Hands-On Design Patterns with Kotlin

Mutexes

Also known as mutual exclusions, mutexes provide a means to protect a shared state.

Let's start with same, old, dreaded counter example:

var counter = 0val jobs = List(10) {    launch {        repeat(1000) {            counter++            yield()        }    }}runBlocking {    jobs.forEach {        it.join()    }    println(counter)}

As you've probably guessed, this prints anything but the result of 10*100. Totally embarrassing.

To solve that, we introduce a mutex:

var counter = 0val mutex = Mutex()val jobs = List(10) {    launch {        repeat(1000) {            mutex.lock()            counter++            mutex.unlock()            yield()        }    }}

Now our example always prints the correct number.

This is good for simple cases. But what if the code within the critical section (that is, between lock() and unlock()) throws an exception?

Then we'll ...