How to add a linked source folder in Android Studio?

In Eclipse I can add a source folder to my android project as a "linked source folder". How do I achieve the same thing in Android Studio?

Or is it possible to add a external folder to build in gradle?

in your build.gradle add the following to the end of the android node

android {
    ....
    ....

    sourceSets {
        main.java.srcDirs += 'src/main/<YOUR DIRECTORY>'
    }

}

The right answer is:

android {
    ....
    ....

    sourceSets {
        main.java.srcDirs += 'src/main/<YOUR DIRECTORY>'
    }
}

Furthermore, if your external source directory is not under src/main, you could use a relative path like this:

sourceSets {
    main.java.srcDirs += 'src/main/../../../<YOUR DIRECTORY>'
}

You can add a source folder to the build script and then sync. Look for sourceSets in the documentation here: http://tools.android.com/tech-docs/new-build-system/user-guide#TOC-Basic-Project

I haven't found a good way of adding test source folders. I have manually added the source to the .iml file. Of course this means it will go away everytime the build script is synched.

While sourceSets allows you to include entire directory structures, there's no way to exclude parts of it in Android Studio (as of version 1.2), as described here: Android Studio Exclude Class from build?

Until Android Studio gets updated to support include/exclude directives for Android sources, Symlinks work quite well. If you're using Windows, native tools such as junction or mklink can accomplish the equivalent of Un*x symlinks. CygWin can also create these with a little coersion. See: Git Symlinks in Windows and How to make symbolic link with cygwin in Windows 7