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kotlinlang

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

gradle will fork a new JVM if it needs to do so for any compilation requirements

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>)

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>)