org.jivesoftware.util
Class Cache<K,V>

java.lang.Object
  org.jivesoftware.util.Cache<K,V>

All Implemented Interfaces:

Map<K,V>

public class Cache<K,V>
extends Object
implements Map<K,V>

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.

Author:

Matt Tucker

Nested Class Summary

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

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 Map<K,org.jivesoftware.util.Cache.CacheObject<V>>	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

Cache(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(Object key)
boolean	containsValue(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.
Set	entrySet()
V	get(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.
String	getName() Returns the name of this cache.
boolean	isEmpty()
Set<K>	keySet()
V	put(K key, V value)
void	putAll(Map<? extends K,? extends V> map)
V	remove(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.
int	size()
Collection<V>	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 Map<K,org.jivesoftware.util.Cache.CacheObject<V>> 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

Cache

public Cache(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 V put(K key,
             V value)

Specified by:

put in interface Map<K,V>

get

public V get(Object key)

Specified by:

get in interface Map<K,V>

remove

public V remove(Object key)

Specified by:

remove in interface Map<K,V>

clear

public void clear()

Specified by:

clear in interface Map<K,V>

size

public int size()

Specified by:

size in interface Map<K,V>

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface Map<K,V>

values

public Collection<V> values()

Specified by:

values in interface Map<K,V>

containsKey

public boolean containsKey(Object key)

Specified by:

containsKey in interface Map<K,V>

putAll

public void putAll(Map<? extends K,? extends V> map)

Specified by:

putAll in interface Map<K,V>

containsValue

public boolean containsValue(Object value)

Specified by:

containsValue in interface Map<K,V>

entrySet

public Set entrySet()

Specified by:

entrySet in interface Map<K,V>

keySet

public Set<K> keySet()

Specified by:

keySet in interface Map<K,V>

getName

public String getName()

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

Returns:

the name of this 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.

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.

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.

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.

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.

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.

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.

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.