how to create a jar file from android studio

I found a way to achieve this, plain simple, using Gradle 2.2.1:

task jar(type: Jar, dependsOn: 'assembleRelease') {
    from fileTree(dir: 'build/intermediates/classes/release')
}

Place this in your library module. It will compile it as release and produce a JAR file in: build/libs.

bash gradlew jar or use your IDE to target that jar Gradle task.