abstractreferencemap.java

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/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.commons.collections.map;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

import org.apache.commons.collections.MapIterator;
import org.apache.commons.collections.keyvalue.DefaultMapEntry;

/**
 * An abstract implementation of a hash-based map that allows the entries to
 * be removed by the garbage collector.
 * <p>
 * This class implements all the features necessary for a subclass reference
 * hash-based map. Key-value entries are stored in instances of the
 * <code>ReferenceEntry</code> class which can be overridden and replaced.
 * The iterators can similarly be replaced, without the need to replace the KeySet,
 * EntrySet and Values view classes.
 * <p>
 * Overridable methods are provided to change the default hashing behaviour, and
 * to change how entries are added to and removed from the map. Hopefully, all you
 * need for unusual subclasses is here.
 * <p>
 * When you construct an <code>AbstractReferenceMap</code>, you can specify what
 * kind of references are used to store the map's keys and values.
 * If non-hard references are used, then the garbage collector can remove
 * mappings if a key or value becomes unreachable, or if the JVM's memory is
 * running low. For information on how the different reference types behave,
 * see {@link Reference}.
 * <p>
 * Different types of references can be specified for keys and values.
 * The keys can be configured to be weak but the values hard,
 * in which case this class will behave like a
 * <a href="http://java.sun.com/j2se/1.4/docs/api/java/util/WeakHashMap.html">
 * <code>WeakHashMap</code></a>. However, you can also specify hard keys and
 * weak values, or any other combination. The default constructor uses
 * hard keys and soft values, providing a memory-sensitive cache.
 * <p>
 * This {@link Map} implementation does <i>not</i> allow null elements.
 * Attempting to add a null key or value to the map will raise a
 * <code>NullPointerException</code>.
 * <p>
 * All the available iterators can be reset back to the start by casting to
 * <code>ResettableIterator</code> and calling <code>reset()</code>.
 * <p>
 * This implementation is not synchronized.
 * You can use {@link java.util.Collections#synchronizedMap} to 
 * provide synchronized access to a <code>ReferenceMap</code>.
 *
 * @see java.lang.ref.Reference
 * @since Commons Collections 3.1 (extracted from ReferenceMap in 3.0)
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Paul Jack
 * @author Stephen Colebourne
 */
public abstract class AbstractReferenceMap extends AbstractHashedMap {

    /** Constant indicating that hard references should be used */
    public static final int HARD = 0;

    /** Constant indicating that soft references should be used */
    public static final int SOFT = 1;

    /** Constant indicating that weak references should be used */
    public static final int WEAK = 2;

    /**
     * The reference type for keys.  Must be HARD, SOFT, WEAK.
     * @serial
     */
    protected int keyType;

    /**
     * The reference type for values.  Must be HARD, SOFT, WEAK.
     * @serial
     */
    protected int valueType;

    /**
     * Should the value be automatically purged when the associated key has been collected?
     */
    protected boolean purgeValues;

    /**
     * ReferenceQueue used to eliminate stale mappings.
     * See purge.
     */
    private transient ReferenceQueue queue;

    //-----------------------------------------------------------------------
    /**
     * Constructor used during deserialization.
     */
    protected AbstractReferenceMap() {
        super();
    }

    /**
     * Constructs a new empty map with the specified reference types,
     * load factor and initial capacity.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param capacity  the initial capacity for the map
     * @param loadFactor  the load factor for the map
     * @param purgeValues  should the value be automatically purged when the 
     *   key is garbage collected 
     */
    protected AbstractReferenceMap(
            int keyType, int valueType, int capacity, 
            float loadFactor, boolean purgeValues) {
        super(capacity, loadFactor);
        verify("keyType", keyType);
        verify("valueType", valueType);
        this.keyType = keyType;
        this.valueType = valueType;
        this.purgeValues = purgeValues;
    }

    /**
     * Initialise this subclass during construction, cloning or deserialization.
     */
    protected void init() {
        queue = new ReferenceQueue();
    }

    //-----------------------------------------------------------------------
    /**
     * Checks the type int is a valid value.
     * 
     * @param name  the name for error messages
     * @param type  the type value to check
     * @throws IllegalArgumentException if the value if invalid
     */
    private static void verify(String name, int type) {
        if ((type < HARD) || (type > WEAK)) {
            throw new IllegalArgumentException(name + " must be HARD, SOFT, WEAK.");
        }
    }

    //-----------------------------------------------------------------------
    /**
     * Gets the size of the map.
     * 
     * @return the size
     */
    public int size() {
        purgeBeforeRead();
        return super.size();
    }

    /**
     * Checks whether the map is currently empty.
     * 
     * @return true if the map is currently size zero
     */
    public boolean isEmpty() {
        purgeBeforeRead();
        return super.isEmpty();
    }

    /**
     * Checks whether the map contains the specified key.
     * 
     * @param key  the key to search for
     * @return true if the map contains the key
     */
    public boolean containsKey(Object key) {
        purgeBeforeRead();
        Entry entry = getEntry(key);
        if (entry == null) {
            return false;
        }
        return (entry.getValue() != null);
    }

    /**
     * Checks whether the map contains the specified value.
     * 
     * @param value  the value to search for
     * @return true if the map contains the value
     */
    public boolean containsValue(Object value) {
        purgeBeforeRead();
        if (value == null) {
            return false;
        }
        return super.containsValue(value);
    }

    /**
     * Gets the value mapped to the key specified.
     * 
     * @param key  the key
     * @return the mapped value, null if no match
     */
    public Object get(Object key) {
        purgeBeforeRead();
        Entry entry = getEntry(key);
        if (entry == null) {
            return null;
        }
        return entry.getValue();
    }


    /**
     * Puts a key-value mapping into this map.
     * Neither the key nor the value may be null.
     * 
     * @param key  the key to add, must not be null
     * @param value  the value to add, must not be null
     * @return the value previously mapped to this key, null if none
     * @throws NullPointerException if either the key or value is null
     */
    public Object put(Object key, Object value) {
        if (key == null) {
            throw new NullPointerException("null keys not allowed");
        }
        if (value == null) {
            throw new NullPointerException("null values not allowed");
        }

        purgeBeforeWrite();
        return super.put(key, value);
    }
    
    /**
     * Removes the specified mapping from this map.
     * 
     * @param key  the mapping to remove
     * @return the value mapped to the removed key, null if key not in map
     */
    public Object remove(Object key) {
        if (key == null) {
            return null;
        }
        purgeBeforeWrite();
        return super.remove(key);
    }

    /**
     * Clears this map.
     */
    public void clear() {
        super.clear();
        while (queue.poll() != null) {} // drain the queue
    }

    //-----------------------------------------------------------------------
    /**
     * Gets a MapIterator over the reference map.
     * The iterator only returns valid key/value pairs.
     * 
     * @return a map iterator
     */
    public MapIterator mapIterator() {
        return new ReferenceMapIterator(this);
    }

    /**
     * Returns a set view of this map's entries.
     * An iterator returned entry is valid until <code>next()</code> is called again.
     * The <code>setValue()</code> method on the <code>toArray</code> entries has no effect.
     *
     * @return a set view of this map's entries
     */
    public Set entrySet() {
        if (entrySet == null) {
            entrySet = new ReferenceEntrySet(this);
        }
        return entrySet;
    }

    /**
     * Returns a set view of this map's keys.
     *
     * @return a set view of this map's keys
     */
    public Set keySet() {
        if (keySet == null) {
            keySet = new ReferenceKeySet(this);
        }
        return keySet;
    }

    /**
     * Returns a collection view of this map's values.
     *
     * @return a set view of this map's values
     */
    public Collection values() {
        if (values == null) {
            values = new ReferenceValues(this);
        }
        return values;
    }