Test library releases using an in project Maven repository

Short, powerful post on how to test the release version of libraries directly within a project without having to deploy them to Maven first.

As a bonus, there will also be an open-source example showcasing all of this in action.

Problem description

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In (Android) library development, the local development setup differs from how customers integrate the library.

Local development uses a module dependency:

dependencies {
    implementation project(':library')
}

Customers integrate through Maven:

dependencies {
    implementation `com.jeroenmols.lib:library:1.0.0`
}

Now because both integration mechanisms are fundamentally different, they can also lead to different results.

Wouldn’t it be great if you could test the Maven version of your library directly in your project?

Let’s do that!

That avoids deploying the library to Maven, makes testing more realistic, and speeds up release testing considerably.

Core idea

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What we’ll try to accomplish is to mimic a local Maven repository within your Android project. That allows integrating the release build similar to an external Maven build:

dependencies {
    // in-project build from current code
    releaseImplementation 'com.jeroenmols.lib:library:local'

    // version from Maven
    releaseImplementation 'com.jeroenmols.lib:library:1.0.0'
}

To accomplish this we’ll:

• add a build flag to toggle between local and external build
• create a release artifact and pom.xml with a special version
• create a Maven like folder structure in app/libs
• add Maven metadata files and symlinks to the build outputs
• include the local Maven repository

Implementation

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In the app level build.gradle file, split the lib dependency between debug and release variant:

dependencies {
    debugImplementation project(':lib')

    def releaseVersion = project.hasProperty("external_version") ? project.external_version : "local"
    releaseImplementation "com.jeroenmols.lib:library:$releaseVersion"

    ...
}

In the lib level build.gradle file, toggle the published version between local and external version:

publishing {
    publications {
        aar(MavenPublication) {
            ...

            groupId = 'com.jeroenmols.lib'
            artifactId = 'library'
            version = project.hasProperty("external_version") ? project.external_version : "local"
        }
    }
}

Create a local Maven repository in the libs folder of the app module:

$ mkdir -p app/libs/<group-id-slash-separated>/<artifact-id>/local
# example: $ mkdir -p app/libs/com/jeroenmols/lib/library/local/

Go into the artifact-id folder and create a new file maven-metadata-local.xml:

$ cd app/libs/<group-id-slash-separated>/<artifact-id>
$ touch maven-metadata-local.xml

# example: $ cd app/libs/com/jeroenmols/lib/library/
#          $ touch maven-metadata-local.xml

Copy the following content into the maven-metadata-local.xml and replace the placeholders with your groupId and artifactId:

<?xml version="1.0" encoding="UTF-8"?>
<metadata>
  <groupId>GROUP ID HERE</groupId>
  <artifactId>ARTIFACT ID HERE</artifactId>
  <versioning>
    <latest>local</latest>
    <release>local</release>
    <versions>
      <version>local</version>
    </versions>
  </versioning>
</metadata>

Create symbolic links to the aar and pom.xml files.

Note that the file names and paths need to match exactly or this won’t work!

// Make sure the output files exist, so we can symlink them
$ ./gradlew generatePomFileForAarPublication assembleRelease

$ cd app/libs/<group-id-slash-separated>/<artifact-id>

$ ln -s <path-to-lib-build>/outputs/aar/<aar-file-name> <artifact-id>-local.aar
$ ln -s <path-to-lib-build>/publications/aar/pom-default.xml <artifact-id>-local.pom
# example: ln -s ../../../../../../../lib/build/outputs/aar/lib-release.aar library-local.aar
#          ln -s ../../../../../../../lib/build/publications/aar/pom-default.xml library-local.pom

Add the local repository to the app level build.gradle file:

repositories {
    maven { url "$projectDir/libs" }
}

And make sure the library dependencies are built when running an app release build by adding to the app level build.gradle file:

afterEvaluate {
    preReleaseBuild.dependsOn(":lib:assembleRelease")
    preReleaseBuild.dependsOn(":lib:generatePomFileForAarPublication")
}

That’s it!

You can now test your release variant locally by changing the build variant of the app project to release in Android studio and clicking run. Or you can test an external Maven version by invoking:

./gradlew assembleRelease -Pexternal_version=1.0.0

Wrap-up

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I hope this neat little trick is helpful to test your libraries. I’ve also open sourced a sample project that demonstrates this concept.

If you’ve made it this far you should probably follow me on Mastodon. Feel free to leave a comment below!