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Achieve modularization in android, I want SDK to work with optional dependencies. Whether dependency is provided or not code should compile in both

I am creating SDK and want to modularize it.
And prevent internal APIs from being exposed on your public interface.
Want optional dependency based on features required. If features is not required we can remove dependency and code and imports should not work.
Tried Ways
Submodule dependencies
Fat AAR
Submodule dependencies
when a library module gets built, the .aar artifact will only include code and resources that are in the library module itself. It won’t include:
any code or resources from database and ui-components
links to its transitive dependencies (these go into the build.gradle)
So when the app module directly includes the library as a gradle dependency, it would crash due to missing classes from database and ui-components on its classpath.
Fat AAR
In the fat .aar solution, code and resources of the submodules are bundled into the main SDK module, hence creating a fat .aar
the fat .aar plugin breaks on almost every minor Android Gradle plugin update! This is because it hooks itself into particular tasks of the Android Gradle plugin and these very often get renamed/moved. However, the project maintainer need to do job at fixing those within a few weeks after the breaking change.
Also, because of the way fat .aar references dependencies from submodules, it can significantly increase the binary size of your SDK.
Is there any other possible solution?

dependencies issue when adding library AAR

I've created an Android library that I want to be able to use in other Android applications. I've created a small test app in the same project as the library and everything works.
To test integration with other apps, I created a new application, and followed this guide to import my library to that new application by creating a new module from the library's AAR file.
build.gradle file:
dependencies {
...
compile project(':my-sdk')
...
}
settings.gradle file:
include ':app', ':my-sdk'
Compilation succeeded and I can use the library's API inside the test app, but when running it I get a runtime exception
java.lang.NoClassDefFoundError: Failed resolution of: Lokhttp3/MediaType;
I assume it's a dependencies issue, since the library uses okhttp as a dependency, which is probably not being packed in the AAR file. I just couldn't find anywhere how to compile this dependency so it would work in other projects.

I fix it adding jar files instead of using maven into my module, then assemble release aar, and in the app project disabling "Instant Run".

Does an Android .aar built with Gradle contain dependencies?

Not sure why I can't find any answers on this. If I convert my library project into an .aar using Gradle in Android Studio, does it retain all the dependencies of that module?
I'm asking because I'm trying to use a Gradle generated .aar locally, but it looks like only some of the original dependencies have been packaged. Namely, it complains that I'm missing 'OkHttp', but if I add it to the main project I get duplicate class errors.

Usually a library does not directly contain its dependencies. This does not matter whether it is an aar or a jar. Instead, the library declares its dependencies in the Gradle build file and they are resolved when someone uses the library.

Multiple AAR files

I am using Android Studio 1.2
I create a private library I want to use that one in another application.
To use it I create an AAR files, but this AAR don't work. I have in my library a dependency to an AAR file.
The AAR files do not the dependencies?
If I use the jar and I includ ans create all the dependencies the project works fine.
NOTE :
I know how to importe the AAR file. The problem is to use an AAR in the AAR..
Thanks.

If I'm understanding your question correctly, there are 3 projects involved:
Library Project 2 --> Library Project 1 --> Application Project
You are editing "Library Project 1" and have added to it's app/build.grade a dependency on the Library Project 2's aar. Something like this: compile 'com.arasthel:gnavdrawer-library:1.1.5'
I am not sure where you are running into an issue, but I'll attempt an answer anyway. If I'm completely off-base, can you please elaborate on how the AAR dependency is not working? Any error messages?, a class/resource not found, etc.
I think it's unlikely you are unable to use a class from Library Project 2 inside Library Project 1, because I just tried this myself and it seems to be working just fine. It's worth noting that the Library Project 1 aar file will NOT include classes or resources from Library Project 2. Library Project 2 will be noted as a dependency in Library Project 1's pom if published using gradle's maven plugin to publish Library Project 1.
My guess is that you are having a problem in the Application Project? Perhaps the class from Library Project 2 is not found in the Application Project?
If that is correct, then there are two possible solutions:
Enable transitive dependencies on the aar dependency in the Application project's app/build.gradle: Instead of compile 'com.example:myLibrary:versionX', make it compile('com.example:myLibrary:versionX'){transitive=true}. I just verified this causes gradle to read Library Project 1's pom and automatically add dependencies found there into the Application Project.
If you would like to use transitive dependencies, your Library Project will need to be generating a pom and publishing it along with the aar. See https://stackoverflow.com/a/30085677/431296 for some additional information on how I have this working.
Manually add the dependency on Library Project 2 to the Application Project - so that your Application has a dependency line for both Libraries. Depending on your specific situation this may or may not be a workable solution.

Add following code to you project build.gradle file, and you should put you AAR file to the libs folder.
repositories {
mavenCentral()
flatDir {
dirs 'libs'
}
}
And finally add compile info to your dependencies:
dependencies {
compile(name:'AARFileName', ext:'aar')
}

Android/Gradle: Where is this dependency embedded?

I'm using the new Android build system that is based on Gradle, together with the early access preview Android Studio. Now, I have two projects: an Android library project, and an Android app project (basically a demo for the library).
In the library project I have added a dependency to the gson library, so my build.gradle file looks like this:
dependencies {
compile 'com.android.support:support-v4:13.0.+'
compile 'com.google.code.gson:gson:2.2.+'
}
Still, everything works fine and dandy and I'm able to use gson in my library and then my app. But I want to understand where this library is embedded. I've opened both the .aar that is built by the library project and the .apk of the demo app. I was expecting to find the jars for the two dependencies in at least one of these, but I didn't.
So where are they?

From Android Tools website:
These items, plus the output of the compilation of the project’s own source code, are sent to dex for bytecode conversion and inclusion in the final APK.
In other words, they are in your *.dex file inside the APK.

As #SharkyXTS said, the code from any external dependencies is compiled into the final .dex file inside your APK. The reason why you can't find any references to these dependencies in the .aar is because there aren't any.
The .aar format is only supported through Maven for now, so dependencies are found through there. I believe there are plans to eventually support local .aar dependencies (without Maven), but the Android plugin isn't quite there yet. You can see this issue for more information.