The value of a manifest attribute must not be null (Key=Start-Class

[bodiam]

kotlinVersion = '1.1.51'
springBootVersion = '2.0.0.M5'
javaVersion = '9'
gradleVersion = 4.2.1

Bug report

Running a ./gradlew clean build results in the following error:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':bootJar'.
> The value of a manifest attribute must not be null (Key=Start-Class).

For a simple project to reproduce: https://github.com/bodiam/booklist

[bodiam]

I tried fixing it by adding

bootRun {
	main = 'com.booklist.backend.BackendApplication'
}

To the build.gradle file, but no effect.

[wilkinsona]

I think your main method is declared at the wrong level. I believe it should be:

diff --git a/src/main/kotlin/com/booklist/backend/BackendApplication.kt b/src/main/kotlin/com/booklist/backend/BackendApplication.kt
index f6efae2..bea523b 100644
--- a/src/main/kotlin/com/booklist/backend/BackendApplication.kt
+++ b/src/main/kotlin/com/booklist/backend/BackendApplication.kt
@@ -16,16 +16,15 @@ import javax.persistence.Id
 @SpringBootApplication
 class BackendApplication {

-    fun main(args: Array<String>) {
-        SpringApplication.run(BackendApplication::class.java, *args)
-    }
-
     @Bean
     fun startup(userService: UserService) = CommandLineRunner {
         userService.register("erik", "erik@gmail.com")
     }

+}

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

 @RestController

With this change in place I can build the fat jar and run it with java -jar.

And, just for the record, setting main on bootRun had no effect as it was the bootJar task that was failing.

[wilkinsona]

I should also have said that I defer to @sdeleuze for all things Kotlin, and the change I've proposed above brings things into line with the main class in his Spring Boot Kotlin demo.

[bodiam]

Hi @wilkinsona , thanks for that, you're right! I've checked it, just to be sure, with Josh' app here: https://github.com/joshlong/flux-flix-service/blob/master/kotlin/flux-flix-service/src/main/kotlin/com/example/fluxflixservice/FluxFlixServiceApplication.kt, and same. Cheers!

[sdeleuze]

Yeah that's the setup I recommend and that we use on our examples and on start.spring.io.

As an alternative, it is possible to declare the main() method inside the application class, but but you would have to use this no very nice syntax to make it a Java static method:

@SpringBootApplication
class BackendApplication {

    companion object {
        @JvmStatic fun main(args: Array<String>) {
            SpringApplication.run(BackendApplication::class.java, *args)
        }
    }

     @Bean
     fun startup(userService: UserService) = CommandLineRunner {
         userService.register("erik", "erik@gmail.com")
     }
}

So probably better to use top level main() method everywhere.

Also see #10511 which will provide an even better syntax in 2.0.0.M6.

[bodiam]

Ah, thanks for that, I'll have a look at #10511 too.