pylucene 3.5.0-3

[pylucene.git] / lucene-java-3.5.0 / lucene / src / java / org / apache / lucene / util / CloseableThreadLocal.java

diff --git a/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/CloseableThreadLocal.java b/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/CloseableThreadLocal.java
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+++ b/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/CloseableThreadLocal.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.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;
+  }
+}