--- /dev/null
+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.io.Closeable;
+import java.lang.ref.WeakReference;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+
+/** Java's builtin ThreadLocal has a serious flaw:
+ * it can take an arbitrarily long amount of time to
+ * dereference the things you had stored in it, even once the
+ * ThreadLocal instance itself is no longer referenced.
+ * This is because there is single, master map stored for
+ * each thread, which all ThreadLocals share, and that
+ * master map only periodically purges "stale" entries.
+ *
+ * While not technically a memory leak, because eventually
+ * the memory will be reclaimed, it can take a long time
+ * and you can easily hit OutOfMemoryError because from the
+ * GC's standpoint the stale entries are not reclaimable.
+ *
+ * This class works around that, by only enrolling
+ * WeakReference values into the ThreadLocal, and
+ * separately holding a hard reference to each stored
+ * value. When you call {@link #close}, these hard
+ * references are cleared and then GC is freely able to
+ * reclaim space by objects stored in it.
+ *
+ * We can not rely on {@link ThreadLocal#remove()} as it
+ * only removes the value for the caller thread, whereas
+ * {@link #close} takes care of all
+ * threads. You should not call {@link #close} until all
+ * threads are done using the instance.
+ *
+ * @lucene.internal
+ */
+
+public class CloseableThreadLocal<T> implements Closeable {
+
+ private ThreadLocal<WeakReference<T>> t = new ThreadLocal<WeakReference<T>>();
+
+ private Map<Thread,T> hardRefs = new HashMap<Thread,T>();
+
+ protected T initialValue() {
+ return null;
+ }
+
+ public T get() {
+ WeakReference<T> weakRef = t.get();
+ if (weakRef == null) {
+ T iv = initialValue();
+ if (iv != null) {
+ set(iv);
+ return iv;
+ } else
+ return null;
+ } else {
+ return weakRef.get();
+ }
+ }
+
+ public void set(T object) {
+
+ t.set(new WeakReference<T>(object));
+
+ synchronized(hardRefs) {
+ hardRefs.put(Thread.currentThread(), object);
+
+ // Purge dead threads
+ for (Iterator<Thread> it = hardRefs.keySet().iterator(); it.hasNext();) {
+ final Thread t = it.next();
+ if (!t.isAlive())
+ it.remove();
+ }
+ }
+ }
+
+ public void close() {
+ // Clear the hard refs; then, the only remaining refs to
+ // all values we were storing are weak (unless somewhere
+ // else is still using them) and so GC may reclaim them:
+ hardRefs = null;
+ // Take care of the current thread right now; others will be
+ // taken care of via the WeakReferences.
+ if (t != null) {
+ t.remove();
+ }
+ t = null;
+ }
+}