Question

i was wondering if there is any ‘easy’ way to update immutable scala collections safely. Consider following code:

class a {
   private var x = Map[Int,Int]()

   def update(p:(Int,Int)) { x = x + (p) }
}

This code is not thread safe, correct? By that i mean that if we have two threads invoking update method and lets say that x is map containing { 1=>2 } and thread A invokes update((3,4)) and only manages to execute the x + (p) part of the code. Then rescheduling occurs and thread B invokes update((13,37)) and successfully updates the variable x. The thread A continues and finishes.

After all this finishes, value x would equal map containing { 1=>2, 3=>4 }, correct? Instead of desired { 1=>2, 3=>4, 13=>37 }. Is there a simple way to fix that? I hope it’s undestandable what I’m asking 🙂

Btw, i know there are solutions like Akka STM but i would prefer not to use those, unless necessary.

Thanks a lot for any answer!

edit: Also, i would prefer solution without locking. Eeeew 🙂

Answer 1

In your case, as Maurício wrote, your collection is already thread safe because it is immutable. The only problem is reassigning the var, which may not be an atomic operation. For this particular problem, the easiest option is to use of the nice classes in java.util.concurrent.atomic, namely AtomicReference.

import java.util.concurrent.atomic.AtomicReference

class a {
  private val x = new AtomicReference(Map[Int,Int]())

  def update(p:(Int,Int)) {
    while (true) {
      val oldMap = x.get // get old value
      val newMap = oldMap + p // update
      if (x.compareAndSet(oldMap, newMap))
        return // exit if update was successful, else repeat
    }
  }
}