.sbt build with .scala files example 

.sbt builds can be supplemented with project/*.scala files. When the build file gets large enough, the first thing to factor out are resolvers and dependencies.

project/Resolvers.scala 

import sbt._
import Keys._

object Resolvers {
  val sunrepo    = "Sun Maven2 Repo" at "http://download.java.net/maven/2"
  val sunrepoGF  = "Sun GF Maven2 Repo" at "http://download.java.net/maven/glassfish" 
  val oraclerepo = "Oracle Maven2 Repo" at "http://download.oracle.com/maven"

  val oracleResolvers = Seq(sunrepo, sunrepoGF, oraclerepo)
}

project/Dependencies.scala 

import sbt._
import Keys._

object Dependencies {
  val logbackVersion = "0.9.16"
  val grizzlyVersion = "1.9.19"

  val logbackcore    = "ch.qos.logback" % "logback-core"     % logbackVersion
  val logbackclassic = "ch.qos.logback" % "logback-classic"  % logbackVersion

  val jacksonjson = "org.codehaus.jackson" % "jackson-core-lgpl" % "1.7.2"

  val grizzlyframwork = "com.sun.grizzly" % "grizzly-framework" % grizzlyVersion
  val grizzlyhttp     = "com.sun.grizzly" % "grizzly-http"      % grizzlyVersion
  val grizzlyrcm      = "com.sun.grizzly" % "grizzly-rcm"       % grizzlyVersion
  val grizzlyutils    = "com.sun.grizzly" % "grizzly-utils"     % grizzlyVersion
  val grizzlyportunif = "com.sun.grizzly" % "grizzly-portunif"  % grizzlyVersion

  val sleepycat = "com.sleepycat" % "je" % "4.0.92"

  val apachenet   = "commons-net"   % "commons-net"   % "2.0"
  val apachecodec = "commons-codec" % "commons-codec" % "1.4"

  val scalatest = "org.scalatest" %% "scalatest" % "3.2.17"
}

These files can be used mange library dependencies in one place.

project/ShellPromptPlugin.scala 

When you want to implement custom commands or tasks, you can organize your build by defining an one-off auto plugin.

import sbt._
import Keys._
import scala.sys.process._

// Shell prompt which show the current project and git branch
object ShellPromptPlugin extends AutoPlugin {
  override def trigger = allRequirements
  override lazy val projectSettings = Seq(
    shellPrompt := buildShellPrompt
  )
  val devnull: ProcessLogger = new ProcessLogger {
    def out(s: => String): Unit = {}
    def err(s: => String): Unit = {}
    def buffer[T] (f: => T): T = f
  }
  def currBranch =
    ("git status -sb" lineStream_! devnull headOption)
      .getOrElse("-").stripPrefix("## ")
  val buildShellPrompt: State => String = {
    case (state: State) =>
      val currProject = Project.extract (state).currentProject.id
      s"""$currProject:$currBranch> """
  }
}

This auto plugin will display the current project name and the git branch.

build.sbt 

Now that we factored out custom settings and dependencies out to project/*.scala, we can make use of them in build.sbt:

import Resolvers._
import Dependencies._

// factor out common settings into a sequence
lazy val buildSettings = Seq(
  organization := "com.example",
  version := "0.1.0",
  scalaVersion := "2.12.18"
)

// Sub-project specific dependencies
lazy val commonDeps = Seq(
  logbackcore,
  logbackclassic,
  jacksonjson,
  scalatest % Test
)

lazy val serverDeps = Seq(
  grizzlyframwork,
  grizzlyhttp,
  grizzlyrcm,
  grizzlyutils,
  grizzlyportunif,
  sleepycat,
  scalatest % Test
)

lazy val pricingDeps = Seq(
  apachenet,
  apachecodec,
  scalatest % Test
)

lazy val cdap2 = (project in file("."))
  .aggregate(common, server, compact, pricing, pricing_service)
  .settings(buildSettings)

lazy val common = (project in file("cdap2-common"))
  .settings(
    buildSettings,
    libraryDependencies ++= commonDeps
  )

lazy val server = (project in file("cdap2-server"))
  .dependsOn(common)
  .settings(
    buildSettings,
    resolvers := oracleResolvers,
    libraryDependencies ++= serverDeps
  )

lazy val pricing = (project in file("cdap2-pricing"))
  .dependsOn(common, compact, server)
  .settings(
    buildSettings,
    libraryDependencies ++= pricingDeps
  )  

lazy val pricing_service = (project in file("cdap2-pricing-service"))
  .dependsOn(pricing, server)
  .settings(buildSettings)

lazy val compatct = (project in file("compact-hashmap"))
  .settings(buildSettings)