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// Portions Copyright [2019] Payara Foundation and/or affiliates
package com.sun.appserv.util.cache;
import java.util.Properties;
import java.util.Map;
/**
* MultiLruCache -- in-memory bounded LRU cache with multiple LRU lists
* Underlying Hashtable is made into logical segments, with each segment
* having its own LRU list.
*/
public class MultiLruCache extends BaseCache {
/* an array of LRU lists; each element in this array is actually
* LruCacheItem[2] with LRU list (lru[0] is head and lru[1] the tail
*/
public static final int LRU_HEAD = 0;
public static final int LRU_TAIL = 1;
public static final int DEFAULT_HASHTABLE_SEGMENT_SIZE = 4096;
int segmentSize;
LruCacheItem[][] lists;
protected int[] listsLength;
int trimCount;
int trimIndex;
Object trimIndexLk = new Object();
/**
* initialize the LRU cache
* @param maxCapacity maximum number of entries this cache may hold
*/
@Override
public void init(int maxCapacity, Properties props) throws Exception {
super.init(maxCapacity, props);
segmentSize = DEFAULT_HASHTABLE_SEGMENT_SIZE;
if (props != null) {
String prop = props.getProperty("MultiLRUSegmentSize");
if (prop != null) {
try {
segmentSize = Integer.parseInt(prop);
} catch (NumberFormatException nfe) {}
}
}
// create the array of LRU lists
int segments = ((maxBuckets / segmentSize) +
(((maxBuckets % segmentSize) != 0) ? 1 : 0));
lists = new LruCacheItem[segments][2];
listsLength = new int[lists.length];
for (int i = 0; i < lists.length; i++) {
lists[i][LRU_HEAD] = null;
lists[i][LRU_TAIL] = null;
listsLength[i] = 0;
}
}
/**
* 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.
*/
@Override
protected CacheItem createItem(int hashCode, Object key,
Object value, int size) {
return new LruCacheItem(hashCode, key, value, size);
}
/**
* remove an lru item from one of the LRU lists
* @param the LRU segment index to trim
* @return the item that was successfully trimmed
*/
protected CacheItem trimLru(int segment) {
LruCacheItem[] list = lists[segment];
LruCacheItem l = null;
l = list[LRU_TAIL];
list[LRU_TAIL] = l.lPrev;
list[LRU_TAIL].lNext = null;
l.lPrev = null;
listsLength[segment]--;
l.isTrimmed = true;
trimCount++;
return l;
}
/**
* this item is just added to the cache
* @param item CacheItem that was created
* @return a overflow item; may be null
*
* Cache bucket is already synchronized by the caller
*/
@Override
protected CacheItem itemAdded(CacheItem item) {
CacheItem overflow = null;
if(! (item instanceof LruCacheItem))
return null;
LruCacheItem lc = (LruCacheItem) item;
int index = getIndex(item.hashCode());
int segment = (index/segmentSize);
LruCacheItem[] list = lists[segment];
// update the LRU
synchronized (list) {
if (list[LRU_HEAD] != null) {
list[LRU_HEAD].lPrev = lc;
lc.lNext = list[LRU_HEAD];
}
else
list[LRU_TAIL] = lc;
list[LRU_HEAD] = lc;
listsLength[segment]++;
if (isThresholdReached()) {
overflow = trimLru(trimIndex);
// go round robin for the next trim
incrementTrimIndex();
}
}
return overflow;
}
/**
* this item is accessed
* @param item CacheItem accessed
*
* Cache bucket is already synchronized by the caller
*/
@Override
protected void itemAccessed(CacheItem item) {
int index = getIndex(item.hashCode());
int segment = (index/segmentSize);
LruCacheItem[] list = lists[segment];
if(! (item instanceof LruCacheItem))
return;
LruCacheItem lc = (LruCacheItem) item;
// update the LRU list
synchronized (list) {
LruCacheItem prev = lc.lPrev;
LruCacheItem next = lc.lNext;
if (prev != null) {
// put the item at the head of LRU list
lc.lPrev = null;
lc.lNext = list[LRU_HEAD];
list[LRU_HEAD].lPrev = lc;
list[LRU_HEAD] = lc;
// patch up the previous neighbors
prev.lNext = next;
if (next != null)
next.lPrev = prev;
else
list[LRU_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
*/
@Override
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
*/
@Override
protected void itemRemoved(CacheItem item) {
if(! (item instanceof LruCacheItem))
return;
LruCacheItem l = (LruCacheItem) item;
int index = getIndex(item.hashCode());
int segment = (index/segmentSize);
LruCacheItem[] list = lists[segment];
// remove the item from the LRU list
synchronized (list) {
// if the item is already trimmed from the LRU list, nothing to do.
if (l.isTrimmed)
return;
LruCacheItem prev = l.lPrev;
LruCacheItem next = l.lNext;
// patch up the neighbors and make sure head/tail are correct
if (prev != null)
prev.lNext = next;
else
list[LRU_HEAD] = next;
if (next != null)
next.lPrev = prev;
else
list[LRU_TAIL] = prev;
listsLength[segment]--;
}
}
/**
* cache has reached threshold so trim its size. subclasses are expected
* to provide a robust cache replacement algorithm.
*/
@Override
protected void handleOverflow() {
LruCacheItem l = null;
}
int getListsLength() {
return lists.length;
}
protected void incrementTrimIndex() {
synchronized (trimIndexLk) {
trimIndex = (trimIndex + 1) % lists.length;
}
}
/**
* 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
*/
@Override
public Object getStatByName(String key) {
Object stat = super.getStatByName(key);
if (stat == null && key != null) {
switch (key) {
case Constants.STAT_MULTILRUCACHE_SEGMENT_SIZE:
stat = segmentSize;
break;
case Constants.STAT_MULTILRUCACHE_TRIM_COUNT:
stat = trimCount;
break;
case Constants.STAT_MULTILRUCACHE_SEGMENT_LIST_LENGTH:
stat = new Integer[lists.length];
for (int i = 0; i < lists.length; i++) {
((Integer[])stat)[i] = listsLength[i];
} break;
default:
break;
}
}
return stat;
}
/**
* get the stats snapshot
* @return a Map of stats
* See also: Constant.java for the keys
*/
@Override
public Map getStats() {
Map stats = super.getStats();
stats.put(Constants.STAT_MULTILRUCACHE_SEGMENT_SIZE, segmentSize);
for (int i = 0; i < lists.length; i++) {
stats.put(Constants.STAT_MULTILRUCACHE_SEGMENT_LIST_LENGTH + "["
+ i + "]:", listsLength[i]);
}
stats.put(Constants.STAT_MULTILRUCACHE_TRIM_COUNT, trimCount);
return stats;
}
/** default CacheItem class implementation ***/
static class LruCacheItem extends BaseCache.CacheItem {
// double linked LRU list
LruCacheItem lNext;
LruCacheItem lPrev;
boolean isTrimmed;
LruCacheItem(int hashCode, Object key, Object value, int size) {
super(hashCode, key, value, size);
}
}
}