Jib MainClassFinder struggles with Scala 2.13 and JDK25

Tooling
1

jib has some tricks to discover the sole main class in a given classpath. It generally works well, but it fails with Scala 2.13 and JDK25. Scala 3 and JDK25 works as expected.

I filed an issue with the jib repo.

The code in question:

Edit:
The problem is that with Scala 2 and JDK25, jib MainClassFinder sees two main classes:

  • example.Main
  • example.Main$

With every other combination only example.Main is seen.

Scala 2.13 Scala 3
Java 17 :white_check_mark: Not Tested
Java 21 :white_check_mark: :white_check_mark:
Java 25 :cross_mark: :white_check_mark:
1 Like
2

How are you creating main?

Generally, it will generate a static forwarder from class C to C$.

I see that in its partial support of deprecated App, Scala 3 produces a synthetic bridge in the App object. Maybe that is not counted?

  public void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x1041) ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
3 
object Main extends App {
  // ...
}

and

object Main {
  def main(args: Array[String]): Unit = {
    // ...   
  }
}

Both of these caused the issue.

4

I would expect explicit main to be identical under Scala 3 as under Scala 2 and show the same behavior, but I did not try it with Jib.

5

Looks like jib MainClassFinder also breaks with explicit main with Scala 3 JDK 25 as well. Curiously extends App does not break MainClassFinder with Scala 3 JDK 25.

6

That is likely per my comment about the additional flags in Scala 3, so

relevantAccess != 0

and the App main is skipped.