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/*
* 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.
*/
package org.apache.lucene.util;
import java.io.Closeable;
import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.atomic.AtomicInteger;
/**
* 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 implements Closeable {
private ThreadLocal> t = new ThreadLocal<>();
// Use a WeakHashMap so that if a Thread exits and is
// GC'able, its entry may be removed:
private Map hardRefs = new WeakHashMap<>();
// Increase this to decrease frequency of purging in get:
private static final int PURGE_MULTIPLIER = 20;
// On each get or set we decrement this; when it hits 0 we
// purge. After purge, we set this to
// PURGE_MULTIPLIER * stillAliveCount. This keeps
// amortized cost of purging linear.
private final AtomicInteger countUntilPurge = new AtomicInteger(PURGE_MULTIPLIER);
protected T initialValue() {
return null;
}
public T get() {
WeakReference weakRef = t.get();
if (weakRef == null) {
T iv = initialValue();
if (iv != null) {
set(iv);
return iv;
} else {
return null;
}
} else {
maybePurge();
return weakRef.get();
}
}
public void set(T object) {
t.set(new WeakReference<>(object));
synchronized (hardRefs) {
hardRefs.put(Thread.currentThread(), object);
maybePurge();
}
}
private void maybePurge() {
if (countUntilPurge.getAndDecrement() == 0) {
purge();
}
}
// Purge dead threads
private void purge() {
synchronized (hardRefs) {
int stillAliveCount = 0;
for (Iterator it = hardRefs.keySet().iterator(); it.hasNext(); ) {
final Thread t = it.next();
if (!t.isAlive()) {
it.remove();
} else {
stillAliveCount++;
}
}
int nextCount = (1 + stillAliveCount) * PURGE_MULTIPLIER;
if (nextCount <= 0) {
// defensive: int overflow!
nextCount = 1000000;
}
countUntilPurge.set(nextCount);
}
}
@Override
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;
}
}