Missing Dependency in dependent project

Question

I have an android project with following structure:

-- Calendar
------app (app module)
----------build.gradle (module level)
------build.gradle (project level)
------Commons(A common project which i reuse across various projects)
----------common (common module in Commons project)
-------------build.gradle (for only common module level)
----------build.gradle(for Commons Project)

Now the problem is that if I compile Commons, deploy it to bintray and then use it as implementation 'com.amitkma.Commons:common:1.0.0' in app module, all the dependencies (which are implemented in common build.gradle) is available to use in app module also. But if I use it like following implementation project(:Commons:common), only dependencies provided using api is available to use.

I want to know what is the difference between api and implementation with respect to module is compiled or used directly like above?

Answer 1

There are two things at play here:

1. The separation of api and implementation when declaring dependencies. In short dependencies in the implementation scope are only visible at runtime because they are an implementation detail. Such libraries will use the runtime scope in their published Maven metadata. More information in the documentation
2. Before Gradle 5.0, the Maven metadata scopes compile and runtime were mixed by Gradle and thus all runtime dependencies did appear on the compile classpath. Gradle 5 changes this

So when you publish your component, the limitation of Gradle 4.x means that your common dependencies are available to app. Note that moving to Gradle 5 will cause a breakage there, as documented. And when you use the project directly, the separation is properly enforced.

The fix is to simply promote dependencies that are part of the common api to the api configuration in common and for the runtime ones, declare them in app as after all they are directly required by it.