In my (2.0.0.M6) kotlin-spring boot app, everything works fine when I do
fun main(args: Array<String>) {
runApplication<MyApplication>(*args)
}
but the mainClass can not be found and IDEA does not allow running the app when I do
fun main(args: Array<String>) = runApplication<RankedApplication>(*args)
this is not critical at all, but I wonder: why?
runApplication does not return Unit.
The signature and return type of main must be exactly:
fun main(Array<String>): Unit
runApplication, though, returns a ConfigurableApplicationContext.
Your "main" method is equivalent to:
fun main(args: Array<String>): ConfigurableApplicationContext {
return runApplication<MyApplication>(*args)
}
since the return type is deduced from the expression - and you can see this is not a valid entry point.
Your first method works because
fun main(args: Array<String>) {
runApplication<MyApplication>(*args)
}
is the same as
fun main(args: Array<String>): Unit {
runApplication<MyApplication>(*args)
return Unit
}
since Unit (a singleton object) is the default return type when none is specified, and is implicitly returned at the end of a method declared to return Unit.
This is also why return without an argument is valid - it just returns Unit.
If you wanted to, you could make a method to convert things to Unit though this is probably overkill.
inline fun Unit(lambda: () -> Any?) {
lambda()
}
...
fun main(args: Array<String>) = Unit { runApplication<MyApplication>(*args) }
It's the same in Java, which is likely where this requirement comes from - the main method's signature must be public static void main(String[]).
fun main(args: Array<String>) {
runApplication<MyApplication>(*args)
}
main returns Unit here implicitely which is correct.
Here
fun main(args: Array<String>) = runApplication<RankedApplication>(*args)
the return type is inferred from the given expression to something other than Unit (because runApplication returns something) which does not work because the main function has to return Unit.
