package com.knowgate.cache;
import java.util.HashMap;
import com.knowgate.debug.DebugFile;
/**
* <p>LRU Cache Policy</p>
* <p>Implementation of a Least Recently Used cache policy.</p>
* @author Simone Bordet <simone.bordet@compaq.com>
* @version 1.3.2.1
*/
public final class LRUCachePolicy {
// Attributes ----------------------------------------------------
/**
* The map holding the cached objects
*/
protected HashMap m_map;
/**
* The linked list used to implement the LRU algorithm
*/
protected LRUList m_list;
/**
* The maximum capacity of this cache
*/
protected int m_maxCapacity;
/**
* The minimum capacity of this cache
*/
protected int m_minCapacity;
// Constructors --------------------------------------------------
/**
* Creates a LRU cache policy object with zero cache capacity.
*
* @see #create
*/
public LRUCachePolicy() {}
/**
* Creates a LRU cache policy object with the specified minimum and maximum capacity.
*
* @see #create
*/
public LRUCachePolicy(int min, int max) {
if (min < 2 || min > max) {
throw new IllegalArgumentException("Illegal cache capacities");
}
m_minCapacity = min;
m_maxCapacity = max;
create();
}
// Service implementation ----------------------------------------------
/**
* Initializes the cache, creating all required objects and initializing their
* values.
* @see #start
* @see #destroy
*/
private void create() {
m_map = new HashMap(211);
m_list = createList();
m_list.m_maxCapacity = m_maxCapacity;
m_list.m_minCapacity = m_minCapacity;
m_list.m_capacity = m_maxCapacity;
}
/**
* Starts this cache that is now ready to be used.
* @see #create
* @see #stop
*/
public void start() {}
/**
* Stops this cache thus {@link #flush}ing all cached objects. <br>
* After this method is called, a call to {@link #start} will restart the cache.
* @see #start
* @see #destroy
*/
public void stop() {
if (m_list != null) {
flush();
}
}
/**
* Destroys the cache that is now unusable. <br>
* To have it working again it must be re-{@link #create}ed and re-{@link #start}ed.
*
* @see #create
*/
public void destroy() {
if (m_map != null) {
m_map.clear();
}
if (m_list != null) {
m_list.clear();
}
}
public java.util.Set keySet() {
return m_map.keySet();
}
public long last(Object key) {
LRUCacheEntry value = (LRUCacheEntry) m_map.get(key);
return value.m_last;
}
public Object get(Object key) {
Object oRetVal;
if (DebugFile.trace) {
DebugFile.writeln("Begin LRUCachePolicy.get(" + key + ")");
DebugFile.incIdent();
}
if (key == null) {
throw new IllegalArgumentException(
"Requesting an object using a null key");
}
Object value = m_map.get(key);
LRUCacheEntry entry;
if (value != null) {
entry = (LRUCacheEntry)value;
m_list.promote(entry);
oRetVal = entry.m_object;
}
else {
if (DebugFile.trace) DebugFile.writeln("LRUCachePolicy.cacheMiss()");
cacheMiss();
oRetVal = null;
}
if (DebugFile.trace) {
DebugFile.decIdent();
DebugFile.writeln("End LRUCachePolicy.get() : " + (oRetVal!=null ? "[Object]" : "null") );
}
return oRetVal;
} // get
public Object peek(Object key) {
if (key == null)
throw new IllegalArgumentException("Requesting an object using a null key");
LRUCacheEntry value = (LRUCacheEntry) m_map.get(key);
if (value == null)
return null;
else
return value.m_object;
} // peek
public void insert(Object key, Object o, long t) {
if (o == null)
throw new IllegalArgumentException("Cannot insert a null object in the cache");
if (key == null)
throw new IllegalArgumentException("Cannot insert an object in the cache with null key");
Object obj = m_map.get(key);
if (obj == null) {
m_list.demote();
LRUCacheEntry entry = createCacheEntry(key, o, t);
m_list.promote(entry);
m_map.put(key, entry);
}
else
{
throw new IllegalStateException("Attempt to put in the cache an object that is already there");
}
} // insert
public void remove(Object key) {
if (key == null)
throw new IllegalArgumentException("Removing an object using a null key");
Object value = m_map.get(key);
if (value != null) {
m_list.remove( (LRUCacheEntry) value);
m_map.remove(key);
}
} // remove
public void flush() {
LRUCacheEntry entry = null;
while ( (entry = m_list.m_tail) != null)
ageOut(entry);
} // flush
public int size() {
return m_list.m_count;
} // size
// Protected -----------------------------------------------------
/**
* Factory method for the linked list used by this cache implementation.
*/
protected LRUList createList() {
return new LRUList();
}
/**
* Callback method called when the cache algorithm ages out of the cache
* the given entry. <br>
* The implementation here is removing the given entry from the cache.
*/
protected void ageOut(LRUCacheEntry entry) {
remove(entry.m_key);
}
/**
* Callback method called when a cache miss happens.
*/
protected void cacheMiss() {
}
/**
* Factory method for cache entries
*/
protected LRUCacheEntry createCacheEntry(Object key, Object value, long t) {
return new LRUCacheEntry(key, value, t);
}
// Private -------------------------------------------------------
// Inner classes -------------------------------------------------
/**
* Double queued list used to store cache entries.
*/
public class LRUList {
/** The maximum capacity of the cache list */
public int m_maxCapacity;
/** The minimum capacity of the cache list */
public int m_minCapacity;
/** The current capacity of the cache list */
public int m_capacity;
/** The number of cached objects */
public int m_count;
/** The head of the double linked list */
public LRUCacheEntry m_head;
/** The tail of the double linked list */
public LRUCacheEntry m_tail;
/** The cache misses happened */
public int m_cacheMiss;
/**
* Creates a new double queued list.
*/
protected LRUList() {
m_head = null;
m_tail = null;
m_count = 0;
}
/**
* Promotes the cache entry <code>entry</code> to the last used position
* of the list. <br>
* If the object is already there, does nothing.
* @param entry the object to be promoted, cannot be null
* @see #demote
* @throws IllegalStateException if this method is called with a full cache
*/
protected void promote(LRUCacheEntry entry) {
if (entry == null) {
throw new IllegalArgumentException("Trying to promote a null object");
}
if (m_capacity < 1) {
throw new IllegalStateException("Can't work with capacity < 1");
}
entry.m_time = System.currentTimeMillis();
if (entry.m_prev == null)
{
if (entry.m_next == null)
{
// entry is new or there is only the head
if (m_count == 0) // cache is empty
{
m_head = entry;
m_tail = entry;
++m_count;
entryAdded(entry);
}
else if (m_count == 1 && m_head == entry) {} // there is only the head and I want to promote it, do nothing
else if (m_count < m_capacity)
{
entry.m_prev = null;
entry.m_next = m_head;
m_head.m_prev = entry;
m_head = entry;
++m_count;
entryAdded(entry);
}
else if (m_count < m_maxCapacity)
{
entry.m_prev = null;
entry.m_next = m_head;
m_head.m_prev = entry;
m_head = entry;
++m_count;
int oldCapacity = m_capacity;
++m_capacity;
entryAdded(entry);
capacityChanged(oldCapacity);
}
else {
throw new IllegalStateException(
"Attempt to put a new cache entry on a full cache");
}
}
else {} // entry is the head, do nothing
}
else
{
if (entry.m_next == null) // entry is the tail
{
LRUCacheEntry beforeLast = entry.m_prev;
beforeLast.m_next = null;
entry.m_prev = null;
entry.m_next = m_head;
m_head.m_prev = entry;
m_head = entry;
m_tail = beforeLast;
}
else // entry is in the middle of the list
{
LRUCacheEntry previous = entry.m_prev;
previous.m_next = entry.m_next;
entry.m_next.m_prev = previous;
entry.m_prev = null;
entry.m_next = m_head;
m_head.m_prev = entry;
m_head = entry;
}
}
}
/**
* Demotes from the cache the least used entry. <br>
* If the cache is not full, does nothing.
* @see #promote
*/
protected void demote() {
if (m_capacity < 1)
throw new IllegalStateException("Can't work with capacity < 1");
if (m_count > m_maxCapacity)
throw new IllegalStateException("Cache list entries number (" + m_count +
") > than the maximum allowed (" +
m_maxCapacity + ")");
if (m_count == m_maxCapacity) {
LRUCacheEntry entry = m_tail;
// the entry will be removed by ageOut
ageOut(entry);
}
}
/**
* Removes from the cache list the specified entry.
*/
protected void remove(LRUCacheEntry entry) throws IllegalArgumentException,IllegalStateException {
if (entry == null)
throw new IllegalArgumentException("LRUList.remove() Cannot remove a null entry from the cache");
if (m_count < 1)
throw new IllegalStateException("LRUList.remove() Trying to remove an entry from an empty cache");
entry.m_key = entry.m_object = null;
if (m_count == 1) {
m_head = m_tail = null;
}
else {
if (entry.m_prev == null) { // the head
m_head = entry.m_next;
if (null!=m_head) m_head.m_prev = null;
entry.m_next = null;
}
else if (entry.m_next == null) { // the tail
m_tail = entry.m_prev;
m_tail.m_next = null;
entry.m_prev = null;
}
else { // in the middle
entry.m_next.m_prev = entry.m_prev;
entry.m_prev.m_next = entry.m_next;
entry.m_prev = null;
entry.m_next = null;
}
}
--m_count;
entryRemoved(entry);
} // remove
/**
* Callback that signals that the given entry has been added to the cache.
*/
protected void entryAdded(LRUCacheEntry entry) {}
/**
* Callback that signals that the given entry has been removed from the cache.
*/
protected void entryRemoved(LRUCacheEntry entry) {}
/**
* Callback that signals that the capacity of the cache is changed.
* @param oldCapacity the capacity before the change happened
*/
protected void capacityChanged(int oldCapacity) {}
protected void clear() {
m_head = null;
m_tail = null;
m_count = 0;
}
public String toString() {
String s = Integer.toHexString(super.hashCode());
s += " size: " + m_count;
for (LRUCacheEntry entry = m_head; entry != null; entry = entry.m_next)
s += "\n" + entry;
return s;
}
}
/**
* Double linked cell used as entry in the cache list.
*/
public final class LRUCacheEntry {
/** Reference to the next cell in the list */
public LRUCacheEntry m_next;
/** Reference to the previous cell in the list */
public LRUCacheEntry m_prev;
/** The key used to retrieve the cached object */
public Object m_key;
/** The cached object */
public Object m_object;
/** The timestamp of the creation */
public long m_time;
/** The timestamp of last modification */
public long m_last;
/**
* Creates a new double linked cell, storing the object we
* want to cache and the key that is used to retrieve it.
*/
protected LRUCacheEntry(Object key, Object object, long t) {
if (key == null)
throw new IllegalArgumentException("LRUCacheEntry() Cannot create a cache entry with a null key");
m_key = key;
m_object = object;
m_next = null;
m_prev = null;
m_time = 0; // Set when inserted in the list.
m_last = t;
}
public String toString() {
return "key: " + m_key + ", object: " +
(m_object == null ? "null" : Integer.toHexString(m_object.hashCode())) +
", entry: " + Integer.toHexString(super.hashCode());
}
} // LRUCacheEntry
} // LRUCachePolicy