Why is the host java version an input to `compileKotlin` ? As long as I set Kotlin `jvmTarget` (and ...

# gradle

[mbonnin]

Why is the host java version an input to compileKotlin ? As long as I set Kotlin jvmTarget (and Java options.release ?), is there any reason 2 different JDKs would produce different class files?

[mbonnin]

Bumped into this: https://youtrack.jetbrains.com/issue/KT-45611 which states that non-JVM doesn't depend on the host java version but JVM might? But not sure why...

[Vampire]

Probably same as with Java? If you compile against different versions, different methods can be selected and written to the class file. For example if you use ByteBuffer#flip which used to return Buffer and now returns ByteBuffer.

[mbonnin]

Isn't that the whole point of --release ?

[mbonnin]

I'd expect the Kotlin compiler to either set --release according to jvmTarget or allow me to configure it

[Vampire]

Isn't that the whole point of --release ?

Yes

I'd expect the Kotlin compiler to either set --release according to jvmTarget or allow me to configure it

If I look at issues like https://github.com/Kotlin/kotlinx.serialization/pull/2350, I'd say it does not.

[Alexander.Likhachev]

Currently the recommended way to deal with different versions is JVM toolchains. https://kotl.in/gradle/jvm/toolchain
The toolchains feature will properly configure Kotlin’s jvmTarget, Java’s targetCompatibility, choose the right Java stdlib, configure publishing attributes, etc. It doesn’t cause the Kotlin compiler to be executed on the old JDK, but configures the compilation to run against that JDK.

Manual configuration of jvmTarget is kinda a low-level configuration key, similar to Java’s --target, not --release. The Kotlin compiler has the argument -Xjdk-release that serves the same needs as --release of the Java compiler, however it’s not yet stabilized

[mbonnin]

@Alexander.Likhachev my goal is to avoid toolchains. I'd like to avoid downloading a jdk11 if I have a jdk21 on my machine (or in CI)

[mbonnin]

Just use jdk21 everywhere, also makes sure I get the nice performance improvements in latest jdks