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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.spark.util
import java.lang.ref.WeakReference
import java.util.concurrent.atomic.AtomicInteger
import scala.collection.mutable
import scala.language.implicitConversions
import org.apache.spark.Logging
/**
* A wrapper of TimeStampedHashMap that ensures the values are weakly referenced and timestamped.
*
* If the value is garbage collected and the weak reference is null, get() will return a
* non-existent value. These entries are removed from the map periodically (every N inserts), as
* their values are no longer strongly reachable. Further, key-value pairs whose timestamps are
* older than a particular threshold can be removed using the clearOldValues method.
*
* TimeStampedWeakValueHashMap exposes a scala.collection.mutable.Map interface, which allows it
* to be a drop-in replacement for Scala HashMaps. Internally, it uses a Java ConcurrentHashMap,
* so all operations on this HashMap are thread-safe.
*
* @param updateTimeStampOnGet Whether timestamp of a pair will be updated when it is accessed.
*/
private[spark] class TimeStampedWeakValueHashMap[A, B](updateTimeStampOnGet: Boolean = false)
extends mutable.Map[A, B]() with Logging {
import TimeStampedWeakValueHashMap._
private val internalMap = new TimeStampedHashMap[A, WeakReference[B]](updateTimeStampOnGet)
private val insertCount = new AtomicInteger(0)
/** Return a map consisting only of entries whose values are still strongly reachable. */
private def nonNullReferenceMap = internalMap.filter { case (_, ref) => ref.get != null }
def get(key: A): Option[B] = internalMap.get(key)
def iterator: Iterator[(A, B)] = nonNullReferenceMap.iterator
override def + [B1 >: B](kv: (A, B1)): mutable.Map[A, B1] = {
val newMap = new TimeStampedWeakValueHashMap[A, B1]
val oldMap = nonNullReferenceMap.asInstanceOf[mutable.Map[A, WeakReference[B1]]]
newMap.internalMap.putAll(oldMap.toMap)
newMap.internalMap += kv
newMap
}
override def - (key: A): mutable.Map[A, B] = {
val newMap = new TimeStampedWeakValueHashMap[A, B]
newMap.internalMap.putAll(nonNullReferenceMap.toMap)
newMap.internalMap -= key
newMap
}
override def += (kv: (A, B)): this.type = {
internalMap += kv
if (insertCount.incrementAndGet() % CLEAR_NULL_VALUES_INTERVAL == 0) {
clearNullValues()
}
this
}
override def -= (key: A): this.type = {
internalMap -= key
this
}
override def update(key: A, value: B) = this += ((key, value))
override def apply(key: A): B = internalMap.apply(key)
override def filter(p: ((A, B)) => Boolean): mutable.Map[A, B] = nonNullReferenceMap.filter(p)
override def empty: mutable.Map[A, B] = new TimeStampedWeakValueHashMap[A, B]()
override def size: Int = internalMap.size
override def foreach[U](f: ((A, B)) => U) = nonNullReferenceMap.foreach(f)
def putIfAbsent(key: A, value: B): Option[B] = internalMap.putIfAbsent(key, value)
def toMap: Map[A, B] = iterator.toMap
/** Remove old key-value pairs with timestamps earlier than `threshTime`. */
def clearOldValues(threshTime: Long) = internalMap.clearOldValues(threshTime)
/** Remove entries with values that are no longer strongly reachable. */
def clearNullValues() {
val it = internalMap.getEntrySet.iterator
while (it.hasNext) {
val entry = it.next()
if (entry.getValue.value.get == null) {
logDebug("Removing key " + entry.getKey + " because it is no longer strongly reachable.")
it.remove()
}
}
}
// For testing
def getTimestamp(key: A): Option[Long] = {
internalMap.getTimeStampedValue(key).map(_.timestamp)
}
def getReference(key: A): Option[WeakReference[B]] = {
internalMap.getTimeStampedValue(key).map(_.value)
}
}
/**
* Helper methods for converting to and from WeakReferences.
*/
private object TimeStampedWeakValueHashMap {
// Number of inserts after which entries with null references are removed
val CLEAR_NULL_VALUES_INTERVAL = 100
/* Implicit conversion methods to WeakReferences. */
implicit def toWeakReference[V](v: V): WeakReference[V] = new WeakReference[V](v)
implicit def toWeakReferenceTuple[K, V](kv: (K, V)): (K, WeakReference[V]) = {
kv match { case (k, v) => (k, toWeakReference(v)) }
}
implicit def toWeakReferenceFunction[K, V, R](p: ((K, V)) => R): ((K, WeakReference[V])) => R = {
(kv: (K, WeakReference[V])) => p(kv)
}
/* Implicit conversion methods from WeakReferences. */
implicit def fromWeakReference[V](ref: WeakReference[V]): V = ref.get
implicit def fromWeakReferenceOption[V](v: Option[WeakReference[V]]): Option[V] = {
v match {
case Some(ref) => Option(fromWeakReference(ref))
case None => None
}
}
implicit def fromWeakReferenceTuple[K, V](kv: (K, WeakReference[V])): (K, V) = {
kv match { case (k, v) => (k, fromWeakReference(v)) }
}
implicit def fromWeakReferenceIterator[K, V](
it: Iterator[(K, WeakReference[V])]): Iterator[(K, V)] = {
it.map(fromWeakReferenceTuple)
}
implicit def fromWeakReferenceMap[K, V](
map: mutable.Map[K, WeakReference[V]]) : mutable.Map[K, V] = {
mutable.Map(map.mapValues(fromWeakReference).toSeq: _*)
}
}
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