dorkbox.collections.LockFreeObjectMap Maven / Gradle / Ivy
/*
* Copyright 2018 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.collections;
import java.io.Serializable;
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
*
* An unordered map. This implementation is a cuckoo hash map using 3 hashes, random walking, and a small stash for problematic
* keys. Null keys are not allowed. Null values are allowed. No allocation is done except when growing the table size.
*
* This map performs very fast get, containsKey, and remove (typically O(1), worst case O(log(n))). Put may be a bit slower,
* depending on hash collisions. Load factors greater than 0.91 greatly increase the chances the map will have to rehash to the
* next higher POT size.
*
* Iteration can be very slow for a map with a large capacity. {@link #clear(int)} and {@link #shrink(int)} can be used to reduce
* the capacity. {@link OrderedMap} provides much faster iteration.
*/
@SuppressWarnings("unchecked")
public final
class LockFreeObjectMap implements Cloneable, Serializable {
public static final String version = Collections.version;
// Recommended for best performance while adhering to the "single writer principle". Must be static-final
private static final AtomicReferenceFieldUpdater mapREF = AtomicReferenceFieldUpdater.newUpdater(
LockFreeObjectMap.class,
ObjectMap.class,
"hashMap");
private volatile ObjectMap 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
LockFreeObjectMap() {
hashMap = new ObjectMap();
}
/**
* 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
LockFreeObjectMap(int initialCapacity) {
hashMap = new ObjectMap(initialCapacity);
}
/**
* 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
LockFreeObjectMap(int initialCapacity, float loadFactor) {
this.hashMap = new ObjectMap(initialCapacity, loadFactor);
}
public
int size() {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.size;
}
public
boolean isEmpty() {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.size == 0;
}
public
boolean containsKey(final K key) {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.containsKey(key);
}
public
boolean containsValue(final V value, boolean identity) {
// use the SWP to get a lock-free get of the value
return mapREF.get(this)
.containsValue(value, identity);
}
@SuppressWarnings("unchecked")
public
V get(final K key) {
// use the SWP to get a lock-free get of the value
return (V) mapREF.get(this)
.get(key);
}
public synchronized
V put(final K key, final V value) {
return hashMap.put(key, value);
}
public synchronized
V remove(final K key) {
return hashMap.remove(key);
}
public synchronized
void putAll(final ObjectMap map) {
this.hashMap.putAll(map);
}
public synchronized
void clear() {
hashMap.clear();
}
/**
* DO NOT MODIFY THE MAP VIA THIS (unless you synchronize around it!) It will result in unknown object visibility!
*
* Returns an iterator for the keys in the map. Remove is supported. Note that the same iterator instance is returned each
* time this method is called. Use the {@link ObjectMap.Entries} constructor for nested or multithreaded iteration.
*/
public
ObjectMap.Keys keys() {
return mapREF.get(this)
.keys();
}
/**
* DO NOT MODIFY THE MAP VIA THIS (unless you synchronize around it!) It will result in unknown object visibility!
*
* Returns an iterator for the values in the map. Remove is supported. Note that the same iterator instance is returned each
* time this method is called. Use the {@link ObjectMap.Entries} constructor for nested or multithreaded iteration.
*/
public
ObjectMap.Values values() {
return mapREF.get(this)
.values();
}
/**
* DO NOT MODIFY THE MAP VIA THIS (unless you synchronize around it!) It will result in unknown object visibility!
*
* Returns an iterator for the entries in the map. Remove is supported. Note that the same iterator instance is returned each
* time this method is called. Use the {@link ObjectMap.Entries} constructor for nested or multithreaded iteration.
*/
public
ObjectMap.Entries entries() {
return mapREF.get(this)
.entries();
}
/**
* Identity equals only!
*/
@Override
public
boolean equals(final Object o) {
return this == o;
}
@Override
public
int hashCode() {
return mapREF.get(this)
.hashCode();
}
@Override
public
String toString() {
return mapREF.get(this)
.toString();
}
/**
* Clears the map and reduces the size of the backing arrays to be the specified capacity, if they are larger. The reduction
* is done by allocating new arrays, though for large arrays this can be faster than clearing the existing array.
* */
public
void clear(final int maximumCapacity) {
mapREF.get(this)
.clear(maximumCapacity);
}
/**
* Reduces the size of the backing arrays to be the specified capacity or less. If the capacity is already less, nothing is
* done.
* If the map contains more items than the specified capacity, the next highest power of two capacity is used instead.
*/
public
void shrink(final int maximumCapacity) {
mapREF.get(this)
.shrink(maximumCapacity);
}
}