Custom Toolchain for Gradle Maven Exec Plugin

Help/Discuss
3

That toolchain setting has nothing to do with the Gradle JVM toolchains despite the same naming.
It is some feature of Maven where you can have toolchains in some XML file in the Maven user home directory and that file you can supply there if it is not in the standard location.
I don’t know if that would help with your issue as I avoid Maven whereever possible.

That task does not support Gradle JVM toolchains as far as I can see.

But from a very quick look at the sources of that plugin, it seems to call the mvn executable of the Maven installation. Those executables if I’m not completely mistaken should consider JAVA_HOME environment variable.
So you might have success with setting the JAVA_HOME environment variable to the toolchain installation you want to be used.
Something like this should probably do (untested):

task('createWebstart', type: MavenExec) {
    environment 'JAVA_HOME', "${javaToolchains.compilerFor { languageVersion = JavaLanguageVersion.of(8) }.get().metadata.installationPath}"
    goals 'clean', 'install'
}