precondition extension functions with @ResponseStatus on Exception for Spring MVC.

preconditions.kt

import org.springframework.http.HttpStatus
import org.springframework.web.bind.annotation.ResponseStatus
import kotlin.contracts.ExperimentalContracts
import kotlin.contracts.contract

@ResponseStatus(HttpStatus.PRECONDITION_FAILED)
class PreconditionException(message: String) : Exception(message)

@ExperimentalContracts
@Throws(EntityNotFoundException::class)
inline fun precondition(check: Boolean, messageExpr: () -> String) {
    contract { returns() implies check }
    if (!check) throw PreconditionException(messageExpr.invoke())
}

@ExperimentalContracts
@Throws(EntityNotFoundException::class)
inline fun precondition(check: Boolean, exception: String) {
    contract { returns() implies check }
    if (!check) throw PreconditionException(exception)
}

contract ensures the the following:

val someThing: String? = someFunction()
precondition(someThing != null, "someThing is required")
funcWithNonNullArguments(someThing)

Can be used without adding !! to funcWithNonNullArguments(someThing)