SBT Tricks

We all pick up SBT tricks: the settings, commands, or clues we wish we’d figured out earlier. This post contains a section of some we use at Underscore. What lesser-known tricks do you make use of?

Changing JVM Options

Mid-session you sometimes want to change JVM flags. The javaOptions setting is good for that. For example…

Trace Typesafe Config Files

project foo
set javaOptions := Seq("-Dconfig.trace=loads")
run

Cheap and Cheerful Profiling

Here is another javaOptions example. Nothing like as good as Mission Control or JProfiler, but still…

project foo
set fork in run := true
set javaOptions in run += "-agentlib:hprof=cpu=samples"
runMain code.MyMain

…and then look in foo/java.hprof.txt.

Triggered Execution

Running ~test:compile rather than ~compile when writing code is often what you really need. Make sure everything in your codebase compiles as you go, rather than checking the tests right before you commit.

Try this if you always want the console cleaned at the the start of each run:

triggeredMessage in ThisBuild := Watched.clearWhenTriggered

You sometimes need test:run or test:runMain when that important application is in src/test/scala.

Testing

This is a better-known trick: testOnly allows you to run a single test suite quickly as you write code:

~testOnly mypackage.MyClass

You can also use * (not _) as a wildcard to run a set of test suites:

~testOnly mypackage.*

Stopping

When you run an application from SBT and hit CTRL-C it, it normally quits to your OS. This is annoying and can be prevented with:

cancelable in Global := true

Latest Dependencies

Ivy dependencies allow you to select “latest.integration” as the revision number:

addSbtPlugin("org.ensime" % "ensime-sbt" % "latest.integration")

This will give you the latest and greatest version. Use it for global plugins (see below) for you development environment. Avoid it for core build dependencies because you want a reproducible build at all times.

The Place for Everything

SBT allows you to configure global settings and plugins for use in all your projects. This content goes in several account-wide configuration files in ~/.sbt:

  • ~/.sbt/0.13/plugins/ folder for the plugins you use everywhere such as ensime-sbt;

  • ~/.sbt/0.13/global.sbt for settings relating to those plugins;

  • ~/.sbtrc for commands you expect to run on startup;

  • /usr/local/etc/sbtopts exists, and you can hope to never have to touch it.

Here are some useful examples of what you might do in each file:

$ cat ~/.sbt/0.13/plugins/build.sbt
addSbtPlugin("net.virtual-void" % "sbt-dependency-graph" % "latest.integration")
addSbtPlugin("com.typesafe.sbteclipse" % "sbteclipse-plugin" % "4.0.0")
addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "latest.integration")
addSbtPlugin("com.orrsella" % "sbt-sublime" % "latest.integration")

$ cat ~/.sbt/0.13/global.sbt
net.virtualvoid.sbt.graph.Plugin.graphSettings
triggeredMessage in ThisBuild := Watched.clearWhenTriggered
cancelable in Global := true

$ cat ~/.sbtrc
alias cd = project

In fact, did you know that SBT will read every .sbt file in a directory when loading a project? Use this to split up complex build files and modularise global settings:

$ cat ~/.sbt/0.13/clear.sbt
// Handy `clear` command:
def clearConsoleCommand = Command.command("clear") { state =>
  val cr = new jline.console.ConsoleReader()
  cr.clearScreen
  state
}

commands += clearConsoleCommand

Problem Solving

SBT-related problems are always something to do with scopes.

You can find the value of a setting in a particular scope using inspect:

inspect project1/libraryDependencies
inspect project2/libraryDependencies

Sometimes you want to make a setting the same for all projects in a build, without having to repeat the configuration for each project:

scalaVersion in ThisBuild := "2.11.7"

Regularly re-reading the scoping rules helps with debugging scopes.

Suggestions from the Community

Via twitter, Miguel Vilá suggests test-quick: run all tests that failed previously, tests that are new, or tests that are affected by changes in existing code.

In the comments, Stefan Schwetschke suggests using the Ammonite REPL as a replacement for the standard REPL (also known as the console). Ammonite features pretty printing, loading of dependencies directly from the REPL, multiline input and many more. Add to global.sbt

libraryDependencies += "com.lihaoyi" % "ammonite-repl" % "0.4.8" % "test" cross CrossVersion.full

initialCommands in (Test, console) := """ammonite.repl.Repl.run("")"""

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