com.jivesoftware.util
Class LRUCache

java.lang.Object
  com.jivesoftware.util.LRUCache

All Implemented Interfaces:

Cache, java.util.Map

public class LRUCache
extends java.lang.Object
implements Cache

Default, non-distributed implementation of the Cache interface. The algorithm for cache is as follows: a HashMap is maintained for fast object lookup. Two linked lists are maintained: one keeps objects in the order they are accessed from cache, the other keeps objects in the order they were originally added to cache. When objects are added to cache, they are first wrapped by a CacheObject which maintains the following pieces of information:

• The size of the object (in bytes).
• A pointer to the node in the linked list that maintains accessed order for the object. Keeping a reference to the node lets us avoid linear scans of the linked list.
• A pointer to the node in the linked list that maintains the age of the object in cache. Keeping a reference to the node lets us avoid linear scans of the linked list.

To get an object from cache, a hash lookup is performed to get a reference to the CacheObject that wraps the real object we are looking for. The object is subsequently moved to the front of the accessed linked list and any necessary cache cleanups are performed. Cache deletion and expiration is performed as needed.

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map

java.util.Map.Entry<K,V>

Field Summary

protected LinkedList	ageList Linked list to maintain time that cache objects were initially added to the cache, most recently added to oldest added.
protected long	cacheHits Maintain the number of cache hits and misses.
protected long	cacheMisses Maintain the number of cache hits and misses.
protected LinkedList	lastAccessedList Linked list to maintain order that cache objects are accessed in, most used to least used.
protected java.util.Map	map The map the keys and values are stored in.
protected long	maxLifetime Maximum length of time objects can exist in cache before expiring.

Constructor Summary

LRUCache(java.lang.String name, int maxSize, long maxLifetime)
Create a new cache and specify the maximum size of for the cache in bytes, and the maximum lifetime of objects.

Method Summary

void	clear()
boolean	containsKey(java.lang.Object key)
boolean	containsValue(java.lang.Object value)
protected void	cullCache() Removes objects from cache if the cache is too full.
protected void	deleteExpiredEntries() Clears all entries out of cache where the entries are older than the maximum defined age.
java.util.Set	entrySet()
java.lang.Object	get(java.lang.Object key)
long	getCacheHits() Returns the number of cache hits.
long	getCacheMisses() Returns the number of cache misses.
int	getCacheSize() Returns the size of the cache contents in bytes.
int	getMaxCacheSize() Returns the maximum size of the cache (in bytes).
long	getMaxLifetime() Returns the maximum number of milleseconds that any object can live in cache.
java.lang.String	getName() Returns the name of this cache.
boolean	isEmpty()
java.util.Set	keySet()
java.lang.Object	put(java.lang.Object key, java.lang.Object value)
void	putAll(java.util.Map map)
java.lang.Object	remove(java.lang.Object key)
void	setMaxCacheSize(int maxCacheSize) Sets the maximum size of the cache.
void	setMaxLifetime(long maxLifetime) Sets the maximum number of milleseconds that any object can live in cache.
void	setName(java.lang.String name) Sets the name of the cache
int	size()
java.util.Collection	values()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

Field Detail

map

protected java.util.Map map

The map the keys and values are stored in.

lastAccessedList

protected LinkedList lastAccessedList

Linked list to maintain order that cache objects are accessed in, most used to least used.

ageList

protected LinkedList ageList

Linked list to maintain time that cache objects were initially added to the cache, most recently added to oldest added.

maxLifetime

protected long maxLifetime

Maximum length of time objects can exist in cache before expiring.

cacheHits

protected long cacheHits

Maintain the number of cache hits and misses. A cache hit occurs every time the get method is called and the cache contains the requested object. A cache miss represents the opposite occurence.

Keeping track of cache hits and misses lets one measure how efficient the cache is; the higher the percentage of hits, the more efficient.

cacheMisses

protected long cacheMisses

Maintain the number of cache hits and misses. A cache hit occurs every time the get method is called and the cache contains the requested object. A cache miss represents the opposite occurence.

Keeping track of cache hits and misses lets one measure how efficient the cache is; the higher the percentage of hits, the more efficient.

Constructor Detail

LRUCache

public LRUCache(java.lang.String name,
                int maxSize,
                long maxLifetime)

Create a new cache and specify the maximum size of for the cache in bytes, and the maximum lifetime of objects.

Parameters:

name - a name for the cache.

maxSize - the maximum size of the cache in bytes. -1 means the cache has no max size.

maxLifetime - the maximum amount of time objects can exist in cache before being deleted. -1 means objects never expire.

Method Detail

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.Object value)

Specified by:

put in interface java.util.Map

get

public java.lang.Object get(java.lang.Object key)

Specified by:

get in interface java.util.Map

remove

public java.lang.Object remove(java.lang.Object key)

Specified by:

remove in interface java.util.Map

clear

public void clear()

Specified by:

clear in interface java.util.Map

size

public int size()

Specified by:

size in interface java.util.Map

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface java.util.Map

values

public java.util.Collection values()

Specified by:

values in interface java.util.Map

containsKey

public boolean containsKey(java.lang.Object key)

Specified by:

containsKey in interface java.util.Map

putAll

public void putAll(java.util.Map map)

Specified by:

putAll in interface java.util.Map

containsValue

public boolean containsValue(java.lang.Object value)

Specified by:

containsValue in interface java.util.Map

entrySet

public java.util.Set entrySet()

Specified by:

entrySet in interface java.util.Map

keySet

public java.util.Set keySet()

Specified by:

keySet in interface java.util.Map

getName

public java.lang.String getName()

Returns the name of this cache. The name is completely arbitrary and used only for display to administrators.

Specified by:

getName in interface Cache

Returns:

the name of this cache.

setName

public void setName(java.lang.String name)

Description copied from interface: Cache

Sets the name of the cache

Specified by:

setName in interface Cache

Parameters:

name - the name of the cache

getCacheHits

public long getCacheHits()

Returns the number of cache hits. A cache hit occurs every time the get method is called and the cache contains the requested object.

Keeping track of cache hits and misses lets one measure how efficient the cache is; the higher the percentage of hits, the more efficient.

Specified by:

getCacheHits in interface Cache

Returns:

the number of cache hits.

getCacheMisses

public long getCacheMisses()

Returns the number of cache misses. A cache miss occurs every time the get method is called and the cache does not contain the requested object.

Keeping track of cache hits and misses lets one measure how efficient the cache is; the higher the percentage of hits, the more efficient.

Specified by:

getCacheMisses in interface Cache

Returns:

the number of cache hits.

getCacheSize

public int getCacheSize()

Returns the size of the cache contents in bytes. This value is only a rough approximation, so cache users should expect that actual VM memory used by the cache could be significantly higher than the value reported by this method.

Specified by:

getCacheSize in interface Cache

Returns:

the size of the cache contents in bytes.

getMaxCacheSize

public int getMaxCacheSize()

Returns the maximum size of the cache (in bytes). If the cache grows larger than the max size, the least frequently used items will be removed. If the max cache size is set to -1, there is no size limit.

Specified by:

getMaxCacheSize in interface Cache

Returns:

the maximum size of the cache (-1 indicates unlimited max size).

setMaxCacheSize

public void setMaxCacheSize(int maxCacheSize)

Sets the maximum size of the cache. If the cache grows larger than the max size, the least frequently used items will be removed. If the max cache size is set to -1, there is no size limit.

Specified by:

setMaxCacheSize in interface Cache

Parameters:

maxCacheSize - the maximum size of this cache (-1 indicates unlimited max size).

getMaxLifetime

public long getMaxLifetime()

Returns the maximum number of milleseconds that any object can live in cache. Once the specified number of milleseconds passes, the object will be automatically expried from cache. If the max lifetime is set to -1, then objects never expire.

Specified by:

getMaxLifetime in interface Cache

Returns:

the maximum number of milleseconds before objects are expired.

setMaxLifetime

public void setMaxLifetime(long maxLifetime)

Sets the maximum number of milleseconds that any object can live in cache. Once the specified number of milleseconds passes, the object will be automatically expried from cache. If the max lifetime is set to -1, then objects never expire.

Specified by:

setMaxLifetime in interface Cache

Parameters:

maxLifetime - the maximum number of milleseconds before objects are expired.

deleteExpiredEntries

protected void deleteExpiredEntries()

Clears all entries out of cache where the entries are older than the maximum defined age.

cullCache

protected final void cullCache()

Removes objects from cache if the cache is too full. "Too full" is defined as within 3% of the maximum cache size. Whenever the cache is is too big, the least frequently used elements are deleted until the cache is at least 10% empty.