Question

How to make Scala object thread-safe.

class Stack {
    case class Node(value: Int, var next: Node)

    private var head: Node = null
    private var sz = 0

    def push(newValue: Int) {
        head = Node(newValue, head)
        sz += 1
    }

    def pop() = {
        val oldNode = head
        head = oldNode.next
        oldNode.next = null
        sz -= 1
    }

    def size = sz    //I am accessing sz from two threads
}

This class is clearly not threadsafe. I want to make it threadsafe.

Thanks in Advance,

HP

Answer 1

To my mind, the easiest way to make this meaningfully thread-safe would be as follows:

class Stack {
    case class Node(value: Int, var next: Node)

    private var head: Node = null
    private var sz : Int = 0

    def push(newValue: Int) {
        synchronized {
            head = Node(newValue, head)
            sz += 1
        }
    }

    def pop() : Option[Int] = {
        synchronized {
            if ( sz >= 1 ) {
                val ret = Some(head.value)
                val oldNode = head
                head = oldNode.next
                oldNode.next = null
                sz -= 1
                ret
            } else {
                None
            }
        }
    }

    def size = synchronized { sz }
}

This implementation would allow you to ensure that push‘s and pop‘s would be atomic, with pop returning a Some wrapping the value it removed from the top of the stack or None if the stack was already empty.

As a note, access to the size is synchronized, but you have no way of guaranteeing that it will be correct at any point after it is returned, since multiple threads are able to access the stack, potentially altering its size. If you really do need to know the size exactly accurately, you would have to go about this differently, synchronizing on the whole stack when you use it.