org.apache.commons.collections.map
public class LRUMap extends AbstractLinkedMap implements BoundedMap, Serializable, Cloneable
Map
implementation with a fixed maximum size which removes
the least recently used entry if an entry is added when full.
The least recently used algorithm works on the get and put operations only. Iteration of any kind, including setting the value by iteration, does not change the order. Queries such as containsKey and containsValue or access via views also do not change the order.
The map implements OrderedMap
and entries may be queried using
the bidirectional OrderedMapIterator
. The order returned is
least recently used to most recently used. Iterators from map views can
also be cast to OrderedIterator
if required.
All the available iterators can be reset back to the start by casting to
ResettableIterator
and calling reset()
.
Note that LRUMap is not synchronized and is not thread-safe.
If you wish to use this map from multiple threads concurrently, you must use
appropriate synchronization. The simplest approach is to wrap this map
using java.util.Collections#synchronizedMap(Map). This class may throw
NullPointerException
's when accessed by concurrent threads.
Since: Commons Collections 3.0 (previously in main package v1.0)
Version: $Revision: 348299 $ $Date: 2005-11-22 23:51:45 +0000 (Tue, 22 Nov 2005) $
Field Summary | |
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protected static int | DEFAULT_MAX_SIZE Default maximum size |
Constructor Summary | |
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LRUMap()
Constructs a new empty map with a maximum size of 100. | |
LRUMap(int maxSize)
Constructs a new, empty map with the specified maximum size.
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LRUMap(int maxSize, boolean scanUntilRemovable)
Constructs a new, empty map with the specified maximum size.
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LRUMap(int maxSize, float loadFactor)
Constructs a new, empty map with the specified initial capacity and
load factor.
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LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable)
Constructs a new, empty map with the specified initial capacity and
load factor.
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LRUMap(Map map)
Constructor copying elements from another map.
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LRUMap(Map map, boolean scanUntilRemovable)
Constructor copying elements from another map.
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Method Summary | |
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protected void | addMapping(int hashIndex, int hashCode, Object key, Object value)
Adds a new key-value mapping into this map.
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Object | clone()
Clones the map without cloning the keys or values.
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protected void | doReadObject(ObjectInputStream in)
Reads the data necessary for put() to work in the superclass. |
protected void | doWriteObject(ObjectOutputStream out)
Writes the data necessary for put() to work in deserialization. |
Object | get(Object key)
Gets the value mapped to the key specified.
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boolean | isFull()
Returns true if this map is full and no new mappings can be added.
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boolean | isScanUntilRemovable()
Whether this LRUMap will scan until a removable entry is found when the
map is full.
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int | maxSize()
Gets the maximum size of the map (the bound).
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protected void | moveToMRU(LinkEntry entry)
Moves an entry to the MRU position at the end of the list.
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protected boolean | removeLRU(LinkEntry entry)
Subclass method to control removal of the least recently used entry from the map.
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protected void | reuseMapping(LinkEntry entry, int hashIndex, int hashCode, Object key, Object value)
Reuses an entry by removing it and moving it to a new place in the map.
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protected void | updateEntry(HashEntry entry, Object newValue)
Updates an existing key-value mapping.
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Parameters: maxSize the maximum size of the map
Throws: IllegalArgumentException if the maximum size is less than one
Parameters: maxSize the maximum size of the map scanUntilRemovable scan until a removeable entry is found, default false
Throws: IllegalArgumentException if the maximum size is less than one
Since: Commons Collections 3.1
Parameters: maxSize the maximum size of the map, -1 for no limit, loadFactor the load factor
Throws: IllegalArgumentException if the maximum size is less than one IllegalArgumentException if the load factor is less than zero
Parameters: maxSize the maximum size of the map, -1 for no limit, loadFactor the load factor scanUntilRemovable scan until a removeable entry is found, default false
Throws: IllegalArgumentException if the maximum size is less than one IllegalArgumentException if the load factor is less than zero
Since: Commons Collections 3.1
The maximum size is set from the map's size.
Parameters: map the map to copy
Throws: NullPointerException if the map is null IllegalArgumentException if the map is empty
Parameters: map the map to copy scanUntilRemovable scan until a removeable entry is found, default false
Throws: NullPointerException if the map is null IllegalArgumentException if the map is empty
Since: Commons Collections 3.1
This implementation checks the LRU size and determines whether to discard an entry or not using LRUMap.
From Commons Collections 3.1 this method uses isFull rather
than accessing size
and maxSize
directly.
It also handles the scanUntilRemovable functionality.
Parameters: hashIndex the index into the data array to store at hashCode the hash code of the key to add key the key to add value the value to add
Returns: a shallow clone
put()
to work in the superclass.put()
to work in deserialization.This operation changes the position of the key in the map to the most recently used position (first).
Parameters: key the key
Returns: the mapped value, null if no match
Returns: true
if the map is full
Returns: true if this map scans
Since: Commons Collections 3.1
Returns: the maximum number of elements the map can hold
This implementation moves the updated entry to the end of the list.
Parameters: entry the entry to update
This method exists for subclasses to override. A subclass may wish to provide cleanup of resources when an entry is removed. For example:
protected boolean removeLRU(LinkEntry entry) { releaseResources(entry.getValue()); // release resources held by entry return true; // actually delete entry }
Alternatively, a subclass may choose to not remove the entry or selectively keep certain LRU entries. For example:
protected boolean removeLRU(LinkEntry entry) { if (entry.getKey().toString().startsWith("System.")) { return false; // entry not removed from LRUMap } else { return true; // actually delete entry } }The effect of returning false is dependent on the scanUntilRemovable flag. If the flag is true, the next LRU entry will be passed to this method and so on until one returns false and is removed, or every entry in the map has been passed. If the scanUntilRemovable flag is false, the map will exceed the maximum size.
NOTE: Commons Collections 3.0 passed the wrong entry to this method. This is fixed in version 3.1 onwards.
Parameters: entry the entry to be removed
This method uses LRUMap, LRUMap and LRUMap.
Parameters: entry the entry to reuse hashIndex the index into the data array to store at hashCode the hash code of the key to add key the key to add value the value to add
This implementation moves the updated entry to the top of the list using LRUMap.
Parameters: entry the entry to update newValue the new value to store