Regression: Main-Class attribute always added to jar manifest when using application plugin

[ashih]

Expected Behavior

Running gradle assemble in Micronaut project produces 2 jars (helloworld-0.1-all.jar and helloworld-0.1.jar), but only helloworld-0.1-all.jar should have a Main-Class manifest entry

helloworld-0.1.jar MANIFEST.MF:

Manifest-Version: 1.0

helloworld-0.1-all.jar MANIFEST.MF:

Manifest-Version: 1.0
Main-Class: example.Application

Current Behavior

Starting with Gradle 6.4, both jars have Main-Class manifest entries

helloworld-0.1.jar MANIFEST.MF:

Manifest-Version: 1.0
Main-Class: example.Application

helloworld-0.1-all.jar MANIFEST.MF:

Manifest-Version: 1.0
Main-Class: example.Application

Running java -jar helloworld-0.1.jar returns

Exception in thread "main" java.lang.NoClassDefFoundError: io/micronaut/runtime/Micronaut
        at example.Application.main(Application.java:24)
Caused by: java.lang.ClassNotFoundException: io.micronaut.runtime.Micronaut

Context

This stopped working with version 6.4. I know it works with 6.3.

I am creating a general purpose build pipeline for gradle projects. The pipeline involves installing the latest gradle version onto a clean Ubuntu image and building a gradle application. This build pipeline is used for Google's open sourced buildpacks. In this buildpack, we run gradle assemble then search in build/libs for a single jar with a Main-Class manifest entry to determine which jar to run. When multiple jars have Main-Class manifest entries, we error.

Steps to Reproduce

Using this gradle sample: https://github.com/micronaut-projects/micronaut-examples/tree/master/hello-world-java

1. Run gradle assemble with Gradle version 6.4
2. Extract data in jars in build/libs (should be the two listed above)
3. Notice they both have Main-Class manifest entries in 6.4 (located at META-INF/MANIFEST.MF), but only one has the manifest entry when built with 6.3.

Your Environment

This is running inside a minimal Ubuntu 18.04 image with Java 11 JDK.
Build scan URL:
https://gradle.com/s/bkpeawim6ffns
(Have not received email to activate but will when I receive it)

[ljacomet]

Thanks for the report.

This is indeed a change that was done as part of the Java Module System support. And it probably should not have been done in a minor.

Looking at options to handle this, will update on this issue.

[ljacomet]

Note that a workaround is available by doing:

tasks.named("jar").configure {
    manifest.attributes.remove("Main-Class")
}

[ashih]

Thanks for looking into this. Does this mean it is an intended change?

[big-guy]

No, it wasn't an intended change.

[ljacomet]

This will be reverted for 6.4.1 and become an intended change in Gradle 7.0 as per #13125

[ashih]

Is there any way to support JPMS while also ensuring that this regression does not happen in 7.0? This output is odd for anyone working with micronaut.