How to use ConcurrentLinkedQueue in Scala?

Question

val nodes = Array.fill[mutable.Buffer[Int]](numNodes){new ArrayBuffer[Int]() with mutable.SynchronizedBuffer[Int]}

def addMutualEdge(i: Int)(j: Int) {nodes(i) += j; nodes(j) += i}

When I compile this, I get deprecation warning:

SynchronizedBuffer is deprecated. Synchronization via traits is deprecated as it is inherently reliable. Consider java.util.concurrent.ConcurrentLinkedQueue as an alternative

How to use java library in the above code?

Answer 1

You may just use ConcurrentLinkedQueue instead of Buffer as it's also mutable:

scala> import java.util.concurrent._
import java.util.concurrent._

scala> val nodes = Array.fill(10){new ConcurrentLinkedQueue[Int]()}
nodes: Array[java.util.concurrent.ConcurrentLinkedQueue[Int]] = Array([], [], [], [], [], [], [], [], [], [])

scala> def addMutualEdge(i: Int)(j: Int) {nodes(i).add(j); nodes(j).add(i)}
addMutualEdge: (i: Int)(j: Int)Unit

It's fastest option as this queue is based on CAS-operations, so no blocking there (in comparision with SynchronizedBuffer). Another option is to synchronize operations directly:

scala> val nodes = Array.fill[mutable.Buffer[Int]](10){new ArrayBuffer[Int]()}
nodes: Array[scala.collection.mutable.Buffer[Int]] = Array(ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer(), ArrayBuffer())

scala> def addMutualEdge(i: Int)(j: Int) = this.synchronized{nodes(i) += j; nodes(j) += i}
addMutualEdge: (i: Int)(j: Int)scala.collection.mutable.Buffer[Int]

You can also use java's Collections.synchronizedList(...) in combination with scala.collection.JavaConverters.asScala

import java.util._
import scala.collection.JavaConverters._
scala> val nodes = Array.fill(10){Collections.synchronizedList(new ArrayBuffer[Int]().asJava).asScala}
nodes: Array[scala.collection.mutable.Buffer[Int]] = Array(Buffer(), Buffer(), Buffer(), Buffer(), Buffer(), Buffer(), Buffer(), Buffer(), Buffer(), Buffer())

Or you can use AtomicReferenceArray:

implicit class RichAtomic[T](a: AtomicReferenceArray[List[T]]) { def apply(i: Int) = (a,i); def update(i: Int, e: List[T]) = a.set(i, e)}
implicit class RichList[T](a: (AtomicReferenceArray[List[T]], Int)) { def ::=(e: T) = while({val lst = a._1.get(a._2);!a._1.compareAndSet(a._2, lst, e :: lst)}){}}
implicit def toList[T](a: (AtomicReferenceArray[List[T]], Int)) = a._1.get(a._2)

val nodes = new AtomicReferenceArray(Array.fill[List[Int]](10){Nil})

scala> def addMutualEdge(i: Int)(j: Int) = {nodes(i) ::= j; nodes(j) ::= i}
addMutualEdge: (i: Int)(j: Int)Unit

Implicits used to provide simillar interface as for just Array. Note, that ::= adds element to the start of list.