staticbucketmap.java

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/*
 *  Copyright 2002-2005 The Apache Software Foundation
 *
 *  Licensed 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;

import java.util.AbstractCollection;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * A StaticBucketMap is an efficient, thread-safe implementation of
 * <code>java.util.Map</code> that performs well in in a highly
 * thread-contentious environment.  The map supports very efficient
 * {@link #get(Object) get}, {@link #put(Object,Object) put}, 
 * {@link #remove(Object) remove} and {@link #containsKey(Object) containsKey}
 * operations, assuming (approximate) uniform hashing and
 * that the number of entries does not exceed the number of buckets.  If the
 * number of entries exceeds the number of buckets or if the hash codes of the
 * objects are not uniformly distributed, these operations have a worst case
 * scenario that is proportional to the number of elements in the map
 * (<i>O(n)</i>).<p>
 *
 * Each bucket in the hash table has its own monitor, so two threads can 
 * safely operate on the map at the same time, often without incurring any 
 * monitor contention.  This means that you don't have to wrap instances
 * of this class with {@link java.util.Collections#synchronizedMap(Map)};
 * instances are already thread-safe.  Unfortunately, however, this means 
 * that this map implementation behaves in ways you may find disconcerting.  
 * Bulk operations, such as {@link #putAll(Map) putAll} or the
 * {@link Collection#retainAll(Collection) retainAll} operation in collection 
 * views, are <i>not</i> atomic.  If two threads are simultaneously 
 * executing 
 *
 * <pre>
 *   staticBucketMapInstance.putAll(map);
 * </pre>
 *
 * and
 *
 * <pre>
 *   staticBucketMapInstance.entrySet().removeAll(map.entrySet());
 * </pre>
 *
 * then the results are generally random.  Those two statement could cancel
 * each other out, leaving <code>staticBucketMapInstance</code> essentially 
 * unchanged, or they could leave some random subset of <code>map</code> in 
 * <code>staticBucketMapInstance</code>.<p>
 *
 * Also, much like an encyclopedia, the results of {@link #size()} and 
 * {@link #isEmpty()} are out-of-date as soon as they are produced.<p>
 *
 * The iterators returned by the collection views of this class are <i>not</i>
 * fail-fast.  They will <i>never</i> raise a 
 * {@link java.util.ConcurrentModificationException}.  Keys and values 
 * added to the map after the iterator is created do not necessarily appear
 * during iteration.  Similarly, the iterator does not necessarily fail to 
 * return keys and values that were removed after the iterator was created.<p>
 *
 * Finally, unlike {@link java.util.HashMap}-style implementations, this
 * class <i>never</i> rehashes the map.  The number of buckets is fixed 
 * at construction time and never altered.  Performance may degrade if 
 * you do not allocate enough buckets upfront.<p>
 *
 * The {@link #atomic(Runnable)} method is provided to allow atomic iterations
 * and bulk operations; however, overuse of {@link #atomic(Runnable) atomic}
 * will basically result in a map that's slower than an ordinary synchronized
 * {@link java.util.HashMap}.
 *
 * Use this class if you do not require reliable bulk operations and 
 * iterations, or if you can make your own guarantees about how bulk 
 * operations will affect the map.<p>
 *
 * @deprecated Moved to map subpackage. Due to be removed in v4.0.
 * @since Commons Collections 2.1
 * @version $Revision: 348273 $ $Date: 2005-11-22 22:24:25 +0000 (Tue, 22 Nov 2005) $
 * 
 * @author <a href="mailto:[e-mail]">Berin Loritsch</a>
 * @author <a href="mailto:[e-mail]">Gerhard Froehlich</a>
 * @author <a href="mailto:[e-mail]">Michael A. Smith</a>
 * @author Paul Jack
 * @author Leo Sutic
 * @author Janek Bogucki
 * @author Kazuya Ujihara
 */
public final class StaticBucketMap implements Map {

    private static final int DEFAULT_BUCKETS = 255;
    private Node[] m_buckets;
    private Lock[] m_locks;

    /**
     * Initializes the map with the default number of buckets (255).
     */
    public StaticBucketMap()
    {
        this( DEFAULT_BUCKETS );
    }

    /**
     * Initializes the map with a specified number of buckets.  The number
     * of buckets is never below 17, and is always an odd number (StaticBucketMap
     * ensures this). The number of buckets is inversely proportional to the
     * chances for thread contention.  The fewer buckets, the more chances for
     * thread contention.  The more buckets the fewer chances for thread
     * contention.
     *
     * @param numBuckets  the number of buckets for this map
     */
    public StaticBucketMap( int numBuckets )
    {
        int size = Math.max( 17, numBuckets );

        // Ensure that bucketSize is never a power of 2 (to ensure maximal distribution)
        if( size % 2 == 0 )
        {
            size--;
        }

        m_buckets = new Node[ size ];
        m_locks = new Lock[ size ];

        for( int i = 0; i < size; i++ )
        {
            m_locks[ i ] = new Lock();
        }
    }

    /**
     * Determine the exact hash entry for the key.  The hash algorithm
     * is rather simplistic, but it does the job:
     *
     * <pre>
     *   He = |Hk mod n|
     * </pre>
     *
     * <p>
     *   He is the entry's hashCode, Hk is the key's hashCode, and n is
     *   the number of buckets.
     * </p>
     */
    private final int getHash( Object key )
    {
        if( key == null ) return 0;
        int hash = key.hashCode();
        hash += ~(hash << 15);
        hash ^= (hash >>> 10);
        hash += (hash << 3);
        hash ^= (hash >>> 6);
        hash += ~(hash << 11);
        hash ^= (hash >>> 16);
        hash %= m_buckets.length;
        return ( hash < 0 ) ? hash * -1 : hash;
    }

    /**
     *  Implements {@link Map#keySet()}.
     */
    public Set keySet()
    {
        return new KeySet();
    }

    /**
     *  Implements {@link Map#size()}.
     */
    public int size()
    {
        int cnt = 0;

        for( int i = 0; i < m_buckets.length; i++ )
        {
            cnt += m_locks[i].size;
        }

        return cnt;
    }

    /**
     *  Implements {@link Map#put(Object, Object)}.
     */
    public Object put( final Object key, final Object value )
    {
        int hash = getHash( key );

        synchronized( m_locks[ hash ] )
        {
            Node n = m_buckets[ hash ];

            if( n == null )
            {
                n = new Node();
                n.key = key;
                n.value = value;
                m_buckets[ hash ] = n;
                m_locks[hash].size++;
                return null;
            }

            // Set n to the last node in the linked list.  Check each key along the way
            //  If the key is found, then change the value of that node and return
            //  the old value.
            for( Node next = n; next != null; next = next.next )
            {
                n = next;

                if( n.key == key || ( n.key != null && n.key.equals( key ) ) )
                {
                    Object returnVal = n.value;
                    n.value = value;
                    return returnVal;
                }
            }

            // The key was not found in the current list of nodes, add it to the end
            //  in a new node.
            Node newNode = new Node();
            newNode.key = key;
            newNode.value = value;
            n.next = newNode;
            m_locks[hash].size++;
        }

        return null;
    }

    /**
     *  Implements {@link Map#get(Object)}.
     */
    public Object get( final Object key )
    {
        int hash = getHash( key );

        synchronized( m_locks[ hash ] )
        {
            Node n = m_buckets[ hash ];

            while( n != null )
            {
                if( n.key == key || ( n.key != null && n.key.equals( key ) ) )
                {
                    return n.value;
                }

                n = n.next;
            }
        }

        return null;
    }

    /**
     * Implements {@link Map#containsKey(Object)}.
     */
    public boolean containsKey( final Object key )
    {
        int hash = getHash( key );

        synchronized( m_locks[ hash ] )
        {
            Node n = m_buckets[ hash ];

            while( n != null )
            {
                if( n.key == key || ( n.key != null && n.key.equals( key ) ) )
                {
                    return true;
                }

                n = n.next;
            }
        }

        return false;
    }

    /**
     * Implements {@link Map#containsValue(Object)}.
     */
    public boolean containsValue( final Object value )
    {
        for( int i = 0; i < m_buckets.length; i++ )
        {
            synchronized( m_locks[ i ] )
            {
                Node n = m_buckets[ i ];

                while( n != null )
                {
                    if( n.value == value || 
                        (n.value != null && n.value.equals( value ) ) )
                    {
                        return true;
                    }

                    n = n.next;
                }
            }
        }

        return false;
    }

    /**
     *  Implements {@link Map#values()}.
     */
    public Collection values()
    {
        return new Values();
    }

    /**
     *  Implements {@link Map#entrySet()}.
     */
    public Set entrySet()
    {
        return new EntrySet();
    }

    /**
     *  Implements {@link Map#putAll(Map)}.
     */
    public void putAll( Map other )
    {
        Iterator i = other.keySet().iterator();

        while( i.hasNext() )
        {
            Object key = i.next();
            put( key, other.get( key ) );
        }
    }

    /**
     *  Implements {@link Map#remove(Object)}.
     */
    public Object remove( Object key )
    {
        int hash = getHash( key );