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.
*/
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;
}
}