com.cedarsoftware.util.cache.LockingLRUCacheStrategy Maven / Gradle / Ivy
package com.cedarsoftware.util.cache;
import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* This class provides a thread-safe Least Recently Used (LRU) cache API that evicts the least recently used items
* once a threshold is met. It implements the Map interface for convenience.
*
* The Locking strategy allows for O(1) access for get(), put(), and remove(). For put(), remove(), and many other
* methods, a write-lock is obtained. For get(), it attempts to lock but does not lock unless it can obtain it right away.
* This 'try-lock' approach ensures that the get() API is never blocking, but it also means that the LRU order is not
* perfectly maintained under heavy load.
*
* LRUCache supports null for both key and value.
* @author John DeRegnaucourt ([email protected])
*
* Copyright (c) Cedar Software 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
*
* License
*
* 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.
*/
public class LockingLRUCacheStrategy implements Map {
private static final Object NULL_ITEM = new Object(); // Sentinel value for null keys and values
private final int capacity;
private final ConcurrentHashMap> cache;
private final Node head;
private final Node tail;
private final Lock lock = new ReentrantLock();
private static class Node {
K key;
V value;
Node prev;
Node next;
Node(K key, V value) {
this.key = key;
this.value = value;
}
}
public LockingLRUCacheStrategy(int capacity) {
this.capacity = capacity;
this.cache = new ConcurrentHashMap<>(capacity);
this.head = new Node<>(null, null);
this.tail = new Node<>(null, null);
head.next = tail;
tail.prev = head;
}
private void moveToHead(Node node) {
if (node.prev == null || node.next == null) {
// Node has been evicted; skip reordering
return;
}
removeNode(node);
addToHead(node);
}
private void addToHead(Node node) {
node.next = head.next;
node.next.prev = node;
head.next = node;
node.prev = head;
}
private void removeNode(Node node) {
if (node.prev != null && node.next != null) {
node.prev.next = node.next;
node.next.prev = node.prev;
}
}
private Node removeTail() {
Node node = tail.prev;
if (node != head) {
removeNode(node);
node.prev = null; // Null out links to avoid GC nepotism
node.next = null; // Null out links to avoid GC nepotism
}
return node;
}
@Override
public V get(Object key) {
Object cacheKey = toCacheItem(key);
Node node = cache.get(cacheKey);
if (node == null) {
return null;
}
// Ben Manes suggestion - use exclusive 'try-lock'
if (lock.tryLock()) {
try {
moveToHead(node);
} finally {
lock.unlock();
}
}
return fromCacheItem(node.value);
}
@SuppressWarnings("unchecked")
@Override
public V put(K key, V value) {
Object cacheKey = toCacheItem(key);
Object cacheValue = toCacheItem(value);
lock.lock();
try {
Node node = cache.get(cacheKey);
if (node != null) {
node.value = (V) cacheValue;
moveToHead(node);
return fromCacheItem(node.value);
} else {
Node newNode = new Node<>(key, (V) cacheValue);
cache.put(cacheKey, newNode);
addToHead(newNode);
if (cache.size() > capacity) {
Node tail = removeTail();
cache.remove(toCacheItem(tail.key));
}
return null;
}
} finally {
lock.unlock();
}
}
@Override
public void putAll(Map m) {
lock.lock();
try {
for (Map.Entry entry : m.entrySet()) {
put(entry.getKey(), entry.getValue());
}
} finally {
lock.unlock();
}
}
@Override
public V remove(Object key) {
Object cacheKey = toCacheItem(key);
lock.lock();
try {
Node node = cache.remove(cacheKey);
if (node != null) {
removeNode(node);
return fromCacheItem(node.value);
}
return null;
} finally {
lock.unlock();
}
}
@Override
public void clear() {
lock.lock();
try {
head.next = tail;
tail.prev = head;
cache.clear();
} finally {
lock.unlock();
}
}
@Override
public int size() {
return cache.size();
}
@Override
public boolean isEmpty() {
return size() == 0;
}
@Override
public boolean containsKey(Object key) {
return cache.containsKey(toCacheItem(key));
}
@Override
public boolean containsValue(Object value) {
Object cacheValue = toCacheItem(value);
lock.lock();
try {
for (Node node = head.next; node != tail; node = node.next) {
if (node.value.equals(cacheValue)) {
return true;
}
}
return false;
} finally {
lock.unlock();
}
}
@Override
public Set> entrySet() {
lock.lock();
try {
Map map = new LinkedHashMap<>();
for (Node node = head.next; node != tail; node = node.next) {
map.put(node.key, fromCacheItem(node.value));
}
return map.entrySet();
} finally {
lock.unlock();
}
}
@Override
public Set keySet() {
lock.lock();
try {
Map map = new LinkedHashMap<>();
for (Node node = head.next; node != tail; node = node.next) {
map.put(node.key, fromCacheItem(node.value));
}
return map.keySet();
} finally {
lock.unlock();
}
}
@Override
public Collection values() {
lock.lock();
try {
Map map = new LinkedHashMap<>();
for (Node node = head.next; node != tail; node = node.next) {
map.put(node.key, fromCacheItem(node.value));
}
return map.values();
} finally {
lock.unlock();
}
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Map)) return false;
Map other = (Map) o;
return entrySet().equals(other.entrySet());
}
@SuppressWarnings("unchecked")
@Override
public String toString() {
lock.lock();
try {
StringBuilder sb = new StringBuilder();
sb.append("{");
for (Node node = head.next; node != tail; node = node.next) {
sb.append((K) fromCacheItem(node.key)).append("=").append((V) fromCacheItem(node.value)).append(", ");
}
if (sb.length() > 1) {
sb.setLength(sb.length() - 2); // Remove trailing comma and space
}
sb.append("}");
return sb.toString();
} finally {
lock.unlock();
}
}
@Override
public int hashCode() {
lock.lock();
try {
int hashCode = 1;
for (Node node = head.next; node != tail; node = node.next) {
Object key = fromCacheItem(node.key);
Object value = fromCacheItem(node.value);
hashCode = 31 * hashCode + (key == null ? 0 : key.hashCode());
hashCode = 31 * hashCode + (value == null ? 0 : value.hashCode());
}
return hashCode;
} finally {
lock.unlock();
}
}
private Object toCacheItem(Object item) {
return item == null ? NULL_ITEM : item;
}
@SuppressWarnings("unchecked")
private T fromCacheItem(Object cacheItem) {
return cacheItem == NULL_ITEM ? null : (T) cacheItem;
}
}