Add or document way of comparing against a gradle coordinate

[simonbasle]

It might be a case of me missing an obvious Gradle feature, but I think it would be beneficial to add support how to compare the jar artifact built by the current build with a baseline jar that is available in a repository (eg. Maven Central), e.g. by providing gradle coordinates to the baseline like groupId:artifactId:baseLineVersion.

This would eliminate the need to create a specific configuration for the base line and add a dependency to the baseline jar in it (which is easy to get wrong, as apparently you also have to force that dependency otherwise Gradle can sometimes resolve the actual dependency version to be the same as the currently built, e.g. if the japicmp task doesn't fail the build).

If not considered, a fallback would be to at least document how to best achieve that, as it seems to me that requiring the user to manually download the jar into a local folder is a lot of boilerplate.

[pettermahlen]

Yes, this is how the Maven plugin for japicmp works, and it's very useful.

[sebersole]

Its actually very easy, e.g.

configurations {
    compatibilityBase {
        description = 'Configuration for the base version of Hibernate we want to verify compatibility against'
    }
}

dependencies {
    compatibilityBase 'org.hibernate:hibernate-core:5.0.0.Final'
}

task generateDesignationsReport(type: me.champeau.gradle.japicmp.JapicmpTask) {
    oldClasspath = configurations.compatibilityBase
    ...
}

[simonbasle]

⚠️ This configuration trick doesn't work for me anymore when the dependency is a different version of the project being built (which makes sense for JAPICMP check). The check ends up comparing the freshly assembled jar to... itself 🤦‍♂️ So it was just silently skipping the check rather than failing visibly 😢

It looks like Gradle (at least in versions as old as 4.10.2) forces to the current version when resolving the dependency.

I even tried force = true in the dependency closure, and dependencyResolution in the Configuration closure, without success :(

The only way I found out of this was to use the download-task plugin de.undercouch.download, using it as an intermediate step between jar and japicmp to download the baseline jar:

jar.finalizedBy(downloadBaseline)
downloadBaseline.finalizedBy(japicmp)

[melix]

Gradle doesn't allow a cycle between the root of the dependency graph and itself, so if what you are comparing is the project artifact with a different version of itself, the dependency graph will eventually contain one node, which is what you are seeing. A workaround is to do the compatibility check in a dedicated subproject.

[simonbasle]

@melix do you think a "hack" around that (like the above workaround of downloading the jar via an http download plugin rather than Gradle's dependency resolution) could be integrated into the plugin?

IMHO it is far from an edge case to want to do the japicmp check against a previous jar (eg. first version in the current maintenance branch), without having to rebuild it, or dedicate a subproject to the check, or jump through too many hoops.

If not, documenting the workaround(s) in the README would also definitely help 😉