content: 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;
    }
  }
  