keys().nextElement();
if ((aKey != null) && (aKey.getObject() != null)) {
cacheItemClass = aKey.getObject().getClass();
}
}
throw ValidationException.nullCacheKeyFoundOnRemoval(this, cacheItemClass);
}
return removeLink((LinkedCacheKey)key).getObject();
}
/**
* Remove the LinkedCacheKey from the linked list.
* @return The removed LinkedCacheKey
*/
protected LinkedCacheKey removeLink(LinkedCacheKey key) {
if (key == null){
return key;
}
synchronized (this.first) {
if (key.getPrevious() == null || key.getNext() == null){
//already removed by a competing thread, just return
return key;
}
key.getPrevious().setNext(key.getNext());
key.getNext().setPrevious(key.getPrevious());
key.setNext(null);
key.setPrevious(null);
}
return key;
}
/**
* INTERNAL:
* This method will be used to update the max cache size, any objects exceeding the max cache size will
* be remove from the cache. Please note that this does not remove the object from the identityMap, except in
* the case of the CacheIdentityMap.
*/
public synchronized void updateMaxSize(int maxSize) {
setMaxSize(maxSize);
ensureFixedSize();
}
}
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