How to create a custom package task to jar a subset of classes in SBT - scala

Question

I am trying to define a separate package task without modifying the original task in compile configuration. This new task will package only a subset of classes conforming an API which we need to be able to share with other teams so they can write plugins for our application. So the end result will be two jars, one with the full application and a second one with a subset of the classes.
I approached this problem by creating a different configuration which I called pluginApi and would redefine the packageBin task within this new configuration so it does not change the original definition of packageBin. This idea was taken from here:
How to create custom "package" task to jar up only specific package in SBT?
In my build.stb I have:
lazy val PluginApi = config("pluginApi") extend(Compile) describedAs("Custom plugin api configuration")
lazy val root = project in file(".") overrideConfigs (PluginApi)
This effectively creates my new configuration and I can call
sbt pluginApi:packageBin
Which generates the complete jar in the same way as compile:packageBin would do. I then try to modify the mappings in the new packageBin task with:
mappings in (PluginApi, packageBin) ~= { (ms: Seq[(File, String)]) =>
ms filter { case (file, toPath) =>
toPath.startsWith("some/path/defining/api")
}
}
but this has no effect. I think the reason is because the call to pluginApi:packageBin is delegated to compile:packageBin rather than it being a cloned task.
I can redefine a new packageBin within the new scope like:
packageBin in PluginApi := {
}
However I would have to rewrite all packageBin functionality instead of reusing existing code. Also, in case that rewriting is unavoidable I am not sure how that implementation would be.
Could somebody provide an example about how to achieve this?

Answer 1

You could have it done as follows
lazy val PluginApi = config("pluginApi").extend(Compile)
inConfig(PluginApi)(Defaults.compileSettings) // you have to have standard
mappings in (PluginApi, packageBin) := {
val original = (mappings in (PluginApi, packageBin)).value
original.filter { case (file, toPath) => toPath.startsWith("some/path/defining/api") }
}
unmanagedSourceDirectories in PluginApi := (unmanagedSourceDirectories in Compile).value
Note that, if you keep your sources in src/main/scala you'll have to override unmanagedSourceDirectories in the newly created configuration.
Normally the unmanagedSourceDirectories contains the configuration name. E.g. src/pluginApi/scala or src/pluginApi/java.

Answer 2

I have had similar problems (with more than one jar per project). Our project uses ant - here you can do it, you just will repeat yourself a lot.
However, I have come to the conclusion that this scenario (2 JARs for one project) actually can be simplified by splitting the project - i.e. making 2 modules out of it.
This way, I don't have to "fight" tools which assume project==artifact (like sbt, maybe maven?, IDEA's default setting,...).
As a bonus point the compiler helps me to verify that my dependencies are correct, i.e. that I did not accidentally make my API package depend on the implementation package - when compiling everything together and only splitting classes apart in the JAR step, you do run the risk of getting an invalid dependency in your setup which you would only see when testing, because during compile time everything is compiled together.