Is there a way inside gradle to have a subproject use java11 and another one java8?

yes, just set the

sourceCompatibility

and the

targetCompatibility

unless you mean different boot classpaths, which is still possible but much more difficult

This can do that for you for Java 8 and earlier https://github.com/nebula-plugins/gradle-java-cross-compile-plugin - it just configures the

jdkHome

property which you can also do yourself (https://github.com/nebula-plugins/gradle-java-cross-compile-plugin/blob/master/src/main/kotlin/nebula/plugin/compile/JavaCrossCompilePlugin.kt#L64-L66)

thx

Java 9 and later have built-in cross compile support that we don't yet configure, and without specific support in the Kotlin plugin/compiler won't work (https://stackoverflow.com/a/43103038)