[jillesvangurp]

Continuations are also the backbone of co-routines in Kotlin and has first class support. One nice feature of the coroutines framework is that they made it really easy to adapt existing asynchronous frameworks in Java (there are many) and on other platforms. The list of supported frameworks includes things like reactive Java, vert.x, spring webflux, Java Futures. And that's just the JVM. If you are using kotlin-js in the browser or on node.js, promises and async/await are covered too. And on IOS, coroutines also do the right thing with e.g. garbage collection and other mechanisms in IOS.

Most of this stuff is supported via extension functions. But for asynchronous things that aren't supported it's really easy to adapt them yourself with the suspendCancellableCoroutine function. This is a suspending function that takes a function with a continuation as the parameter. Inside the function you do whatever needs doing and handle your callbacks, futures, awaits, or whatever and call continuation.resume, continuation.resumeWithException, or continuation.invokeOnCancellation as needed (which sadly is not a feature every async framework or language supports).

With a few small extension functions you can pretend most things are nice kotlin suspend functions and not deal with the spaghetti code typically needed otherwise. E.g. Spring Flux is an absolute PITA in Java because of this and essentially effortless in Kotlin. Your code looks like normal kotlin code. Nothing special about it. All the flux madness is shoveled under the carpet.

In the same way the new virtual threads in Java are supported trivially because all of that is exposed via the existing Theading and Threadpool APIs. So, you can just dispatch your co-routine asyncs and launches as virtual threads via a dispatcher that you create with a little extension function on Threadpool. Most of this stuff "just works" out of the box. There's a lot of confusion on this topic in the Java community. For Kotlin this is just yet another way to do async stuff that joins a long list of existing other ways. It has its pros and cons for different use cases and is there if you need it. No big deal but very nice to have around. I've not found any use for it yet and we're on Spring Boot and java 21. We already have everything asynchronous and non blocking.