Does Kotlin val give the same visibility guarantees as java final?

Question

I have a class like this:

class MyClass {
    private val myLock = ReentrantLock()
    ...
}

In java I would make this field final to provide memory guarantees: https://stackoverflow.com/a/27254652/2674303

Does val provides the same guarantees ?

Answer 1

Yes, at least on Kotlin/JVM. For other platforms, probably not.

While I couldn't find authoritative documentation on this (The Kotlin/JVM spec still hasn't been written), a private val declared in the way your example does, compiles to a final field in bytecode. You can see this by inspecting the .class file. It is unlikely that they will change this to generate a non-final field in the future, because as you pointed out, final fields have certain guarantees, and removing final will be a breaking change.

Of course, if the val doesn't have a backing field, like val x get() = 1, then no field is generated.