Package com.mckoi.util

Class Cache

java.lang.Object

com.mckoi.util.Cache

Direct Known Subclasses:

DataCellCache.DCCache

public class Cache
extends java.lang.Object

Represents a cache of Objects. A Cache is similar to a Hashtable, in that you can 'add' and 'get' objects from the container given some key. However a cache may remove objects from the container when it becomes too full.

The cache scheme uses a doubly linked-list hashtable. The most recently accessed objects are moved to the start of the list. The end elements in the list are wiped if the cache becomes too full.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
(package private) static class	Cache.ListNode	An element in the linked list structure.

Field Summary

Fields

Modifier and Type	Field	Description
private int	current_cache_size	The current cache size.
private long	get_total
private Cache.ListNode	list_end	A pointer to the end of the list.
private Cache.ListNode	list_start	A pointer to the start of the list.
private int	max_cache_size	The maximum number of DataCell objects that can be stored in the cache at any one time.
private Cache.ListNode[]	node_hash	The array of ListNode objects arranged by hashing value.
private long	total_gets	Some statistics about the hashing algorithm.
private int	wipe_to	The number of nodes that should be left available when the cache becomes too full and a clean up operation occurs.

Constructor Summary

Constructors

Constructor	Description
Cache()
Cache(int max_size)
Cache(int max_size, int clean_percentage)
Cache(int hash_size, int max_size, int clean_percentage)	The Constructors.

Method Summary

Modifier and Type	Method	Description
private void	bringToHead(Cache.ListNode node)	Brings 'node' to the start of the list.
protected void	checkClean()	This is called whenever at Object is put into the cache.
protected int	clean()	Cleans away some old elements in the cache.
void	clear()
private void	clearHash()	Clears the entire hashtable of all entries.
private Cache.ListNode	createListNode()	Creates a new ListNode.
java.lang.Object	get(java.lang.Object key)	If the cache contains the cell with the given key, this method will return the object.
private Cache.ListNode	getFromHash(java.lang.Object key)	Finds the node with the given key in the hash table and returns it.
protected int	getHashSize()	Deprecated.
int	nodeCount()	Returns the number of nodes that are currently being stored in the cache.
protected void	notifyGetWalks(long total_walks, long total_get_ops)	Notifies that some statistical information about the hash map has updated.
protected void	notifyWipingNode(java.lang.Object ob)	Notifies that the given object has been wiped from the cache by the clean up procedure.
void	put(java.lang.Object key, java.lang.Object ob)	Puts an Object into the cache with the given key.
private Cache.ListNode	putIntoHash(Cache.ListNode node)	Puts the node with the given key into the hash table.
java.lang.Object	remove(java.lang.Object key)	Ensures that there is no cell with the given key in the cache.
void	removeAll()	Clear the cache of all the entries.
private Cache.ListNode	removeFromHash(java.lang.Object key)	Removes the given node from the hash table.
protected boolean	shouldWipeMoreNodes()	Returns true if the clean-up method that periodically cleans up the cache, should clean up more elements from the cache.

Methods inherited from class java.lang.Object

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

Field Detail

max_cache_size

private int max_cache_size

The maximum number of DataCell objects that can be stored in the cache at any one time.

current_cache_size

private int current_cache_size

The current cache size.

wipe_to

private int wipe_to

The number of nodes that should be left available when the cache becomes too full and a clean up operation occurs.

node_hash

private final Cache.ListNode[] node_hash

The array of ListNode objects arranged by hashing value.

list_start

private Cache.ListNode list_start

A pointer to the start of the list.

list_end

private Cache.ListNode list_end

A pointer to the end of the list.

total_gets

private long total_gets

Some statistics about the hashing algorithm.

get_total

private long get_total

Constructor Detail

Cache

public Cache(int hash_size,
             int max_size,
             int clean_percentage)

The Constructors. It takes a maximum size the cache can grow to, and the percentage of the cache that is wiped when it becomes too full.

Cache

public Cache(int max_size,
             int clean_percentage)

Cache

public Cache(int max_size)

Cache

public Cache()

Method Detail

getHashSize

protected final int getHashSize()

Deprecated.

Creates the HashMap object to store objects in this cache. This is available to be overwritten.

checkClean

protected void checkClean()

This is called whenever at Object is put into the cache. This method should determine if the cache should be cleaned and call the clean method if appropriate.

shouldWipeMoreNodes

protected boolean shouldWipeMoreNodes()

Returns true if the clean-up method that periodically cleans up the cache, should clean up more elements from the cache.

notifyWipingNode

protected void notifyWipingNode(java.lang.Object ob)

Notifies that the given object has been wiped from the cache by the clean up procedure.

notifyGetWalks

protected void notifyGetWalks(long total_walks,
                              long total_get_ops)

Notifies that some statistical information about the hash map has updated. This should be used to compile statistical information about the number of walks a 'get' operation takes to retreive an entry from the hash.

This method is called every 8192 gets.

getFromHash

private Cache.ListNode getFromHash(java.lang.Object key)

Finds the node with the given key in the hash table and returns it. Returns 'null' if the value isn't in the hash table.

putIntoHash

private Cache.ListNode putIntoHash(Cache.ListNode node)

Puts the node with the given key into the hash table.

removeFromHash

private Cache.ListNode removeFromHash(java.lang.Object key)

Removes the given node from the hash table. Returns 'null' if the entry wasn't found in the hash.

clearHash

private void clearHash()

Clears the entire hashtable of all entries.

nodeCount

public final int nodeCount()

Returns the number of nodes that are currently being stored in the cache.

put

public final void put(java.lang.Object key,
                      java.lang.Object ob)

Puts an Object into the cache with the given key.

get

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

If the cache contains the cell with the given key, this method will return the object. If the cell is not in the cache, it returns null.

remove

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

Ensures that there is no cell with the given key in the cache. This is useful for ensuring the cache does not contain out-dated information.

removeAll

public void removeAll()

Clear the cache of all the entries.

clear

public void clear()

createListNode

private final Cache.ListNode createListNode()

Creates a new ListNode. If there is a free ListNode on the 'recycled_nodes' then it obtains one from there, else it creates a new blank one.

clean

protected final int clean()

Cleans away some old elements in the cache. This method walks from the end, back 'wipe_count' elements putting each object on the recycle stack.

bringToHead

private final void bringToHead(Cache.ListNode node)

Brings 'node' to the start of the list. Only nodes at the end of the list are cleaned.