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?

Solution 1

1. You're duplicating topics....

2. 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).

Solution 2

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

import java.util.concurrent.ConcurrentHashMap

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

Comments

[4lex1v]

possible duplicate of Thread safe map which preserves the insertion order

[om-nom-nom]

@AlexIv not a duplicate in complete sense -- rahul asks about List too

[4lex1v]

@om-nom-nom anyway it could be modified, but create a new question is excess

[tysonjh]

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") }

[Patrick White]

For anyone new viewing this, the SynchronizedMap trait has been deprecated as not safe enough. Recommends using Java thread safe collections.

[Patrick White]

It's a compiler warning when you build after mixing in this trait - not sure about a link

[Jeremy Rodi]

@Gepp: scala-lang.org/api/2.12.3/scala/collection/mutable/…: "(Since version 2.11.0) Synchronization via traits is deprecated as it is inherently unreliable. Consider java.util.concurrent.ConcurrentHashMap as an alternative."

[ianpojman]

TrieMap is mutable, thread-safe, lock-free, and is not deprecated

[Saurabh Gangamwar]

Is it still valid in 2021?