What is "implementation" in Kotlin Gradle dependencies?

Question

I'm using Android Studio 3.0 Preview to start new Kotlin project. As I try to add dependencies in build.gradle I saw implementation scope instead of usual compile.

androidTestImplementation('com.android.support.test.espresso:espresso-core:2.2.2', {
    exclude group: 'com.android.support', module: 'support-annotations'
})
implementation 'com.android.support:appcompat-v7:25.3.1'
testImplementation 'junit:junit:4.12'

There's also androidTestImplementation and testImplementation scope.

In the end, I add compile to add third party dependencies and it works.

compile 'io.reactivex.rxjava2:rxandroid:2.0.1'

So my questions are..

• What is implementation, androidTestImplementation, and testImplementation scope?
• Is it any different than compile, testCompile, and androidTestCompile?
• Which one I should use for my Kotlin project?

Edit: My bad, this question is not Kotlin specific. It's the new Android Gradle Plugin configuration.

Answer 1

This is not specific to Kotlin, but has to do with the new Gradle plugin for Android.

compile, provided and apk are now deprecated.
Use implementation or api instead of compile, compileOnly instead of provided, and runtimeOnly instead of apk.

The reason for this is to speed up multi-module builds. Given module A which depends on module B which in turn depends on module C, a change in module C would trigger a recompile of module A as well. If A does not use C directly, there is no need for A to recompile when C changes.

The implementation configuration ensures exactly this: if you specify implementation project(':C') in B, you cannot access C from A and you avoid building unnecessary modules. In a large multi-module project this can save a lot of time.

See Migrate to the new Gradle plugin for more information.