Hey i want to create a web application with ktor 1.5.4. But what JDK is the best? Can i use the Latest? Is kotlin 1.5.1 compatible or do I have to use the 1.4.3?

you can use latest, for my project i'm use JDK 14 with compile target 12

And which JRE do you use?

I have not currently released the project on production (only for hobby)

Ok thank you

i think yes, i use Ktor 1.5.3 with Kotlin 1.5.0

Ktor doesn't use it yet unless you use EAP, but it will work mostly fine if you use it. The only issue I'm aware of is https://youtrack.jetbrains.com/issue/KT-46340

FYI, Ktor is not updated for Kotlin 1.5.0 yet. It's working for me with Kotlin/JVM 1.5.0, it's not working on Kotlin/JS 1.5.0 though (Apparently some JS-specific things were renamed in Kotlin 1.5.0).