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/*
* Copyright 2015 dorkbox, llc
*
* 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 dorkbox.util.collections;
import java.io.Serializable;
import java.util.*;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
/**
* This class uses the "single-writer-principle" for lock-free publication.
*
* Since there are only 2 methods to guarantee that modifications can only be called one-at-a-time (either it is only called by
* one thread, or only one thread can access it at a time) -- we chose the 2nd option -- and use 'synchronized' to make sure that only
* one thread can access this modification methods at a time. Getting or checking the presence of values can then happen in a lock-free
* manner.
*
* According to my benchmarks, this is approximately 25% faster than ConcurrentHashMap for (all types of) reads, and a lot slower for
* contended writes.
*
* This data structure is for many-read/few-write scenarios
*/
public final
class LockFreeHashMap implements Map, Cloneable, Serializable {
// Recommended for best performance while adhering to the "single writer principle". Must be static-final
private static final AtomicReferenceFieldUpdater mapREF = AtomicReferenceFieldUpdater.newUpdater(
LockFreeHashMap.class,
HashMap.class,
"hashMap");
private volatile HashMap hashMap;
// synchronized is used here to ensure the "single writer principle", and make sure that ONLY one thread at a time can enter this
// section. Because of this, we can have unlimited reader threads all going at the same time, without contention (which is our
// use-case 99% of the time)
/**
* Constructs an empty HashMap with the default initial capacity
* (16) and the default load factor (0.75).
*/
public
LockFreeHashMap() {
hashMap = new HashMap();
}
/**
* Constructs an empty HashMap with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
*
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public
LockFreeHashMap(int initialCapacity) {
hashMap = new HashMap(initialCapacity);
}
/**
* Constructs a new HashMap with the same mappings as the
* specified Map. The HashMap is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified Map.
*
* @param map the map whose mappings are to be placed in this map
*
* @throws NullPointerException if the specified map is null
*/
public
LockFreeHashMap(Map map) {
this.hashMap = new HashMap(map);
}
/**
* Constructs an empty HashMap with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
*
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public
LockFreeHashMap(int initialCapacity, float loadFactor) {
this.hashMap = new HashMap(initialCapacity, loadFactor);
}
@SuppressWarnings("unchecked")
public
Map getMap() {
// use the SWP to get a lock-free get of the map. It's values are only valid at the moment this method is called.
return Collections.unmodifiableMap(mapREF.get(this));
}
@Override
public
int size() {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.size();
}
@Override
public
boolean isEmpty() {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.isEmpty();
}
@Override
public
boolean containsKey(final Object key) {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.containsKey(key);
}
@Override
public
boolean containsValue(final Object value) {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.containsValue(value);
}
@Override
public
V get(final Object key) {
// use the SWP to get a lock-free get of the value
return (V) mapREF.get(this)
.get(key);
}
@Override
public synchronized
V put(final K key, final V value) {
return hashMap.put(key, value);
}
@Override
public synchronized
V remove(final Object key) {
return hashMap.remove(key);
}
@Override
public synchronized
void putAll(final Map map) {
this.hashMap.putAll(map);
}
@Override
public synchronized
void clear() {
hashMap.clear();
}
@Override
public
Set keySet() {
return getMap().keySet();
}
@Override
public
Collection values() {
return getMap().values();
}
@Override
public
Set> entrySet() {
return getMap().entrySet();
}
@Override
public
boolean equals(final Object o) {
return mapREF.get(this)
.equals(o);
}
@Override
public
int hashCode() {
return mapREF.get(this)
.hashCode();
}
@Override
public
String toString() {
return mapREF.get(this)
.toString();
}
}