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Cristal-ise is a description-driven software platform originally developed to track the construction of the CMS ECAL detector of the LHC at CERN. This is its core library, known as the kernel, which manages business objects called Items. Items are entirely configured from data, called descriptions, held in other Items. Every change of a state in an Item is a consequence of an execution of an activity in that Item's lifecycle, meaning that Cristal-ise applications are completely traceable, even in their design. It also supports extensive versioning of Item description data, giving the system a high level of flexibility.

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/**
 * This file is part of the CRISTAL-iSE kernel.
 * Copyright (c) 2001-2015 The CRISTAL Consortium. All rights reserved.
 *
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published
 * by the Free Software Foundation; either version 3 of the License, or (at
 * your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; with out even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library; if not, write to the Free Software Foundation,
 * Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
 *
 * http://www.fsf.org/licensing/licenses/lgpl.html
 */
package org.cristalise.kernel.utils;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;

/*******************************************************************************
 * WeakReferences are reaped if no strong references are left next time the gc has a chance. 
 * The ClusterStorageManager caches can optionally use this one, for high volume imports etc
 *
 * $Revision: 1.5 $ $Date: 2004/10/29 13:29:09 $
 ******************************************************************************/
public class WeakCache extends AbstractMap {

    private final Map> hash = new HashMap>();
    private final int minSize;
    private final LinkedList hardCache = new LinkedList();
    private final ReferenceQueue queue = new ReferenceQueue();

    public WeakCache() {
        this(0);
    }

    public WeakCache(int minSize) {
        this.minSize = minSize;
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#get(java.lang.Object)
	 */
	@Override
    public V get(Object key) {
        V result = null;
        WeakValue weak_ref = hash.get(key);
        if (weak_ref != null) {
            result = weak_ref.get();
            if (result == null)
                hash.remove(key);
            else
                if (minSize > 0) { // add to hard cache so it's not reaped for a while
                    hardCache.addFirst(result);
                    if (hardCache.size() > minSize) // trim last one off
                        hardCache.removeLast();
                }
        }
        return result;
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#put(K, V)
	 */
	@Override
	public V put(K key, V value) {
        processQueue();
        if (minSize > 0) {
            hardCache.addFirst(value);
            if (hardCache.size() > minSize)
                hardCache.removeLast();
        }
        hash.put(key, new WeakValue(key, value, queue));
        return value;
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#remove(java.lang.Object)
	 */
	@Override
	public V remove(Object key) {
        processQueue();
        if (hash.containsKey(key)) return hash.remove(key).get();
        return null;
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#clear()
	 */
	@Override
	public void clear() {
        hardCache.clear();
        while(queue.poll()!=null);
        hash.clear();
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#size()
	 */
    @Override
	public int size() {
        processQueue();
        return hash.size();
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#keySet()
	 */
    @Override
	public Set keySet() {
        processQueue();
        return hash.keySet();
    }

    /* (non-Javadoc)
	 * @see org.cristalise.kernel.utils.NonStrongRefCache#entrySet()
	 */
    @Override
	public Set> entrySet() {
        // Would have to create another Map to do this - too expensive
        // Throwing runtime expensive is dangerous, but better than nulls
        throw new UnsupportedOperationException();
    }

    private static class WeakValue extends WeakReference {
        private final Object key;
        private WeakValue(Object key, V value, ReferenceQueue q) {
            super(value, q);
            this.key = key;
        }
    }

    /**
     * Look for values that have been reaped, and remove their keys from the cache
     */
    private void processQueue() {
    	WeakValue sv;
        while ((sv = (WeakValue) queue.poll()) != null) {
            hash.remove(sv.key);
        }
    }

}