.scala Build Definition

.scala Build Definition

This page assumes you've read previous pages in the Getting Started Guide, especially .sbt build definition and more about settings.

sbt is recursive

build.sbt is so simple, it 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 project inside your project which knows how to build your project. The project inside project can (in theory) 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 project's base directory

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

    build.sbt           # build.sbt is part of the source code for the
                        #   build definition project inside project/

    project/            # base directory of the build definition project

        Build.scala     # a source file in the project/ project,
                        #   that is, a source file in the build definition

        build.sbt       # this is part of a build definition for a project
                        #   in project/project ; build definition's build
                        #   definition

        project/        # base directory of the build definition project
                        #   for the build definition

            Build.scala # source file in the project/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 Build.scala are conventions only. This also means that multiple files are allowed.

.scala source files in the build definition project

.sbt files are merged into their sibling project directory. Looking back at the project layout:

hello/                  # your project's base directory

    build.sbt           # build.sbt is part of the source code for the
                        #   build definition project inside project/

    project/            # base directory of the build definition project

        Build.scala     # a source file in the project/ project,
                        #   that is, a source file in the build definition

The Scala expressions in build.sbt are compiled alongside and merged with Build.scala (or any other .scala files in the project/ directory).

``.sbt`` files in the base directory for a project become part of the ``project`` build definition project also located in that base directory.

The .sbt file format is a convenient shorthand for adding settings to the build definition project.

Relating build.sbt to Build.scala

To mix .sbt and .scala files in your build definition, you need to understand how they relate.

The following two files illustrate. First, if your project is in hello, create hello/project/Build.scala as follows:

import sbt._
import Keys._

object HelloBuild extends Build {

    val sampleKeyA = SettingKey[String]("sample-a", "demo key A")
    val sampleKeyB = SettingKey[String]("sample-b", "demo key B")
    val sampleKeyC = SettingKey[String]("sample-c", "demo key C")
    val sampleKeyD = SettingKey[String]("sample-d", "demo key D")

    override lazy val settings = super.settings ++
        Seq(sampleKeyA := "A: in Build.settings in Build.scala", resolvers := Seq())

    lazy val root = Project(id = "hello",
                            base = file("."),
                            settings = Project.defaultSettings ++ Seq(sampleKeyB := "B: in the root project settings in Build.scala"))

Now, create hello/build.sbt as follows:

sampleKeyC in ThisBuild := "C: in build.sbt scoped to ThisBuild"

sampleKeyD := "D: in build.sbt"

Start up the sbt interactive prompt. Type inspect sample-a and you should see (among other things):

[info] Setting: java.lang.String = A: in Build.settings in Build.scala
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}/*:sample-a

and then inspect sample-c and you should see:

[info] Setting: java.lang.String = C: in build.sbt scoped to ThisBuild
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}/*:sample-c

Note that the "Provided by" shows the same scope for the two values. That is, sampleKeyC in ThisBuild in a .sbt file is equivalent to placing a setting in the Build.settings list in a .scala file. sbt takes build-scoped settings from both places to create the build definition.

Now, inspect sample-b:

[info] Setting: java.lang.String = B: in the root project settings in Build.scala
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}hello/*:sample-b

Note that sample-b is scoped to the project ({file:/home/hp/checkout/hello/}hello) rather than the entire build ({file:/home/hp/checkout/hello/}).

As you've probably guessed, inspect sample-d matches sample-b:

[info] Setting: java.lang.String = D: in build.sbt
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}hello/*:sample-d

sbt appends the settings from .sbt files to the settings from Build.settings and Project.settings which means .sbt settings take precedence. Try changing Build.scala so it sets key sample-c or sample-d, which are also set in build.sbt. The setting in build.sbt should "win" over the one in Build.scala.

One other thing you may have noticed: sampleKeyC and sampleKeyD were available inside build.sbt. That's because sbt imports the contents of your Build object into your .sbt files. In this case import HelloBuild._ was implicitly done for the build.sbt file.

In summary:

  • In .scala files, you can add settings to Build.settings for sbt to find, and they are automatically build-scoped.
  • In .scala files, you can add settings to Project.settings for sbt to find, and they are automatically project-scoped.
  • Any Build object you write in a .scala file will have its contents imported and available to .sbt files.
  • The settings in .sbt files are appended to the settings in .scala files.
  • The settings in .sbt files are project-scoped unless you explicitly specify another scope.

When to use .scala files

In .scala files, you are not limited to a series of settings expressions. You can write any Scala code including val, object, and method definitions.

One recommended approach is to define settings in ``.sbt`` files, using ``.scala`` files when you need to factor out a ``val`` or ``object`` or method definition.

Because the .sbt format allows only single expressions, it doesn't give you a way to share code among expressions. When you need to share code, you need a .scala file so you can set common variables or define methods.

There's one build definition, which is a nested project inside your main project. .sbt and .scala files are compiled together to create that single definition.

.scala files are also required to define multiple projects in a single build. More on that is coming up in Multi-Project Builds.

(A disadvantage of using .sbt files in a multi-project build is that they'll be spread around in different directories; for that reason, some people prefer to put settings in their .scala files if they have sub-projects. This will be clearer after you see how multi-project builds work.)

The build definition project in interactive mode

You can switch the sbt interactive prompt to have the build definition project in project/ as the current project. To do so, type reload plugins.

> reload plugins
[info] Set current project to default-a0e8e4 (in build file:/home/hp/checkout/hello/project/)
> show sources
[info] ArrayBuffer(/home/hp/checkout/hello/project/Build.scala)
> reload return
[info] Loading project definition from /home/hp/checkout/hello/project
[info] Set current project to hello (in build file:/home/hp/checkout/hello/)
> show sources
[info] ArrayBuffer(/home/hp/checkout/hello/hw.scala)

As shown above, you use reload return to leave the build definition project and return to your regular project.

Reminder: it's all immutable

It would be wrong to think that the settings in build.sbt are added to the settings fields in Build and Project objects. Instead, the settings list from Build and Project, and the settings from build.sbt, are concatenated into another immutable list which is then used by sbt. The Build and Project objects are "immutable configuration" forming only part of the complete build definition.

In fact, there are other sources of settings as well. They are appended in this order:

  • Settings from Build.settings and Project.settings in your .scala files.
  • Your user-global settings; for example in ~/.sbt/build.sbt you can define settings affecting all your projects.
  • Settings injected by plugins, see using plugins coming up next.
  • Settings from .sbt files in the project.
  • Build definition projects (i.e. projects inside project) have settings from global plugins (~/.sbt/plugins) added. Using plugins explains this more.

Later settings override earlier ones. The entire list of settings forms the build definition.