Issue #4461: only recognize instance main methods in instantiable classes - #4532
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… in instantiable classes Since Java 25 support (GoogleContainerTools#4432), MainClassFinder accepts instance main methods. Scala 2 compiles `object Main` into a `Main$` class with an instance main(String[]) but a private constructor, so both Main and Main$ were reported as main classes and main class inference failed. JEP 512 only launches an instance main if the class can be instantiated through a non-private no-args constructor. Apply that rule: instance mains now count only for concrete classes with such a constructor. Static mains are unaffected.
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Fixes #4461 🛠️
Problem
Since Java 25 support was added in #4432,
MainClassFinderaccepts instancemainmethods. Scala 2 compilesobject Maininto two classes:Main, with a static forwardermain, andMain$, the module class, which has an instancemain(String[])and a private constructor. Both classes were reported as main classes, so main class inference failed with multiple candidates on Scala 2.13 + JDK 25.Solution
JEP 512 only launches an instance
mainif the class can be instantiated through a non-private constructor with no parameters.MainClassVisitornow records static and instance mains separately, along with whether the class is concrete (not abstract, not an interface) and has a non-private no-args<init>. An instancemaincounts only when both hold. Static mains, and class files older than Java 25, are unaffected. I described the cause and this approach in the issue before opening the PR.Tests
New fixtures in
class-finder-tests/java25-instance-main-requirements(class file version 69):ScalaObjectMain+ScalaObjectMain$: finds exactlyScalaObjectMain(the case from Scala2 JDK25 MainClassFinder finds the singleton implementation class name(Main$) along with the correct class name(Main) #4461)PrivateConstructorInstanceMain: instance main with only a private constructor, not a main classAbstractInstanceMain: instance main in an abstract class, not a main classThese class files were generated with ASM to match the structure Scala 2 emits (private constructor,
MODULE$field, instancemain), since the build doesn't include a Scala or JDK 25 compiler. I'm happy to share the generator or swap inscalacoutput if you prefer.MainClassFinderTesttests pass with it./gradlew :jib-core:testpasses (606 tests), plusverifyGoogleJavaFormatand checkstyle, on JDK 11🤖 Generated with Claude Code