IntelliJ Idea 2017.3 unable to start Kotlin Spring Boot App - @Configuration class may not be final

First of all, it's all due to class Kotlin class definition:

The open annotation on a class is the opposite of Java's final: it allows others to inherit from this class. By default, all classes in Kotlin are final

so if you are free to modify your source code, you can make your class not final, just adding open to it's signature as follows:

@SpringBootApplication
open class DemoApplication

fun main(args: Array<String>) {
    SpringApplication.run(DemoApplication::class.java, *args)
}

or one of the possible solutions, according to this article:

The @SpringBootApplication is a convenience annotation that marks the class with the @Configuration, @EnableAutoConfiguration and @ComponentScan annotations. It is the @Configuration annotation that forces the use of the open keyword.

Is to try to remove the @SpringBootApplication annotation and annotate your class with @EnableAutoConfiguration and @ComponentScan to solve this issue.

Fixed upgrading IntelliJ's Kotlin pluging to 1.2.21: https://plugins.jetbrains.com/plugin/6954-kotlin/update/42501

If you get this error on IntelliJ, even with kotlin-spring plugin, you may want to check if the annotation processing has been enabled in IntelliJ.