| /* |
| * 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. |
| */ |
| package org.apache.jackrabbit.oak.commons.cache; |
| |
| import java.util.LinkedHashMap; |
| import java.util.Map; |
| import java.util.concurrent.atomic.AtomicInteger; |
| |
| public class Cache<K, V extends Cache.Value> { |
| |
| int maxMemoryBytes; |
| AtomicInteger memoryUsed = new AtomicInteger(); |
| private final Backend<K, V> backend; |
| |
| private final Map<K, V> map = new LinkedHashMap<K, V>(1024, 0.75f, true) { |
| |
| private static final long serialVersionUID = 1L; |
| |
| @Override |
| protected boolean removeEldestEntry(Map.Entry<K, V> eldest) { |
| if (memoryUsed.get() < maxMemoryBytes) { |
| return false; |
| } |
| int memory = eldest.getValue().getMemory(); |
| memoryUsed.addAndGet(-memory); |
| return true; |
| } |
| |
| }; |
| |
| private Cache(Backend<K, V> backend, int maxMemoryBytes) { |
| this.backend = backend; |
| this.maxMemoryBytes = maxMemoryBytes; |
| } |
| |
| public void put(K key, V value) { |
| int memory = value.getMemory(); |
| // only add elements that are smaller than half the cache size |
| if (memory < maxMemoryBytes / 2) { |
| memoryUsed.addAndGet(memory); |
| synchronized (map) { |
| map.put(key, value); |
| } |
| } |
| } |
| |
| /** |
| * Get the element in the cache if one exists, or add it to the cache if not. |
| * |
| * @param key the key |
| * @param value the value |
| * @return the cached element |
| */ |
| public V replace(K key, V value) { |
| synchronized (map) { |
| if (map.containsKey(key)) { |
| return map.get(key); |
| } |
| } |
| put(key, value); |
| return value; |
| } |
| |
| public V get(K key) { |
| synchronized (map) { |
| V value = map.get(key); |
| if (value != null) { |
| // object was in the cache - good |
| return value; |
| } |
| } |
| // synchronize on the backend when not in the cache |
| // to ensure only one thread accessed the backend |
| // and loads the object |
| synchronized (backend) { |
| // another thread might have added it in the meantime |
| V value; |
| synchronized (map) { |
| value = map.get(key); |
| } |
| if (value == null) { |
| value = backend.load(key); |
| put(key, value); |
| } |
| return value; |
| } |
| } |
| |
| /** |
| * A cacheable object. |
| */ |
| public interface Value { |
| |
| /** |
| * Get the memory required in bytes. The method must always return the |
| * same value once the element is in the cache. |
| * |
| * @return the memory used in bytes |
| */ |
| int getMemory(); |
| |
| } |
| |
| /** |
| * A cache backend that can load objects from persistent storage. |
| * |
| * @param <K> the key class |
| * @param <V> the value class |
| */ |
| public interface Backend<K, V> { |
| |
| /** |
| * Load the object. The method does not need to be synchronized |
| * (it is synchronized in the cache) |
| * |
| * @param key the key |
| * @return the value |
| */ |
| V load(K key); |
| |
| } |
| |
| public static <K, V extends Cache.Value> Cache<K, V> newInstance(Backend<K, V> backend, int maxMemoryBytes) { |
| return new Cache<K, V>(backend, maxMemoryBytes); |
| } |
| |
| public void clear() { |
| synchronized (map) { |
| map.clear(); |
| } |
| } |
| |
| public int size() { |
| return map.size(); |
| } |
| |
| public int getMemoryUsed() { |
| return memoryUsed.get(); |
| } |
| |
| public int getMemoryMax() { |
| return maxMemoryBytes; |
| } |
| |
| } |