1. Multi-project builds

Multi-project builds 

This page introduces multiple subprojects in a single build.

Please read the earlier pages in the Getting Started Guide first, in particular you need to understand build.sbt before reading this page.

Multiple subprojects 

It can be useful to keep multiple related subprojects in a single build, especially if they depend on one another and you tend to modify them together.

Each subproject in a build has its own source directories, generates its own jar file when you run package, and in general works like any other project.

A project is defined by declaring a lazy val of type Project. For example, :

lazy val util = (project in file("util"))

lazy val core = (project in file("core"))

The name of the val is used as the subproject’s ID, which is used to refer to the subproject at the sbt shell.

Optionally the base directory may be omitted if it’s the same as the name of the val.

lazy val util = project

lazy val core = project

Build-wide settings 

To factor out common settings across multiple subprojects, define the settings scoped to ThisBuild. ThisBuild acts as a special subproject name that you can use to define default value for the build. When you define one or more subprojects, and when the subproject does not define scalaVersion key, it will look for ThisBuild / scalaVersion.

The limitation is that the right-hand side needs to be a pure value or settings scoped to Global or ThisBuild, and there are no default settings scoped to subprojects. (See Scopes)

ThisBuild / organization := "com.example"
ThisBuild / version      := "0.1.0-SNAPSHOT"
ThisBuild / scalaVersion := "2.12.18"

lazy val core = (project in file("core"))
    // other settings

lazy val util = (project in file("util"))
    // other settings

Now we can bump up version in one place, and it will be reflected across subprojects when you reload the build.

Common settings 

Another way to factor out common settings across multiple projects is to create a sequence named commonSettings and call settings method on each project.

lazy val commonSettings = Seq(
  target := { baseDirectory.value / "target2" }

lazy val core = (project in file("core"))
    // other settings

lazy val util = (project in file("util"))
    // other settings


Projects in the build can be completely independent of one another, but usually they will be related to one another by some kind of dependency. There are two types of dependencies: aggregate and classpath.


Aggregation means that running a task on the aggregate project will also run it on the aggregated projects. For example,

lazy val root = (project in file("."))
  .aggregate(util, core)

lazy val util = (project in file("util"))

lazy val core = (project in file("core"))

In the above example, the root project aggregates util and core. Start up sbt with two subprojects as in the example, and try compile. You should see that all three projects are compiled.

In the project doing the aggregating, the root project in this case, you can control aggregation per-task. For example, to avoid aggregating the update task:

lazy val root = (project in file("."))
  .aggregate(util, core)
    update / aggregate := false


update / aggregate is the aggregate key scoped to the update task. (See scopes.)

Note: aggregation will run the aggregated tasks in parallel and with no defined ordering between them.

Classpath dependencies 

A project may depend on code in another project. This is done by adding a dependsOn method call. For example, if core needed util on its classpath, you would define core as:

lazy val core = project.dependsOn(util)

Now code in core can use classes from util. This also creates an ordering between the projects when compiling them; util must be updated and compiled before core can be compiled.

To depend on multiple projects, use multiple arguments to dependsOn, like dependsOn(bar, baz).

Per-configuration classpath dependencies 

core dependsOn(util) means that the compile configuration in core depends on the compile configuration in util. You could write this explicitly as dependsOn(util % "compile->compile").

The -> in "compile->compile" means “depends on” so "test->compile" means the test configuration in core would depend on the compile configuration in util.

Omitting the ->config part implies ->compile, so dependsOn(util % "test") means that the test configuration in core depends on the Compile configuration in util.

A useful declaration is "test->test" which means test depends on test. This allows you to put utility code for testing in util/src/test/scala and then use that code in core/src/test/scala, for example.

You can have multiple configurations for a dependency, separated by semicolons. For example, dependsOn(util % "test->test;compile->compile").

Inter-project dependencies 

On extremely large projects with many files and many subprojects, sbt can perform less optimally at continuously watching files that have changed and use a lot of disk and system I/O.

sbt has trackInternalDependencies and exportToInternal settings. These can be used to control whether to trigger compilation of a dependent subprojects when you call compile. Both keys will take one of three values: TrackLevel.NoTracking, TrackLevel.TrackIfMissing, and TrackLevel.TrackAlways. By default they are both set to TrackLevel.TrackAlways.

When trackInternalDependencies is set to TrackLevel.TrackIfMissing, sbt will no longer try to compile internal (inter-project) dependencies automatically, unless there are no *.class files (or JAR file when exportJars is true) in the output directory.

When the setting is set to TrackLevel.NoTracking, the compilation of internal dependencies will be skipped. Note that the classpath will still be appended, and dependency graph will still show them as dependencies. The motivation is to save the I/O overhead of checking for the changes on a build with many subprojects during development. Here’s how to set all subprojects to TrackIfMissing.

ThisBuild / trackInternalDependencies := TrackLevel.TrackIfMissing
ThisBuild / exportJars := true

lazy val root = (project in file("."))

The exportToInternal setting allows the dependee subprojects to opt out of the internal tracking, which might be useful if you want to track most subprojects except for a few. The intersection of the trackInternalDependencies and exportToInternal settings will be used to determine the actual track level. Here’s an example to opt-out one project:

lazy val dontTrackMe = (project in file("dontTrackMe"))
    exportToInternal := TrackLevel.NoTracking

Default root project 

If a project is not defined for the root directory in the build, sbt creates a default one that aggregates all other projects in the build.

Because project hello-foo is defined with base = file("foo"), it will be contained in the subdirectory foo. Its sources could be directly under foo, like foo/Foo.scala, or in foo/src/main/scala. The usual sbt directory structure applies underneath foo with the exception of build definition files.

At the sbt interactive prompt, type projects to list your projects and project <projectname> to select a current project. When you run a task like compile, it runs on the current project. So you don’t necessarily have to compile the root project, you could compile only a subproject.

You can run a task in another project by explicitly specifying the project ID, such as subProjectID/compile.

Common code 

The definitions in .sbt files are not visible in other .sbt files. In order to share code between .sbt files, define one or more Scala files in the project/ directory of the build root.

See organizing the build for details.

Appendix: Subproject build definition files 

Any .sbt files in foo, say foo/build.sbt, will be merged with the build definition for the entire build, but scoped to the hello-foo project.

If your whole project is in hello, try defining a different version (version := "0.6") in hello/build.sbt, hello/foo/build.sbt, and hello/bar/build.sbt. Now show version at the sbt interactive prompt. You should get something like this (with whatever versions you defined):

> show version
[info] hello-foo/*:version
[info]  0.7
[info] hello-bar/*:version
[info]  0.9
[info] hello/*:version
[info]  0.5

hello-foo/*:version was defined in hello/foo/build.sbt, hello-bar/*:version was defined in hello/bar/build.sbt, and hello/*:version was defined in hello/build.sbt. Remember the syntax for scoped keys. Each version key is scoped to a project, based on the location of the build.sbt. But all three build.sbt are part of the same build definition.

Style choices:

  • Each subproject’s settings can go into *.sbt files in the base directory of that project, while the root build.sbt declares only minimum project declarations in the form of lazy val foo = (project in file("foo")) without the settings.
  • We recommend putting all project declarations and settings in the root build.sbt file in order to keep all build definition under a single file. However, it’s up to you.

Note: You cannot have a project subdirectory or project/*.scala files in the sub-projects. foo/project/Build.scala would be ignored.