synchronize property getters/setters

Question

So I'm trying to get a handle on how Kotlin handles synchronization of properties. If I have this class:

class Foo {
    var a = 0
    var b = 0
}

and I want to make sure that a & b have all access synchronized. How would I do it? I've tried using the annotation:

class Foo {
    @Synchronized
    var a = 0
    @Synchronized
    var b = 0
}

but Kotlin gives me a compiler error:

This annotation is not applicable to target 'member property with backing field'

I'm just trying to get a better handle on thread safety in Kotlin. It's something that I've always been OCD about in Java, and I'm trying to figure out how to properly handle it in Kotlin.

score 42 · Answer 1 · answered Aug 02 '17 at 09:07

42

You can't synchronize on a property but you can synchronize on the accessor methods. Use the following syntax:

class Foo {
    var a = 0
        @Synchronized get
        @Synchronized set   

    //or

    @get:Synchronized @set:Synchronized
    var b = 0
}

answered Aug 02 '17 at 09:07

Kirill Rakhman

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has this syntax always been in Kotlin? if not, what version was it introduced? – ycomp Dec 20 '18 at 07:19
I think it was always there. – Kirill Rakhman Dec 20 '18 at 09:16
Does this lock on `Foo` or on the created setter and getter functions? – stk Jul 29 '20 at 10:48
1

@stk instance functions aren't objects so you can't lock on them. Therefore it must lock on the `Foo` instance. – Kirill Rakhman Jul 30 '20 at 11:09

score 6 · Answer 2 · answered Jun 08 '20 at 10:30

Followed by the @Kirill Rakhman's answer, use of Delegates could be more performant if there is tons of contention to the same object since it locks on the delegate rather than the whole object.

class Foo {
    var a by Synchronize(0)
    var b by Synchronize(0)
}

class Synchronize<T>(defaultValue: T): ReadWriteProperty<Any, T> {
    private var backingField = defaultValue

    override fun getValue(thisRef: Any, property: KProperty<*>): T {
        return synchronized(this) {
            backingField
        }
    }

    override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
        synchronized(this) {
            backingField = value
        }
    }
}

References: Kotlin Delegated Properties Reddit Discussion

Why this is not part of sdk ? It looks quite useful ... Any issues with this approach ? — Renetik, Nov 05 '20 at 03:55
I am using this and no issue till now. Maybe it will be added later. Hope so. — Rishav Chudal, Nov 17 '20 at 06:23

synchronize property getters/setters

2 Answers2