sbt cheatsheet

This document contains various references and notes about sbt build definition. One of the motivation for this document is that the official sbt documentation is not very informative.

Resources

• Official sbt documentation
• sbt.Keys defines all the tasks and settings keys, including their documentation.

Logging

• Get reference to the logger via streams.value.log.
• Log and error via streams.value.log.error("My error")
  • Note that this will not cause the task to fail, this will only log an [error] message. IF you want to fail a task, refer to Fail a task.

Fail a task

• Simply throw a java.lang.RuntimeException.
  • This will always work - it will crash the whole sbt process with the RuntimeException. But it is not the official recommended way.

Inspect tasks and settings

• inspect tree project/assembly
  • This prints all the tasks and settings that will be executed when project / assembly is run.
  • It may be helpful to increase the width of the ASCI tree with set asciiGraphWidth := 150.
• See help inspect.

See all the transitive dependencies

• print dependencyTree
  • This shows all the transitive dependencies for the default Compile scope.
• print Test/dependencyTree
  • This show all the transitive dependencies for the Test scope.

Debugging sbt tasks

• There is --jvm-debug cmd line option to sbt which allows to attach a debugger to the sbt process.
  • This option is not very handy, as you will still not be able to add a breakpoint to various task definitions inside build.sbt, only to some classes in the project directory. Moreover, once you run reload, the debugging will not work for sure.
    • This is because sbt internally compiles everything from build.sbt a various anonymous classes and the debugger does not see their sources.
• It is better to either use println directly, or to use streams.value.log.info to log messages.

Exclude internal sbt project from a fat jar

• Fat jar for fat-project is assembled via the fat-project / assembly task.
• There is fat-project / assembly / assemblyExcludedJars setting which is of type Def.ClassPath.
• To exclude internal project one must:
  • Set project / Compile / exportJars := true. This ensures that project / Compile / exportedProducts is a path to the jar.
  • In fat-project / assembly / assemblyExcludedJars get the path to the project jar via project / Compile / exportedProducts.
  • Declare dependency on project / Compile / packageBin from fat-project / assembly.
• Note that this is complicated because assemblyExcludedJars setting only works if it points to an already existing jar file.