The Commons Collections StaticBucketMap.java source code
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* 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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