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Lucene example source code file (DoubleBarrelLRUCache.java)

This example Lucene source code file (DoubleBarrelLRUCache.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

atomicinteger, cloneablekey, concurrenthashmap, concurrenthashmap, doublebarrellrucache, doublebarrellrucache, map, map, object, override, suppresswarnings, threading, threads, util, v, v

The Lucene DoubleBarrelLRUCache.java source code

package org.apache.lucene.util;

/**
 * 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.
 */

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.Map;

/**
 * Simple concurrent LRU cache, using a "double barrel"
 * approach where two ConcurrentHashMaps record entries.
 *
 * <p>At any given time, one hash is primary and the other
 * is secondary.  {@link #get} first checks primary, and if
 * that's a miss, checks secondary.  If secondary has the
 * entry, it's promoted to primary (<b>NOTE: the key is
 * cloned at this point).  Once primary is full, the
 * secondary is cleared and the two are swapped.</p>
 *
 * <p>This is not as space efficient as other possible
 * concurrent approaches (see LUCENE-2075): to achieve
 * perfect LRU(N) it requires 2*N storage.  But, this
 * approach is relatively simple and seems in practice to
 * not grow unbounded in size when under hideously high
 * load.</p>
 *
 * @lucene.internal
 */

final public class DoubleBarrelLRUCache<K extends DoubleBarrelLRUCache.CloneableKey,V> {

  public static abstract class CloneableKey {
    @Override
    abstract public Object clone();
  }

  private final Map<K,V> cache1;
  private final Map<K,V> cache2;
  private final AtomicInteger countdown;
  private volatile boolean swapped;
  private final int maxSize;

  public DoubleBarrelLRUCache(int maxSize) {
    this.maxSize = maxSize;
    countdown = new AtomicInteger(maxSize);
    cache1 = new ConcurrentHashMap<K,V>();
    cache2 = new ConcurrentHashMap<K,V>();
  }

  @SuppressWarnings("unchecked") 
  public V get(K key) {
    final Map<K,V> primary;
    final Map<K,V> secondary;
    if (swapped) {
      primary = cache2;
      secondary = cache1;
    } else {
      primary = cache1;
      secondary = cache2;
    }

    // Try primary first
    V result = primary.get(key);
    if (result == null) {
      // Not found -- try secondary
      result = secondary.get(key);
      if (result != null) {
        // Promote to primary
        put((K) key.clone(), result);
      }
    }
    return result;
  }

  public void put(K key, V value) {
    final Map<K,V> primary;
    final Map<K,V> secondary;
    if (swapped) {
      primary = cache2;
      secondary = cache1;
    } else {
      primary = cache1;
      secondary = cache2;
    }
    primary.put(key, value);

    if (countdown.decrementAndGet() == 0) {
      // Time to swap

      // NOTE: there is saturation risk here, that the
      // thread that's doing the clear() takes too long to
      // do so, while other threads continue to add to
      // primary, but in practice this seems not to be an
      // issue (see LUCENE-2075 for benchmark & details)

      // First, clear secondary
      secondary.clear();

      // Second, swap
      swapped = !swapped;

      // Third, reset countdown
      countdown.set(maxSize);
    }
  }
}

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