Scala - Mutable thread safe collections

Question

I need a mutable thread safe Map and a mutable thread safe List in Scala. I know that the immutable collections are thread safe by default. But, I need to update my collections very often because of which I couldn't use immutable. Also I need my threadsafe mutable Map to maintain the insertion order.

Right now am using the map below

val map = scala.collection.mutable.LinkedHashMap[String,Any]()

This map maintains the insertion order and is mutable. How do I make it thread safe?

possible duplicate of [Thread safe map which preserves the insertion order](http://stackoverflow.com/questions/17540467/thread-safe-map-which-preserves-the-insertion-order) — 4lex1v, Jul 09 '13 at 07:11
@AlexIv not a duplicate in complete sense -- rahul asks about List too — om-nom-nom, Jul 09 '13 at 16:10
@om-nom-nom anyway it could be modified, but create a new question is excess — 4lex1v, Jul 09 '13 at 16:17

score 44 · Answer 1 · answered Apr 06 '18 at 09:32

44

Fast hint for those coming to this in 2018 or later:

import java.util.concurrent.ConcurrentHashMap

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

answered Apr 06 '18 at 09:32

akauppi

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Is it still valid in 2021? – Saurabh Gangamwar Oct 17 '21 at 12:36
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Yes, it is valid. The "synchronized" collections in Scala are deprecated since Scala 2.11.0. It is recommended to use `java.util.concurrent.` package. – Iraj Hedayati Jul 25 '22 at 21:15

score 21 · Accepted Answer · edited May 23 '17 at 12:09

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You're duplicating topics....

As was mentioned by AlexIv in his answer, there's a trait you can mix in if you want thread safety. There's another way though:

val synchronizedMap = new scala.collection.mutable.LinkedHashMap[String, Any]() with scala.collection.mutable.SynchronizedMap[String, Any]

That should give you the map with synchronization on each access. Easy, but might not meet the performance requirements. If so, it would be probably easier to create a custom class extending the LinkedHashMap, mixing in the concurrent.Map trait (as was suggested) and provide the implementation of relevant methods, i.e: putIfAbsent, remove replace (2 overloads).

edited May 23 '17 at 12:09

Community

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answered Jul 09 '13 at 07:11

Patryk Ćwiek

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If you can stand a bit more work, use the `synchronized` method on the mutable collection directly where you need thread safety. It'll save you the performance hit that this mixin brings. E.g. `someMap.synchronized { someMap += ("someKey", "anyValue") }` – tysonjh Mar 06 '14 at 14:32
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For anyone new viewing this, the SynchronizedMap trait has been deprecated as not safe enough. Recommends using Java thread safe collections. – Patrick White Aug 20 '15 at 01:20
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It's a compiler warning when you build after mixing in this trait - not sure about a link – Patrick White Oct 10 '15 at 19:34
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@Gepp: http://www.scala-lang.org/api/2.12.3/scala/collection/mutable/SynchronizedMap.html: "(Since version 2.11.0) Synchronization via traits is deprecated as it is inherently unreliable. Consider `java.util.concurrent.ConcurrentHashMap` as an alternative." – Jeremy Rodi Nov 08 '17 at 22:13
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TrieMap is mutable, thread-safe, lock-free, and is not deprecated – ianpojman Nov 05 '20 at 23:05
> @deprecated("Synchronization via traits is deprecated as it is inherently unreliable. Consider java.util.concurrent.ConcurrentHashMap as an alternative.", "2.11.0") trait SynchronizedMap[A, B] extends Map[A, B] { it's deprecated already. – linehrr Oct 04 '22 at 17:47

score 0 · Answer 3 · answered Jan 20 '23 at 15:41

To elaborate on the comment for a Map you can use

import scala.collection.concurrent.TrieMap

Is not deprecated and is mutable and thread-safe.

It should drop in for your scala.collection.mutable.LinkedHashMap as they share the same interface of scala.collection.mutable.MapLike and scala.collection.mutable.Map

Sadly I don't know about the List

Scala - Mutable thread safe collections

3 Answers3

Linked