001// ***************************************************************************************************************************
002// * Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements.  See the NOTICE file *
003// * distributed with this work for additional information regarding copyright ownership.  The ASF licenses this file        *
004// * to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance            *
005// * with the License.  You may obtain a copy of the License at                                                              *
006// *                                                                                                                         *
007// *  http://www.apache.org/licenses/LICENSE-2.0                                                                             *
008// *                                                                                                                         *
009// * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an  *
010// * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the License for the        *
011// * specific language governing permissions and limitations under the License.                                              *
012// ***************************************************************************************************************************
013package org.apache.juneau.internal;
014
015import java.util.concurrent.*;
016
017/**
018 * Simple in-memory cache of objects.
019 *
020 * <p>
021 * Essentially just a wrapper around a ConcurrentHashMap.
022 *
023 * @param <K> The key type.
024 * @param <V> The value type.
025 */
026public class Cache<K,V> {
027   private final boolean nocache;
028   private final int maxSize;
029   private final ConcurrentHashMap<K,V> cache;
030
031   /**
032    * Constructor.
033    *
034    * @param disabled If <jk>true</jk> then the cache is disabled.
035    * @param maxSize The maximum size of the cache.  If this threshold is reached, the cache is flushed.
036    */
037   public Cache(boolean disabled, int maxSize) {
038      this.nocache = disabled;
039      this.maxSize = maxSize;
040      if (! nocache)
041         cache = new ConcurrentHashMap<>();
042      else
043         cache = null;
044   }
045
046   /**
047    * Retrieves the value with the specified key from this cache.
048    *
049    * @param key The key.
050    * @return The value, or <jk>null</jk> if the value is not in the cache, or the cache is disabled.
051    */
052   public V get(K key) {
053      if (nocache || key == null)
054         return null;
055      return cache.get(key);
056   }
057
058   /**
059    * Adds the value with the specified key to this cache.
060    *
061    * @param key The key.
062    * @param value The value.
063    * @return
064    *    Either the value already in the cache if it already exists, or the same value passed in.
065    *    Always returns the same value if the cache is disabled.
066    */
067   public V put(K key, V value) {
068      if (nocache || key == null)
069         return value;
070
071      // Prevent OOM in case of DDOS
072      if (cache.size() > maxSize)
073         cache.clear();
074
075      while (true) {
076         V v = cache.get(key);
077         if (v != null)
078            return v;
079         cache.putIfAbsent(key, value);
080         return value;
081      }
082   }
083}