'Android / Execution failed for task checkDebugAarMetadata / A failure occurred while executing CheckAarMetadataWorkAction

TL'DR: In this Android Kotlin library I updated from Gradle 5.6.4 to 6.6.1 (commit d5d8d2). Now I cannot build a project depending on the aar anymore.

Test setup

I build and deploy the aar to mavenLocal ...

$ ./gradlew clean :roadsigns:assemble
$ ./gradlew publishToMavenLocal

... and then reference the deployed library artifact in the sample Android app module. First I add mavenLocal() in the root build.gradle file:

allprojects {
    repositories {
        google()
        mavenCentral()
        jcenter()
        mavenLocal() // <-- Add this
    }
}

I reference the mavenLocal() dependency directly:

dependencies {
    implementation
    // implementation project(":roadsigns")
    implementation "info.metadude.kotlin.library.roadsigns:roadsigns:$version"
    implementation Libs.kotlinStdlib
    // ...

The error

When I build the sample app then I get the following build error:

$ ./gradlew clean assembleDebug

Execution failed for task ':checkDebugAarMetadata'.
> Multiple task action failures occurred:

   > A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
      > A dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties) does
        not specify an aarFormatVersion value, which is a required value.
        Dependency: info.metadude.kotlin.library.roadsigns:roadsigns:4.0.0.
        AAR metadata file: /home/USERNAME/.m2/repository/info/metadude/kotlin/library/roadsigns/roadsigns/4.0.0/roadsigns-4.0.0-javadoc.jar.

   > A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
      > A dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties) does
        not specify an aarFormatVersion value, which is a required value.
        Dependency: info.metadude.kotlin.library.roadsigns:roadsigns:4.0.0.
        AAR metadata file: /home/USERNAME/.m2/repository/info/metadude/kotlin/library/roadsigns/roadsigns/4.0.0/roadsigns-4.0.0-sources.jar.

   > A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
      > A dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties) does
        not specify an aarFormatVersion value, which is a required value.
        Dependency: info.metadude.kotlin.library.roadsigns:roadsigns:4.0.0.
        AAR metadata file: /home/USERNAME/.m2/repository/info/metadude/kotlin/library/roadsigns/roadsigns/4.0.0/roadsigns-4.0.0.aar.

Context information

When I try the same with Gradle 5.6.4 then there is no error.

In the Android Kotlin library I am using:

I using Java 8 (OpenJDK) on my machine and verified that the same error occurs on a different computer.

Experiments

When I deploy the library with Gradle 5 and build the app with Gradle 5 or Gradle 6 in both cases it works.

The question

What changed from Gradle 5 to Gradle 6 which causes the aar to be broken (?)

Findings

I took a look at the files being deployed in mavenLocal():

~/.m2/repository/info/metadude/kotlin/library/roadsigns
└── roadsigns
├── 4.0.0
│   ├── roadsigns-4.0.0.aar
│   ├── roadsigns-4.0.0-javadoc.jar
│   ├── roadsigns-4.0.0.module
│   ├── roadsigns-4.0.0.pom
│   └── roadsigns-4.0.0-sources.jar
└── maven-metadata-local.xml
I compared the files deployed with Gradle 5 and Gradle 6. Interestingly, the roadsigns-4.0.0.aar files are binary equivalent. The pom file differs, though:
Also there is a roadsigns-4.0.0.module file only when I deploy with Gradle 6.
When I manually remove the "new" do_not_remove: published-with-gradle-metadata part from the file deployed with Gradle 6 then the app builds successfully! The question remains ... what's going on here?!

Solution 1:^[1]

The problem very likely comes from the plugins you are using to publish the library (digital.wup:android-maven-publish:3.6.3) and the quite abandoned bintray Gradle plugin which hasn't been updated to support Gradle metadata.

I bet the .module file that takes precedence over the content of the pom.xml files in Gradle 6 consumer projects is improperly populated because of how these two plugins work.

Fortunately, there's now built-in maven-publish support in the Android Gradle Plugin (so first-party), and it's documented here: https://developer.android.com/studio/build/maven-publish-plugin

With it, you can publish to mavenLocal, bintray, and any other proper maven repo.

You should not have any issues if you use that instead of any third party alternatives.

Solution 2:^[2]

I was running into a lot of these errors:

...
The minCompileSdk (31) specified in a
dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties)
is greater than this module's compileSdkVersion (android-30).
Dependency: androidx.lifecycle:lifecycle-livedata-core:2.4.0.
AAR metadata file: C:\Users\CUR_USER\.gradle\caches\transforms-3\4e8a32482e233cfbbc450e03aabed8dd\transformed\lifecycle-livedata-core-2.4.0\META-INF\com\android\build\gradle\aar-metadata.properties.
...

What I did to fix it was

open build.gradle (app)
change android . compileSdk to 31 from 30
change android . defaultConfig . targetSdk to 31 from 30
File -> Sync Project with Gradle Files

Solution 3:^[3]

I have the same problem. I found a temporary solution: Add the aar suffix to the error module and make it transitive (or you will lose dependencies) e.g.:

implementation "androidx.lifecycle:lifecycle-extensions:2.2.0"

reports "not specify an aarFormatVersion" error, compilation failed.

change to:

implementation ("androidx.lifecycle:lifecycle-extensions:2.2.0@aar") {
    transitive = true
}

compile error disappeared.

AS Louis CAD says, you can use built-in 'maven-publish' to solve this issues.

Solution 4:^[4]

My solution is download the library package, copy LICENSE and other files to src/main-> new "Java Resources Folder" -> new "META-INF" folder

implementation 'org.apache.ftpserver:ftpserver-core:1.2.0'

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution	Source
Solution 1
Solution 2	Jacksonkr
Solution 3
Solution 4	a442509097