In a scala object, is an immutable val thread safe?

Question

object Users {
  val userCountByAgeMap = readFromFile(); // read from file returns immutable map
}

As in above example in scala, Users will be Singleton object and userCountByAgeMap will be initialized lazily.

So is this initialization atomic? i.e. one and only one thread will be able to initialize it.
Suppose userCountByAgeMap is initialized by Thread A, will it be visible to Thread B.

If initialization is not atomic/memory visibility not ensured, will making userCountByAgeMap variable as lazy val fix it?

Answer 1

In Scala, an object is initialized in a static block so thread safety is guaranteed by JVM (Java static initializers are thread safe). You can use JAD decompiler to analyzer bytecode. Here is the code:

object Users {
  val userCountByAgeMap = Map.empty[String, Int]
}

And decompiled Users$.class file:

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   Users.scala

import scala.Predef$;
import scala.collection.immutable.Map;
import scala.collection.immutable.Map$;

public final class Users$
{

    public Map userCountByAgeMap()
    {
        return userCountByAgeMap;
    }

    private Users$()
    {
        userCountByAgeMap = Predef$.MODULE$.Map().empty();
    }

    public static final Users$ MODULE$ = this;
    private final Map userCountByAgeMap;

    static 
    {
        new Users$();
    }
}

As you're using immutable Map which automatically gives you thread safety. So it's ok to access this field from different threads.