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Developer guide

Getting started

Create a fork of the repository and clone it to create a local copy.

Branch to work against

sbt uses two or three branches for development: Use the default branch set on Github for bug fixes.

Next minor branch: 1.$MINOR.x, where $MINOR is next minor version (e.g. 1.10.x during 1.9.x series)
Development branch: develop
Stable branch: 1.$MINOR.x, where $MINOR is current minor version (e.g. 1.9.x during 1.9.x series)

The develop branch represents sbt 2.x, the next major sbt series. Next minor branch is where new features should be added as long as it is binary compatible with sbt 1.x. The stable branch represents the current stable sbt release. Only bug fixes are back-ported to the stable branch.

Note on supported JDK version for the sbt build

The sbt build itself currently doesn't support any JDK beyond version 21. You may run into deprecation warnings (which would become build errors due to build configuration) if you use any later JDK version to build sbt.

If you're using Metals as IDE, also check the Java Version setting. The default at the time of writing this is 17, but this may change in the future, or you may have set it to a later version yourself. (Be aware that Metals may download a JDK in the background if you haven't switch to a local JDK matching the version before changing this setting. Also, this setting is currently only available as a User setting, so you can't set it for a single workspace. Don't forget to switch back if you need to for other projects).

Instruction to build just sbt

Sbt has a number of sub-modules. If the change you are making is just contained in sbt/sbt (not one of the sub-modules), you can use the publishLocal command to publish the sbt project to your local machine. This is helpful for testing your changes.

$ sbt
sbt:sbtRoot> publishLocal

Instruction to build sbtn

$ sbt nativeImage

On macOS, the following can be used to target ARM64:

$ ARCHS=arm64 sbt nativeImage

Instruction to build all modules from source

When working on a change that requires changing one or more sub modules, the source code for these modules can be pulled in by running the following script (instead of building them from Maven for example and not being able to change the source code for these sub-modules).

Install the current stable binary release of sbt (see [Setup]), which will be used to build sbt from source.

Get the source code.

$ mkdir sbt-modules
$ cd sbt-modules
$ for i in sbt io librarymanagement zinc; do \
  git clone https://github.com/sbt/$i.git && (cd $i; git checkout -b develop origin/develop)
done
$ cd sbt
$ ./sbt-allsources.sh

To build and publish all components locally,

$ ./sbt-allsources.sh
sbt:sbtRoot> publishLocalAllModule

Using the locally built sbt

The publishLocal command above will build and publish version 1.$MINOR.$PATCH-SNAPSHOT (e.g. 1.1.2-SNAPSHOT) to your local ivy repository.

To use the locally built sbt, set the version in build.properties file in your project to 1.$MINOR.$PATCH-SNAPSHOT then launch sbt (this can be the sbt launcher installed in your machine).

$ cd $YOUR_OWN_PROJECT
$ sbt
> compile

Clearing out boot and local cache

sbt consists of lots of JAR files. When running sbt locally, these JAR artifacts are cached in the boot directory under $HOME/.sbt/boot/scala-2.12.6/org.scala-sbt/sbt/1.$MINOR.$PATCH-SNAPSHOT directory.

In order to see a change you've made to sbt's source code, this cache should be cleared. To clear this out run: reboot dev command from sbt's session of your test application.

By default sbt uses a snapshot version (this is a scala convention for quick local changes- it tells users that this version could change). One drawback of -SNAPSHOT version is that it's slow to resolve as it tries to hit all the resolvers. This is important when testing performance, so that the slowness of the resolution does not impact sbt.

You can workaround that by using a version name like 1.$MINOR.$PATCH-LOCAL1. A non-SNAPSHOT artifacts will now be cached under $HOME/.ivy/cache/ directory, so you need to clear that out using sbt-dirty-money's cleanCache task.

Running sbt "from source" - `sbtOn`

In addition to locally publishing a build of sbt, there is an alternative, experimental launcher within sbt/sbt to be able to run sbt "from source", that is to compile sbt and run it from its resulting classfiles rather than from published jar files.

Such a launcher is available within sbt/sbt's build through a custom sbtOn command that takes as its first argument the directory on which you want to run sbt, and the remaining arguments are passed to that sbt instance. For example:

I have setup a minimal sbt build in the directory /s/t, to run sbt on that directory I call:

> sbtOn /s/t
[info] Packaging /d/sbt/scripted/sbt/target/scala-2.12/scripted-sbt_2.12-1.2.0-SNAPSHOT.jar ...
[info] Done packaging.
[info] Running (fork) sbt.RunFromSourceMain /s/t
Listening for transport dt_socket at address: 5005
[info] Loading settings from idea.sbt,global-plugins.sbt ...
[info] Loading global plugins from /Users/dnw/.dotfiles/.sbt/1.0/plugins
[info] Loading project definition from /s/t/project
[info] Set current project to t (in build file:/s/t/)
[info] sbt server started at local:///Users/dnw/.sbt/1.0/server/ce9baa494c7598e4d59b/sock
> show baseDirectory
[info] /s/t
> exit
[info] shutting down sbt server
[success] Total time: 19 s, completed 25-Apr-2018 15:04:58

Please note that this alternative launcher does not have feature parity with sbt/launcher. (Meta) contributions welcome! :-D

Updating Scala version

See https://github.com/sbt/sbt/pull/6522 for the list of files to change for Scala version upgrade.

Diagnosing build failures

Globally included plugins can interfere building sbt; if you are getting errors building sbt, try disabling all globally included plugins and try again.

Running Tests

sbt has a suite of unit tests and integration tests, also known as scripted tests.

Unit / Functional tests

Various functional and unit tests are defined throughout the project. To run all of them, run sbt test. You can run a single test suite with sbt testOnly

Integration tests

Scripted integration tests reside in sbt-app/src/sbt-test and are written using the same testing infrastructure sbt plugin authors can use to test their own plugins with sbt. You can read more about this style of tests here.

You can run the integration tests with the sbt scripted sbt command. To run a single test, such as the test in sbt-app/src/sbt-test/project/global-plugin, simply run:

sbt "scripted project/global-plugin"

Random tidbits

Clean history

Make sure you document each commit and squash them appropriately. You can use the following guides as a reference:

Scala's documentation on Git Hygiene
Play's documentation on Working with Git

Import statements

You'd need alternative DSL import since you can't rely on sbt package object.

// for slash syntax
import sbt.SlashSyntax0._

// for IO
import sbt.io.syntax._

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