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Commons Collections example source code file (StaticBucketMap.java)

This example Commons Collections source code file (StaticBucketMap.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

entryiterator, entryset, iterator, iterator, keyiterator, lock, lock, map, node, node, object, object, staticbucketmap, util, values

The Commons Collections StaticBucketMap.java source code

/*
 *  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.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;

import org.apache.commons.collections.KeyValue;

/**
 * A StaticBucketMap is an efficient, thread-safe implementation of
 * <code>java.util.Map 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)).

* * 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 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 essentially * unchanged, or they could leave some random subset of <code>map in * <code>staticBucketMapInstance.

* * 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 * fail-fast. They will <i>never 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 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> * * @since Commons Collections 3.0 (previously in main package v2.1) * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $ * * @author Berin Loritsch * @author Gerhard Froehlich * @author Michael A. Smith * @author Paul Jack * @author Leo Sutic * @author Janek Bogucki * @author Kazuya Ujihara */ public final class StaticBucketMap implements Map { /** The default number of buckets to use */ private static final int DEFAULT_BUCKETS = 255; /** The array of buckets, where the actual data is held */ private Node[] buckets; /** The matching array of locks */ private Lock[] 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--; } buckets = new Node[size]; locks = new Lock[size]; for (int i = 0; i < size; i++) { 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 %= buckets.length; return (hash < 0) ? hash * -1 : hash; } /** * Gets the current size of the map. * The value is computed fresh each time the method is called. * * @return the current size */ public int size() { int cnt = 0; for (int i = 0; i < buckets.length; i++) { cnt += locks[i].size; } return cnt; } /** * Checks if the size is currently zero. * * @return true if empty */ public boolean isEmpty() { return (size() == 0); } /** * Gets the value associated with the key. * * @param key the key to retrieve * @return the associated value */ public Object get(final Object key) { int hash = getHash(key); synchronized (locks[hash]) { Node n = buckets[hash]; while (n != null) { if (n.key == key || (n.key != null && n.key.equals(key))) { return n.value; } n = n.next; } } return null; } /** * Checks if the map contains the specified key. * * @param key the key to check * @return true if found */ public boolean containsKey(final Object key) { int hash = getHash(key); synchronized (locks[hash]) { Node n = buckets[hash]; while (n != null) { if (n.key == key || (n.key != null && n.key.equals(key))) { return true; } n = n.next; } } return false; } /** * Checks if the map contains the specified value. * * @param value the value to check * @return true if found */ public boolean containsValue(final Object value) { for (int i = 0; i < buckets.length; i++) { synchronized (locks[i]) { Node n = buckets[i]; while (n != null) { if (n.value == value || (n.value != null && n.value.equals(value))) { return true; } n = n.next; } } } return false; } //----------------------------------------------------------------------- /** * Puts a new key value mapping into the map. * * @param key the key to use * @param value the value to use * @return the previous mapping for the key */ public Object put(final Object key, final Object value) { int hash = getHash(key); synchronized (locks[hash]) { Node n = buckets[hash]; if (n == null) { n = new Node(); n.key = key; n.value = value; buckets[hash] = n; 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; locks[hash].size++; } return null; } /** * Removes the specified key from the map. * * @param key the key to remove * @return the previous value at this key */ public Object remove(Object key) { int hash = getHash(key); synchronized (locks[hash]) { Node n = buckets[hash]; Node prev = null; while (n != null) { if (n.key == key || (n.key != null && n.key.equals(key))) { // Remove this node from the linked list of nodes. if (null == prev) { // This node was the head, set the next node to be the new head. buckets[hash] = n.next; } else { // Set the next node of the previous node to be the node after this one. prev.next = n.next; } locks[hash].size--; return n.value; } prev = n; n = n.next; } } return null; } //----------------------------------------------------------------------- /** * Gets the key set. * * @return the key set */ public Set keySet() { return new KeySet(); } /** * Gets the values. * * @return the values */ public Collection values() { return new Values(); } /** * Gets the entry set. * * @return the entry set */ public Set entrySet() { return new EntrySet(); } //----------------------------------------------------------------------- /** * Puts all the entries from the specified map into this map. * This operation is <b>not atomic and may have undesired effects. * * @param map the map of entries to add */ public void putAll(Map map) { Iterator i = map.keySet().iterator(); while (i.hasNext()) { Object key = i.next(); put(key, map.get(key)); } } /** * Clears the map of all entries. */ public void clear() { for (int i = 0; i < buckets.length; i++) { Lock lock = locks[i]; synchronized (lock) { buckets[i] = null; lock.size = 0; } } } /** * Compares this map to another, as per the Map specification. * * @param obj the object to compare to * @return true if equal */ public boolean equals(Object obj) { if (obj == this) { return true; } if (obj instanceof Map == false) { return false; } Map other = (Map) obj; return entrySet().equals(other.entrySet()); } /** * Gets the hash code, as per the Map specification. * * @return the hash code */ public int hashCode() { int hashCode = 0; for (int i = 0; i < buckets.length; i++) { synchronized (locks[i]) { Node n = buckets[i]; while (n != null) { hashCode += n.hashCode(); n = n.next; } } } return hashCode; } //----------------------------------------------------------------------- /** * The Map.Entry for the StaticBucketMap. */ private static final class Node implements Map.Entry, KeyValue { protected Object key; protected Object value; protected Node next; public Object getKey() { return key; } public Object getValue() { return value; } public int hashCode() { return ((key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode())); } public boolean equals(Object obj) { if (obj == this) { return true; } if (obj instanceof Map.Entry == false) { return false; } Map.Entry e2 = (Map.Entry) obj; return ( (key == null ? e2.getKey() == null : key.equals(e2.getKey())) && (value == null ? e2.getValue() == null : value.equals(e2.getValue()))); } public Object setValue(Object obj) { Object retVal = value; value = obj; return retVal; } } /** * The lock object, which also includes a count of the nodes in this lock. */ private final static class Lock { public int size; } //----------------------------------------------------------------------- private class EntryIterator implements Iterator { private ArrayList current = new ArrayList(); private int bucket; private Map.Entry last; public boolean hasNext() { if (current.size() > 0) return true; while (bucket < buckets.length) { synchronized (locks[bucket]) { Node n = buckets[bucket]; while (n != null) { current.add(n); n = n.next; } bucket++; if (current.size() > 0) return true; } } return false; } protected Map.Entry nextEntry() { if (!hasNext()) throw new NoSuchElementException(); last = (Map.Entry)current.remove(current.size() - 1); return last; } public Object next() { return nextEntry(); } public void remove() { if (last == null) throw new IllegalStateException(); StaticBucketMap.this.remove(last.getKey()); last = null; } } private class ValueIterator extends EntryIterator { public Object next() { return nextEntry().getValue(); } } private class KeyIterator extends EntryIterator { public Object next() { return nextEntry().getKey(); } } private class EntrySet extends AbstractSet { public int size() { return StaticBucketMap.this.size(); } public void clear() { StaticBucketMap.this.clear(); } public Iterator iterator() { return new EntryIterator(); } public boolean contains(Object obj) { Map.Entry entry = (Map.Entry) obj; int hash = getHash(entry.getKey()); synchronized (locks[hash]) { for (Node n = buckets[hash]; n != null; n = n.next) { if (n.equals(entry)) return true; } } return false; } public boolean remove(Object obj) { if (obj instanceof Map.Entry == false) { return false; } Map.Entry entry = (Map.Entry) obj; int hash = getHash(entry.getKey()); synchronized (locks[hash]) { for (Node n = buckets[hash]; n != null; n = n.next) { if (n.equals(entry)) { StaticBucketMap.this.remove(n.getKey()); return true; } } } return false; } } private class KeySet extends AbstractSet { public int size() { return StaticBucketMap.this.size(); } public void clear() { StaticBucketMap.this.clear(); } public Iterator iterator() { return new KeyIterator(); } public boolean contains(Object obj) { return StaticBucketMap.this.containsKey(obj); } public boolean remove(Object obj) { int hash = getHash(obj); synchronized (locks[hash]) { for (Node n = buckets[hash]; n != null; n = n.next) { Object k = n.getKey(); if ((k == obj) || ((k != null) && k.equals(obj))) { StaticBucketMap.this.remove(k); return true; } } } return false; } } private class Values extends AbstractCollection { public int size() { return StaticBucketMap.this.size(); } public void clear() { StaticBucketMap.this.clear(); } public Iterator iterator() { return new ValueIterator(); } } /** * Prevents any operations from occurring on this map while the * given {@link Runnable} executes. This method can be used, for * instance, to execute a bulk operation atomically: * * <pre> * staticBucketMapInstance.atomic(new Runnable() { * public void run() { * staticBucketMapInstance.putAll(map); * } * }); * </pre> * * It can also be used if you need a reliable iterator: * * <pre> * staticBucketMapInstance.atomic(new Runnable() { * public void run() { * Iterator iterator = staticBucketMapInstance.iterator(); * while (iterator.hasNext()) { * foo(iterator.next(); * } * } * }); * </pre> * * <b>Implementation note: This method requires a lot of time * and a ton of stack space. Essentially a recursive algorithm is used * to enter each bucket's monitor. If you have twenty thousand buckets * in your map, then the recursive method will be invoked twenty thousand * times. You have been warned. * * @param r the code to execute atomically */ public void atomic(Runnable r) { if (r == null) throw new NullPointerException(); atomic(r, 0); } private void atomic(Runnable r, int bucket) { if (bucket >= buckets.length) { r.run(); return; } synchronized (locks[bucket]) { atomic(r, bucket + 1); } } }

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