Best practices for mixing in Scala concurrent.Map

Question

The ScalaDoc says this about concurrentMap: "Deprecated (Since version 2.10.0) Use scala.collection.concurrent.Map instead." Unfortunately, the rest of the Scala docs has not been updated and still references concurrentMap.

I tried to mix in concurrent.Map into a HashMap, with the following results:

scala> val mmap = new mutable.HashMap[String, String] with collection.concurrent.Map[String, String]
<console>:16: error: object creation impossible, since:
it has 4 unimplemented members.
/** As seen from anonymous class $anon, the missing signatures are as follows.
 *  For convenience, these are usable as stub implementations.
 */
  def putIfAbsent(k: String,v: String): Option[String] = ???
  def remove(k: String,v: String): Boolean = ???
  def replace(k: String,v: String): Option[String] = ???
  def replace(k: String,oldvalue: String,newvalue: String): Boolean = ???

       val mmap = new mutable.HashMap[String, String] with collection.concurrent.Map[String, String]

So we see that instead of a simple mixin, some methods must also be implemented. Is this the best way to use concurrent.Map, or is there a better way?

Answer 1

The scala.collection.concurrent.Map trait is not meant to be mixed-in with an existing mutable Scala Map to obtain a thread-safe version of the map instance. The SynchronizedMap mixin existed for this purpose before 2.11, but is now deprecated.

Currently, Scala has the scala.collection.concurrent.TrieMap implementation for the scala.collection.concurrent.Map interface, but can wrap Java classes as well.

The scala.collection.concurrent.Map, in versions prior to 2.10 known as scala.collection.mutable.ConcurrentMap, interface is used when you:

• want to implement your own concurrent, thread-safe Map from scratch

• want to wrap an existing Java concurrent map implementation:

E.g:

import scala.collection._
import scala.collection.convert.decorateAsScala._
import java.util.concurrent.ConcurrentHashMap

val map: concurrent.Map[String, String] = new ConcurrentHashMap().asScala

• want to write generic code that works concurrent maps, and don't want to commit to a specific implementation:

E.g.:

import scala.collection._

def foo(map: concurrent.Map[String, String]) = map.putIfAbsent("", "")

foo(new concurrent.TrieMap)
foo(new java.util.concurrent.ConcurrentSkipListMap().asScala)

• you could implement your own wrapper around a single-threaded mutable map implementation by using synchronized (but you would need to ensure that your program is accessing the mutable map only through this wrapper and never directly).

E.g.:

class MySynchronizedMap[K, V](private val underlying: mutable.Map[K, V])
extends concurrent.Map[K, V] {
  private val monitor = new AnyRef
  def putIfAbsent(k: K,v: V): Option[String] = monitor.synchronized {
    underlying.get(k) match {
      case s: Some[V] => s
      case None =>
        underlying(k) = v
        None
    }
  }
  def remove(k: K, v: V): Boolean = monitor.synchronized {
    underlying.get(k) match {
      case Some(v0) if v == v0 => underlying.remove(k); true
      case None => false
    }
  }
  // etc.
}

Answer 2

Unless you want to implement a concurrent mutable hash map yourself, you have to use scala.collection.concurrent.TrieMap.

Answer 3

Taken from this comment directly: https://stackoverflow.com/a/49689669/7082628

In 2018, apparently you can just do this:

import java.util.concurrent.ConcurrentHashMap

val m: ConcurrentHashMap[String,MyClass] = new ConcurrentHashMap

Answer 4

By "simple mixin", perhaps you're asking if the trait can be used as a decorator as shown here for SynchronizedMap, and the answer is apparently not.

Implementations include TrieMap and the wrapper for Java's ConcurrentMap (of which there are two implementations). (Java also offers ConcurrentSkipListSet as Set.)

Also see this roll-your-own question.

They have you covered on the conversion side of things, if that's what you were used to:

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

scala> import collection.JavaConverters._
import collection.JavaConverters._

scala> val m = new ConcurrentHashMap[String, Int]
m: java.util.concurrent.ConcurrentHashMap[String,Int] = {}

scala> val mm = m.asScala
mm: scala.collection.concurrent.Map[String,Int] = Map()

scala> mm.replace("five",5)
res0: Option[Int] = None

scala> mm.getClass
res1: Class[_ <: scala.collection.concurrent.Map[String,Int]] = class scala.collection.convert.Wrappers$JConcurrentMapWrapper

Answer 5

Update for 2021 and Scala 2.13:

You need to use different implicit conversion when wrapping the Java concurrent map implementation:

import java.util.concurrent.ConcurrentHashMap
import scala.collection.concurrent
import scala.jdk.CollectionConverters._

val map: concurrent.Map[String, String] = new ConcurrentHashMap().asScala