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package com.sun.appserv.util.cache;
import java.util.ArrayList;
import java.util.Map;
import java.util.Properties;
import java.util.ResourceBundle;
/**
* LRUCache
* in-memory bounded cache with an LRU list
*/
public class LruCache extends BaseCache {
// the item never expires
public static final long NO_TIMEOUT = -1;
// LRU list
protected LruCacheItem head;
protected LruCacheItem tail;
// the number of times the cache was trimmed
protected int trimCount;
protected int listSize;
protected long timeout = NO_TIMEOUT;
protected int defaultMaxEntries = Constants.DEFAULT_MAX_ENTRIES;
protected boolean isUnbounded = false;
/**
* default constructor
*/
public LruCache() { }
/**
* constructor with specified max entries.
* @param defaultMaxEntries specifies the default max entries
* to use when the maxEntries is <= 0.
*/
public LruCache(int defaultMaxEntries) {
this.defaultMaxEntries = defaultMaxEntries;
}
/**
* initialize the cache
* @param maxEntries maximum number of entries expected in the cache
* @param loadFactor the load factor
* @param timeout to be used to trim the expired entries
* @param props opaque list of properties for a given cache implementation
* @throws a generic Exception if the initialization failed
*/
public void init(int maxEntries, long timeout, float loadFactor, Properties props) {
// if the max entries is <= 0 then set the default max entries
if (maxEntries <= 0) {
maxEntries = defaultMaxEntries;
// mark this cache unbounded
isUnbounded = true;
}
setTimeout(timeout);
super.init(maxEntries, loadFactor, props);
}
/**
* sets the timeout value
* @param timeout to be used to trim the expired entries
*/
public void setTimeout(long timeout) {
// accept a positive timeout
if (timeout > 0)
this.timeout = timeout;
}
/**
* create new item
* @param hashCode for the entry
* @param key Object key
* @param value Object value
* @param size size in bytes of the item
*
* subclasses may override to provide their own CacheItem extensions
* e.g. one that permits persistence.
*/
protected CacheItem createItem(int hashCode, Object key,
Object value, int size) {
return new LruCacheItem(hashCode, key, value, size);
}
/**
* trim one item from the LRU list
* @param currentTime of this operation
* @return the item trimmed from cache
*
* list synchronization is handled by the caller
*/
protected CacheItem trimLru(long currentTime) {
if(tail == null)
return null;
LruCacheItem trimItem = tail;
if (tail != head) {
tail = trimItem.lPrev;
if(tail == null) {
//TODO:: print out a warning message here
tail = head = null;
} else {
tail.lNext = null;
trimItem.lPrev = null;
}
} else {
tail = head = null;
}
trimItem.isTrimmed = true;
trimItem.lPrev = null;
trimCount++;
listSize--;
return trimItem;
}
/**
/**
* this item is just added to the cache
* @param item CacheItem that was created
* @return a overflow item; may be null
*
* this function checks if adding the new item results in a overflow;
* if so, it returns the item to be removed.
*
* Cache bucket is already synchronized by the caller
*/
protected CacheItem itemAdded(CacheItem item) {
boolean updateThreshold = false;
CacheItem overflow = null;
if(! (item instanceof LruCacheItem))
return null;
LruCacheItem lc = (LruCacheItem) item;
// set the timestamp
lc.lastAccessed = System.currentTimeMillis();
// update the LRU
synchronized (this) {
if (head != null) {
head.lPrev = lc;
lc.lNext = head;
lc.lPrev = null;
head = lc;
}
else {
head = tail = lc;
lc.lPrev = lc.lNext = null;
}
listSize++;
if (isThresholdReached()) {
if (!isUnbounded)
overflow = trimLru(lc.lastAccessed);
else
updateThreshold = true;
}
}
// update the base cache threshold if needed
if (updateThreshold)
super.handleOverflow();
return overflow;
}
/**
* this item is accessed
* @param item CacheItem accessed
*
* Cache bucket is already synchronized by the caller
*/
protected void itemAccessed(CacheItem item) {
if(head == null)
return;
if(item == null)
return;
if(! (item instanceof LruCacheItem))
return;
LruCacheItem lc = (LruCacheItem) item;
synchronized (this) {
// if the item is already trimmed from the LRU list, nothing to do.
if (lc.isTrimmed)
return;
// update the timestamp
lc.lastAccessed = System.currentTimeMillis();
LruCacheItem prev = lc.lPrev;
LruCacheItem next = lc.lNext;
// update the LRU list
if (prev != null) {
// put the item at the head of LRU list
lc.lPrev = null;
lc.lNext = head;
head.lPrev = lc;
head = lc;
// patch up the previous neighbors
prev.lNext = next;
if (next != null)
next.lPrev = prev;
else
tail = prev;
}
}
}
/**
* item value has been refreshed
* @param item CacheItem that was refreshed
* @param oldSize size of the previous value that was refreshed
* Cache bucket is already synchronized by the caller
*/
protected void itemRefreshed(CacheItem item, int oldSize) {
itemAccessed(item);
}
/**
* item value has been removed from the cache
* @param item CacheItem that was just removed
*
* Cache bucket is already synchronized by the caller
*/
protected void itemRemoved(CacheItem item) {
if(! (item instanceof LruCacheItem))
return;
LruCacheItem l = (LruCacheItem) item;
// remove the item from the LRU list
synchronized (this) {
LruCacheItem prev = l.lPrev;
LruCacheItem next = l.lNext;
// if the item is already trimmed from the LRU list, nothing to do.
if (l.isTrimmed)
return;
// patch up the neighbors and make sure head/tail are correct
if (prev != null)
prev.lNext = next;
else
head = next;
if (next != null)
next.lPrev = prev;
else
tail = prev;
l.lPrev = l.lNext = null;
listSize--;
}
}
/**
* trim the expired entries from the cache.
* @param maxCount maximum number of invalid entries to trim
* specify Integer.MAX_VALUE to trim all invalid entries
* This call is to be scheduled by a thread managed by the container.
*
* NOTE: this algorithm assumes that all the entries in the cache have
* identical timeout (otherwise traversing from tail won't be right).
*/
public void trimExpiredEntries(int maxCount) {
int count = 0;
LruCacheItem item;
long currentTime = System.currentTimeMillis();
ArrayList list = new ArrayList();
synchronized (this) {
// traverse LRU list till we reach a valid item; remove them at once
for (item = tail; item != null && count < maxCount;
item = item.lPrev) {
if (timeout != NO_TIMEOUT &&
(item.lastAccessed + timeout) <= currentTime) {
item.isTrimmed = true;
list.add(item);
count++;
} else {
break;
}
}
// if there was at least one invalid item then item != tail.
if (item != tail) {
if (item != null)
item.lNext = null;
else
head = null;
tail = item;
}
listSize -= count;
trimCount += count;
}
// trim the items from the BaseCache from the old tail backwards
for (int index=list.size()-1; index >= 0; index--) {
trimItem((LruCacheItem) list.get(index));
}
}
/**
* get generic stats from subclasses
*/
/**
* get the desired statistic counter
* @param key to corresponding stat
* @return an Object corresponding to the stat
* See also: Constant.java for the key
*/
public Object getStatByName(String key) {
Object stat = super.getStatByName(key);
if (stat == null && key != null) {
if (key.equals(Constants.STAT_LRUCACHE_LIST_LENGTH))
stat = Integer.valueOf(listSize);
else if (key.equals(Constants.STAT_LRUCACHE_TRIM_COUNT))
stat = Integer.valueOf(trimCount);
}
return stat;
}
public Map getStats() {
Map stats = super.getStats();
stats.put(Constants.STAT_LRUCACHE_LIST_LENGTH,
Integer.valueOf(listSize));
stats.put(Constants.STAT_LRUCACHE_TRIM_COUNT,
Integer.valueOf(trimCount));
return stats;
}
/** default CacheItem class implementation ***/
protected static class LruCacheItem extends CacheItem {
// double linked LRU list
protected LruCacheItem lNext;
protected LruCacheItem lPrev;
protected boolean isTrimmed;
protected long lastAccessed;
protected LruCacheItem(int hashCode, Object key, Object value, int size) {
super(hashCode, key, value, size);
}
/**
* Return the next item
*/
public LruCacheItem getLNext() {
return lNext;
}
/**
* Reset the next item reference
*/
public void setLNext(LruCacheItem item) {
lNext = item;
}
/**
* Return the previous item
*/
public LruCacheItem getLPrev() {
return lPrev;
}
/**
* Reset the previous item reference
*/
public void setLPrev(LruCacheItem item) {
lPrev = item;
}
/**
* Return true if this item is trimmed
*/
public boolean isTrimmed() {
return isTrimmed;
}
/**
* Set the trimmed flag
*/
public void setTrimmed(boolean value) {
isTrimmed = value;
}
/**
* Return the last accessed timestamp
*/
public long getLastAccessed() {
return lastAccessed;
}
/**
* Reset the last accessed timestamp
*/
public void setLastAccessed(long l) {
lastAccessed = l;
}
}
}