Organizing the build 

This page discusses the organization of the build structure.

Please read the earlier pages in the Getting Started Guide first, in particular you need to understand build.sbt, task graph, Library dependencies, and Multi-project builds before reading this page.

sbt is recursive 

build.sbt conceals how sbt really works. sbt builds are defined with Scala code. That code, itself, has to be built. What better way than with sbt?

The project directory is another build inside your build, which knows how to build your build. To distinguish the builds, we sometimes use the term proper build to refer to your build, and meta-build to refer to the build in project. The projects inside the metabuild can do anything any other project can do. Your build definition is an sbt project.

And the turtles go all the way down. If you like, you can tweak the build definition of the build definition project, by creating a project/project/ directory.

Here’s an illustration.

hello/                     # your build's root project's base directory

    Hello.scala            # a source file in your build's root project
                           #   (could be in src/main/scala too)

    build.sbt              # build.sbt is part of the source code for
                           #   meta-build's root project inside project/;
                           #   the build definition for your build

    project/               # base directory of meta-build's root project

        Dependencies.scala # a source file in the meta-build's root project,
                           #   that is, a source file in the build definition
                           #   the build definition for your build

        assembly.sbt       # this is part of the source code for
                           #   meta-meta-build's root project in project/project;
                           #   build definition's build definition

        project/           # base directory of meta-meta-build's root project;
                           #   the build definition project for the build definition

            MetaDeps.scala # source file in the root project of
                           #   meta-meta-build in project/project/

Don’t worry! Most of the time you are not going to need all that. But understanding the principle can be helpful.

By the way: any time files ending in .scala or .sbt are used, naming them build.sbt and Dependencies.scala are conventions only. This also means that multiple files are allowed.

Tracking dependencies in one place 

One way of using the fact that .scala files under project becomes part of the build definition is to create project/Dependencies.scala to track dependencies in one place.

import sbt._

object Dependencies {
  // Versions
  lazy val akkaVersion = "2.3.8"

  // Libraries
  val akkaActor = "com.typesafe.akka" %% "akka-actor" % akkaVersion
  val akkaCluster = "com.typesafe.akka" %% "akka-cluster" % akkaVersion
  val specs2core = "org.specs2" %% "specs2-core" % "2.4.17"

  // Projects
  val backendDeps =
    Seq(akkaActor, specs2core % Test)

The Dependencies object will be available in build.sbt. To use the vals under it easier, import Dependencies._.

import Dependencies._

lazy val commonSettings = Seq(
  version := "0.1.0",
  scalaVersion := "2.12.1"

lazy val backend = (project in file("backend")).
  settings(commonSettings: _*).
    libraryDependencies ++= backendDeps

This technique is useful when you have a multi-project build that’s getting large, and you want to make sure that subprojects to have consistent dependencies.

When to use .scala files 

In .scala files, you can write any Scala code, including top-level classes and objects.

The recommended approach is to define most settings in a multi-project build.sbt file, and using project/*.scala files for task implementations or to share values, such as keys. The use of .scala files also depends on how comfortable you or your team are with Scala.

Defining auto plugins 

For more advanced users, another way of organizing your build is to define one-off auto plugins in project/*.scala. By defining triggered plugins, auto plugins can be used as a convenient way to inject custom tasks and commands across all subprojects.


sbt Reference Manual
    1. Organizing the build