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diff --git a/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/DoubleBarrelLRUCache.java b/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/DoubleBarrelLRUCache.java
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+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</b>: 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);
+    }
+  }
+}