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SBT: The Missing Tutorial

Create a New Project

$ sbt new sbt/scala-seed.g8
$ cd hello
$ sbt
...
> run
> exit

Giter8 templates

Layout

sbt uses the same directory structure as Maven for source files by default (all paths are relative to the base directory):

src/
  main/
    resources/
       <files to include in main jar here>
    scala/
       <main Scala sources>
    java/
       <main Java sources>
  test/
    resources
       <files to include in test jar here>
    scala/
       <test Scala sources>
    java/
       <test Java sources>

Other directories in src/ will be ignored. Additionally, all hidden directories will be ignored.

Source code can be placed in the project’s base directory as hello/app.scala, which may be for small projects, though for normal projects people tend to keep the projects in the src/main/ directory to keep things neat.

Build Definition

The build definition goes in a file called build.sbt, located in the project’s base directory. The “base directory” is the directory containing the project. In addition to build.sbt, the project directory can contain .scala files that defines helper objects and one-off plugins.

build.sbt
project/
  Dependencies.scala

.gitignore (or equivalent for other version control systems) should contain:

target/

As part of your build definition, specify the version of sbt that your build uses. This allows people with different versions of the sbt launcher to build the same projects with consistent results. To do this, create a file named project/build.properties that specifies the sbt version as follows:

sbt.version=1.0.2

A build definition is defined in build.sbt, and it consists of a set of projects (of type Project). Because the term project can be ambiguous, we often call it a subproject.

lazy val root = (project in file("."))
  .settings(
    name := "Hello",
    scalaVersion := "2.12.3"
  )

Each subproject is configured by key-value pairs.

build.sbt may also be interspersed with vals, lazy vals, and defs. Top-level objects and classes are not allowed in build.sbt. Those should go in the project/ directory as Scala source files.

There are three flavors of key:

SettingKey[T]: a key for a value computed once (the value is computed when loading the subproject, and kept around).
TaskKey[T]: a key for a value, called a task, that has to be recomputed each time, potentially with side effects.
InputKey[T]: a key for a task that has command line arguments as input. Check out Input Tasks for more details.

Built-in Keys

The built-in keys are just fields in an object called Keys. A build.sbt implicitly has an import sbt.Keys._, so sbt.Keys.name can be referred to as name.

Adding Library Dependencies

To depend on third-party libraries, there are two options. The first is to drop jars in lib/ (unmanaged dependencies) and the other is to add managed dependencies, which will look like this in build.sbt:

val derby = "org.apache.derby" % "derby" % "10.4.1.3"

lazy val commonSettings = Seq(
  organization := "com.example",
  version := "0.1.0-SNAPSHOT",
  scalaVersion := "2.12.3"
)

lazy val root = (project in file("."))
  .settings(
    commonSettings,
    name := "Hello",
    libraryDependencies += derby
  )

The libraryDependencies key involves two complexities: += rather than :=, and the % method. += appends to the key’s old value rather than replacing it. The % method is used to construct an Ivy module ID from strings.