Customized java.util.HashMap: operate in a multithreaded environment where the large majority of method calls are read-only, instead of structural changes.
/*
* 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.
*/
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
/**
* <p>A customized implementation of <code>java.util.HashMap</code> designed
* to operate in a multithreaded environment where the large majority of
* method calls are read-only, instead of structural changes. When operating
* in "fast" mode, read calls are non-synchronized and write calls perform the
* following steps:</p>
* <ul>
* <li>Clone the existing collection
* <li>Perform the modification on the clone
* <li>Replace the existing collection with the (modified) clone
* </ul>
* <p>When first created, objects of this class default to "slow" mode, where
* all accesses of any type are synchronized but no cloning takes place. This
* is appropriate for initially populating the collection, followed by a switch
* to "fast" mode (by calling <code>setFast(true)</code>) after initialization
* is complete.</p>
*
* <p><strong>NOTE</strong>: If you are creating and accessing a
* <code>HashMap</code> only within a single thread, you should use
* <code>java.util.HashMap</code> directly (with no synchronization), for
* maximum performance.</p>
*
* <p><strong>NOTE</strong>: <i>This class is not cross-platform.
* Using it may cause unexpected failures on some architectures.</i>
* It suffers from the same problems as the double-checked locking idiom.
* In particular, the instruction that clones the internal collection and the
* instruction that sets the internal reference to the clone can be executed
* or perceived out-of-order. This means that any read operation might fail
* unexpectedly, as it may be reading the state of the internal collection
* before the internal collection is fully formed.
* For more information on the double-checked locking idiom, see the
* <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html">
* Double-Checked Locking Idiom Is Broken Declaration</a>.</p>
*
* @since Commons Collections 1.0
* @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
*
* @author Craig R. McClanahan
* @author Stephen Colebourne
*/
public class FastHashMap extends HashMap {
/**
* The underlying map we are managing.
*/
protected HashMap map = null;
/**
* Are we currently operating in "fast" mode?
*/
protected boolean fast = false;
// Constructors
// ----------------------------------------------------------------------
/**
* Construct an empty map.
*/
public FastHashMap() {
super();
this.map = new HashMap();
}
/**
* Construct an empty map with the specified capacity.
*
* @param capacity the initial capacity of the empty map
*/
public FastHashMap(int capacity) {
super();
this.map = new HashMap(capacity);
}
/**
* Construct an empty map with the specified capacity and load factor.
*
* @param capacity the initial capacity of the empty map
* @param factor the load factor of the new map
*/
public FastHashMap(int capacity, float factor) {
super();
this.map = new HashMap(capacity, factor);
}
/**
* Construct a new map with the same mappings as the specified map.
*
* @param map the map whose mappings are to be copied
*/
public FastHashMap(Map map) {
super();
this.map = new HashMap(map);
}
// Property access
// ----------------------------------------------------------------------
/**
* Returns true if this map is operating in fast mode.
*
* @return true if this map is operating in fast mode
*/
public boolean getFast() {
return (this.fast);
}
/**
* Sets whether this map is operating in fast mode.
*
* @param fast true if this map should operate in fast mode
*/
public void setFast(boolean fast) {
this.fast = fast;
}
// Map access
// ----------------------------------------------------------------------
// These methods can forward straight to the wrapped Map in 'fast' mode.
// (because they are query methods)
/**
* Return the value to which this map maps the specified key. Returns
* <code>null</code> if the map contains no mapping for this key, or if
* there is a mapping with a value of <code>null</code>. Use the
* <code>containsKey()</code> method to disambiguate these cases.
*
* @param key the key whose value is to be returned
* @return the value mapped to that key, or null
*/
public Object get(Object key) {
if (fast) {
return (map.get(key));
} else {
synchronized (map) {
return (map.get(key));
}
}
}
/**
* Return the number of key-value mappings in this map.
*
* @return the current size of the map
*/
public int size() {
if (fast) {
return (map.size());
} else {
synchronized (map) {
return (map.size());
}
}
}
/**
* Return <code>true</code> if this map contains no mappings.
*
* @return is the map currently empty
*/
public boolean isEmpty() {
if (fast) {
return (map.isEmpty());
} else {
synchronized (map) {
return (map.isEmpty());
}
}
}
/**
* Return <code>true</code> if this map contains a mapping for the
* specified key.
*
* @param key the key to be searched for
* @return true if the map contains the key
*/
public boolean containsKey(Object key) {
if (fast) {
return (map.containsKey(key));
} else {
synchronized (map) {
return (map.containsKey(key));
}
}
}
/**
* Return <code>true</code> if this map contains one or more keys mapping
* to the specified value.
*
* @param value the value to be searched for
* @return true if the map contains the value
*/
public boolean containsValue(Object value) {
if (fast) {
return (map.containsValue(value));
} else {
synchronized (map) {
return (map.containsValue(value));
}
}
}
// Map modification
// ----------------------------------------------------------------------
// These methods perform special behaviour in 'fast' mode.
// The map is cloned, updated and then assigned back.
// See the comments at the top as to why this won't always work.
/**
* Associate the specified value with the specified key in this map.
* If the map previously contained a mapping for this key, the old
* value is replaced and returned.
*
* @param key the key with which the value is to be associated
* @param value the value to be associated with this key
* @return the value previously mapped to the key, or null
*/
public Object put(Object key, Object value) {
if (fast) {
synchronized (this) {
HashMap temp = (HashMap) map.clone();
Object result = temp.put(key, value);
map = temp;
return (result);
}
} else {
synchronized (map) {
return (map.put(key, value));
}
}
}
/**
* Copy all of the mappings from the specified map to this one, replacing
* any mappings with the same keys.
*
* @param in the map whose mappings are to be copied
*/
public void putAll(Map in) {
if (fast) {
synchronized (this) {
HashMap temp = (HashMap) map.clone();
temp.putAll(in);
map = temp;
}
} else {
synchronized (map) {
map.putAll(in);
}
}
}
/**
* Remove any mapping for this key, and return any previously
* mapped value.
*
* @param key the key whose mapping is to be removed
* @return the value removed, or null
*/
public Object remove(Object key) {
if (fast) {
synchronized (this) {
HashMap temp = (HashMap) map.clone();
Object result = temp.remove(key);
map = temp;
return (result);
}
} else {
synchronized (map) {
return (map.remove(key));
}
}
}
/**
* Remove all mappings from this map.
*/
public void clear() {
if (fast) {
synchronized (this) {
map = new HashMap();
}
} else {
synchronized (map) {
map.clear();
}
}
}
// Basic object methods
// ----------------------------------------------------------------------
/**
* Compare the specified object with this list for equality. This
* implementation uses exactly the code that is used to define the
* list equals function in the documentation for the
* <code>Map.equals</code> method.
*
* @param o the object to be compared to this list
* @return true if the two maps are equal
*/
public boolean equals(Object o) {
// Simple tests that require no synchronization
if (o == this) {
return (true);
} else if (!(o instanceof Map)) {
return (false);
}
Map mo = (Map) o;
// Compare the two maps for equality
if (fast) {
if (mo.size() != map.size()) {
return (false);
}
Iterator i = map.entrySet().iterator();
while (i.hasNext()) {
Map.Entry e = (Map.Entry) i.next();
Object key = e.getKey();
Object value = e.getValue();
if (value == null) {
if (!(mo.get(key) == null && mo.containsKey(key))) {
return (false);
}
} else {
if (!value.equals(mo.get(key))) {
return (false);
}
}
}
return (true);
} else {
synchronized (map) {
if (mo.size() != map.size()) {
return (false);
}
Iterator i = map.entrySet().iterator();
while (i.hasNext()) {
Map.Entry e = (Map.Entry) i.next();
Object key = e.getKey();
Object value = e.getValue();
if (value == null) {
if (!(mo.get(key) == null && mo.containsKey(key))) {
return (false);
}
} else {
if (!value.equals(mo.get(key))) {
return (false);
}
}
}
return (true);
}
}
}
/**
* Return the hash code value for this map. This implementation uses
* exactly the code that is used to define the list hash function in the
* documentation for the <code>Map.hashCode</code> method.
*
* @return suitable integer hash code
*/
public int hashCode() {
if (fast) {
int h = 0;
Iterator i = map.entrySet().iterator();
while (i.hasNext()) {
h += i.next().hashCode();
}
return (h);
} else {
synchronized (map) {
int h = 0;
Iterator i = map.entrySet().iterator();
while (i.hasNext()) {
h += i.next().hashCode();
}
return (h);
}
}
}
/**
* Return a shallow copy of this <code>FastHashMap</code> instance.
* The keys and values themselves are not copied.
*
* @return a clone of this map
*/
public Object clone() {
FastHashMap results = null;
if (fast) {
results = new FastHashMap(map);
} else {
synchronized (map) {
results = new FastHashMap(map);
}
}
results.setFast(getFast());
return (results);
}
// Map views
// ----------------------------------------------------------------------
/**
* Return a collection view of the mappings contained in this map. Each
* element in the returned collection is a <code>Map.Entry</code>.
*/
public Set entrySet() {
return new EntrySet();
}
/**
* Return a set view of the keys contained in this map.
*/
public Set keySet() {
return new KeySet();
}
/**
* Return a collection view of the values contained in this map.
*/
public Collection values() {
return new Values();
}
// Map view inner classes
// ----------------------------------------------------------------------
/**
* Abstract collection implementation shared by keySet(), values() and entrySet().
*/
private abstract class CollectionView implements Collection {
public CollectionView() {
}
protected abstract Collection get(Map map);
protected abstract Object iteratorNext(Map.Entry entry);
public void clear() {
if (fast) {
synchronized (FastHashMap.this) {
map = new HashMap();
}
} else {
synchronized (map) {
get(map).clear();
}
}
}
public boolean remove(Object o) {
if (fast) {
synchronized (FastHashMap.this) {
HashMap temp = (HashMap) map.clone();
boolean r = get(temp).remove(o);
map = temp;
return r;
}
} else {
synchronized (map) {
return get(map).remove(o);
}
}
}
public boolean removeAll(Collection o) {
if (fast) {
synchronized (FastHashMap.this) {
HashMap temp = (HashMap) map.clone();
boolean r = get(temp).removeAll(o);
map = temp;
return r;
}
} else {
synchronized (map) {
return get(map).removeAll(o);
}
}
}
public boolean retainAll(Collection o) {
if (fast) {
synchronized (FastHashMap.this) {
HashMap temp = (HashMap) map.clone();
boolean r = get(temp).retainAll(o);
map = temp;
return r;
}
} else {
synchronized (map) {
return get(map).retainAll(o);
}
}
}
public int size() {
if (fast) {
return get(map).size();
} else {
synchronized (map) {
return get(map).size();
}
}
}
public boolean isEmpty() {
if (fast) {
return get(map).isEmpty();
} else {
synchronized (map) {
return get(map).isEmpty();
}
}
}
public boolean contains(Object o) {
if (fast) {
return get(map).contains(o);
} else {
synchronized (map) {
return get(map).contains(o);
}
}
}
public boolean containsAll(Collection o) {
if (fast) {
return get(map).containsAll(o);
} else {
synchronized (map) {
return get(map).containsAll(o);
}
}
}
public Object[] toArray(Object[] o) {
if (fast) {
return get(map).toArray(o);
} else {
synchronized (map) {
return get(map).toArray(o);
}
}
}
public Object[] toArray() {
if (fast) {
return get(map).toArray();
} else {
synchronized (map) {
return get(map).toArray();
}
}
}
public boolean equals(Object o) {
if (o == this) return true;
if (fast) {
return get(map).equals(o);
} else {
synchronized (map) {
return get(map).equals(o);
}
}
}
public int hashCode() {
if (fast) {
return get(map).hashCode();
} else {
synchronized (map) {
return get(map).hashCode();
}
}
}
public boolean add(Object o) {
throw new UnsupportedOperationException();
}
public boolean addAll(Collection c) {
throw new UnsupportedOperationException();
}
public Iterator iterator() {
return new CollectionViewIterator();
}
private class CollectionViewIterator implements Iterator {
private Map expected;
private Map.Entry lastReturned = null;
private Iterator iterator;
public CollectionViewIterator() {
this.expected = map;
this.iterator = expected.entrySet().iterator();
}
public boolean hasNext() {
if (expected != map) {
throw new ConcurrentModificationException();
}
return iterator.hasNext();
}
public Object next() {
if (expected != map) {
throw new ConcurrentModificationException();
}
lastReturned = (Map.Entry)iterator.next();
return iteratorNext(lastReturned);
}
public void remove() {
if (lastReturned == null) {
throw new IllegalStateException();
}
if (fast) {
synchronized (FastHashMap.this) {
if (expected != map) {
throw new ConcurrentModificationException();
}
FastHashMap.this.remove(lastReturned.getKey());
lastReturned = null;
expected = map;
}
} else {
iterator.remove();
lastReturned = null;
}
}
}
}
/**
* Set implementation over the keys of the FastHashMap
*/
private class KeySet extends CollectionView implements Set {
protected Collection get(Map map) {
return map.keySet();
}
protected Object iteratorNext(Map.Entry entry) {
return entry.getKey();
}
}
/**
* Collection implementation over the values of the FastHashMap
*/
private class Values extends CollectionView {
protected Collection get(Map map) {
return map.values();
}
protected Object iteratorNext(Map.Entry entry) {
return entry.getValue();
}
}
/**
* Set implementation over the entries of the FastHashMap
*/
private class EntrySet extends CollectionView implements Set {
protected Collection get(Map map) {
return map.entrySet();
}
protected Object iteratorNext(Map.Entry entry) {
return entry;
}
}
}
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