Lru Hashtable
// LruHashtable - a Hashtable that expires least-recently-used objects
//
// Copyright (C) 1996 by Jef Poskanzer <jef@acme.com>. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
// SUCH DAMAGE.
//
// Visit the ACME Labs Java page for up-to-date versions of this and other
// fine Java utilities: http://www.acme.com/java/
//
// moved to the net.matuschek.util package by Daniel Matuschek
//
import java.util.Enumeration;
import java.util.Hashtable;
/**
* A Hashtable that expires least-recently-used objects.
*
* <p>Use just like java.util.Hashtable, except that the initial-capacity
* parameter is required. Instead of growing bigger than that size,
* it will throw out objects that haven't been looked at in a while.
* </p>
*
* @author Jef Poskanzer
* @author Daniel Matuschek
* @version $Id: LruHashtable.java,v 1.3 2002/05/31 14:45:56 matuschd Exp $
*
* @see java.util.Hashtable
*/
public class LruHashtable extends Hashtable
{
// Number of buckets.
private static final int nBuckets = 2;
// Load factor.
private float loadFactor;
// When count exceeds this threshold, expires the old table.
private int threshold;
// Capacity of each bucket.
private int eachCapacity;
// The tables.
private Hashtable oldTable;
private Hashtable newTable;
/// Constructs a new, empty hashtable with the specified initial
// capacity and the specified load factor.
// Unlike a plain Hashtable, an LruHashtable will never grow or
// shrink from this initial capacity.
// @param initialCapacity the initial number of buckets
// @param loadFactor a number between 0.0 and 1.0, it defines
// the threshold for expiring old entries
// @exception IllegalArgumentException If the initial capacity
// is less than or equal to zero.
// @exception IllegalArgumentException If the load factor is
// less than or equal to zero.
public LruHashtable( int initialCapacity, float loadFactor )
{
// We have to call a superclass constructor, but we're not actually
// going to use it at all. The only reason we want to extend Hashtable
// is for type conformance. So, make a parent hash table of minimum
// size and then ignore it.
super( 1 );
if ( initialCapacity <= 0 || loadFactor <= 0.0 )
throw new IllegalArgumentException();
this.loadFactor = loadFactor;
threshold = (int) ( initialCapacity * loadFactor ) - 1;
eachCapacity = initialCapacity / nBuckets + 1;
oldTable = new Hashtable( eachCapacity, loadFactor );
newTable = new Hashtable( eachCapacity, loadFactor );
}
/// Constructs a new, empty hashtable with the specified initial
// capacity.
// Unlike a plain Hashtable, an LruHashtable will never grow or
// shrink from this initial capacity.
// @param initialCapacity the initial number of buckets
public LruHashtable( int initialCapacity )
{
this( initialCapacity, 0.75F );
}
/// Returns the number of elements contained in the hashtable.
public int size()
{
return newTable.size() + oldTable.size();
}
/// Returns true if the hashtable contains no elements.
public boolean isEmpty()
{
return size() == 0;
}
/// Returns an enumeration of the hashtable's keys.
// @see LruHashtable#elements
// @see Enumeration
public synchronized Enumeration keys()
{
return new LruHashtableEnumerator( oldTable, newTable, true );
}
/// Returns an enumeration of the elements. Use the Enumeration methods
// on the returned object to fetch the elements sequentially.
// @see LruHashtable#keys
// @see Enumeration
public synchronized Enumeration elements()
{
return new LruHashtableEnumerator( oldTable, newTable, false );
}
/// Returns true if the specified object is an element of the hashtable.
// This operation is more expensive than the containsKey() method.
// @param value the value that we are looking for
// @exception NullPointerException If the value being searched
// for is equal to null.
// @see LruHashtable#containsKey
public synchronized boolean contains( Object value )
{
if ( newTable.contains( value ) )
return true;
if ( oldTable.contains( value ) )
{
// We would like to move the object from the old table to the
// new table. However, we need keys to re-add the objects, and
// there's no good way to find all the keys for the given object.
// We'd have to enumerate through all the keys and check each
// one. Yuck. For now we just punt. Anyway, contains() is
// probably not a commonly-used operation.
return true;
}
return false;
}
/// Returns true if the collection contains an element for the key.
// @param key the key that we are looking for
// @see LruHashtable#contains
public synchronized boolean containsKey( Object key )
{
if ( newTable.containsKey( key ) )
return true;
if ( oldTable.containsKey( key ) )
{
// Move object from old table to new table.
Object value = oldTable.get( key );
newTable.put( key, value );
oldTable.remove( key );
return true;
}
return false;
}
/// Gets the object associated with the specified key in the
// hashtable.
// @param key the specified key
// @returns the element for the key or null if the key
// is not defined in the hash table.
// @see LruHashtable#put
public synchronized Object get( Object key )
{
Object value;
value = newTable.get( key );
if ( value != null )
return value;
value = oldTable.get( key );
if ( value != null )
{
// Move object from old table to new table.
newTable.put( key, value );
oldTable.remove( key );
return value;
}
return null;
}
/// Puts the specified element into the hashtable, using the specified
// key. The element may be retrieved by doing a get() with the same key.
// The key and the element cannot be null.
// @param key the specified key in the hashtable
// @param value the specified element
// @exception NullPointerException If the value of the element
// is equal to null.
// @see LruHashtable#get
// @return the old value of the key, or null if it did not have one.
public synchronized Object put( Object key, Object value )
{
Object oldValue = newTable.put( key, value );
if ( oldValue != null )
return oldValue;
oldValue = oldTable.get( key );
if ( oldValue != null )
oldTable.remove( key );
else
{
if ( size() >= threshold )
{
// Rotate the tables.
oldTable = newTable;
newTable = new Hashtable( eachCapacity, loadFactor );
}
}
return oldValue;
}
/// Removes the element corresponding to the key. Does nothing if the
// key is not present.
// @param key the key that needs to be removed
// @return the value of key, or null if the key was not found.
public synchronized Object remove( Object key )
{
Object oldValue = newTable.remove( key );
if ( oldValue == null )
oldValue = oldTable.remove( key );
return oldValue;
}
/// Clears the hash table so that it has no more elements in it.
public synchronized void clear()
{
newTable.clear();
oldTable.clear();
}
/// Creates a clone of the hashtable. A shallow copy is made,
// the keys and elements themselves are NOT cloned. This is a
// relatively expensive operation.
public synchronized Object clone()
{
LruHashtable n = (LruHashtable) super.clone();
n.newTable = (Hashtable) n.newTable.clone();
n.oldTable = (Hashtable) n.oldTable.clone();
return n;
}
// toString() can be inherited.
}
class LruHashtableEnumerator implements Enumeration
{
Enumeration oldEnum;
Enumeration newEnum;
boolean old;
LruHashtableEnumerator( Hashtable oldTable, Hashtable newTable, boolean keys )
{
if ( keys )
{
oldEnum = oldTable.keys();
newEnum = newTable.keys();
}
else
{
oldEnum = oldTable.elements();
newEnum = newTable.elements();
}
old = true;
}
public boolean hasMoreElements()
{
boolean r;
if ( old )
{
r = oldEnum.hasMoreElements();
if ( ! r )
{
old = false;
r = newEnum.hasMoreElements();
}
}
else
r = newEnum.hasMoreElements();
return r;
}
public Object nextElement()
{
if ( old )
return oldEnum.nextElement();
return newEnum.nextElement();
}
}
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