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Developing Scala in IntelliJ IDEA

Use the latest IntelliJ release.

Make sure the Scala plugin is installed. (It may come preinstalled; if not, install it before proceeding.)

Initial Setup

Do not attempt to import our sbt build into IntelliJ; it won't work.

Instead, create IntelliJ project files as follows.

  • Run sbt intellij

The project files are created as copies of the .SAMPLE files, which are under version control. The actual IntelliJ project files are in .gitignore so that local changes are ignored.

Then to start coding:

  • Open src/intellij/scala.ipr in IntelliJ
  • Wait for the project to index
  • On the Build menu, choose Build Project

Everything (library, compiler etc) should build within a few minutes.

Troubleshooting

Recent versions of IntelliJ are able to find a JDK on your system and select it automatically. If that doesn't happen:

  • In File → Project Structure → Project → Project SDK, create an SDK entry named "1.8" containing the Java 1.8 SDK

Note that 8 is the safest choice. If you're having trouble, you might check to see if IntelliJ selected some later version.

Switching Branches

If you often work on both the 2.12.x and 2.13.x branches, the safest approach is to have a separate clone of the repository for each branch.

(But if you find that switching works even in the same clone, consider submitting an update to this readme with any advice you have on this.)

IntelliJ and sbt

Note that compilation IntelliJ is performed in a single pass (no bootstrap), like the sbt build.

Note that the output directory when compiling in IntelliJ is the same as for the sbt build. This allows building incrementally in IntelliJ and directly using the changes using the command-line scripts in build/quick/bin/.

Running JUnit Tests

JUnit tests can be executed by right-clicking on a test class or test method and selecting "Run" or "Debug". The debugger will allow you to stop at breakpoints within the Scala library.

It is possible to invoke the Scala compiler from a JUnit test (passing the source code as a string) and inspect the generated bytecode, see for example scala.issues.BytecodeTest. Debugging such a test is an easy way to stop at breakpoints within the Scala compiler.

Running the Compiler and REPL

You can create run/debug configurations to run the compiler and REPL directly within IntelliJ, which might accelerate development and debugging of the compiler.

To debug the Scala codebase you can also use "Remote" debug configuration and pass the corresponding arguments to the jvm running the compiler / program.

To run the compiler create an "Application" configuration with

  • Main class: scala.tools.nsc.Main
  • Program arguments: -usejavacp -cp sandbox -d sandbox sandbox/Test.scala
  • Working directory: the path of your checkout
  • Use classpath of module: compiler

To run the REPL create an "Application" configuration with

  • Main class: scala.tools.nsc.MainGenericRunner
  • Program arguments: -usejavacp
  • Working directory: the path of your checkout
  • Use classpath of module: repl-frontend

Dependencies

For every module in the IntelliJ project there is a corresponding -deps library, for example compiler-deps provides JARs for the compiler codebase. The .jar files in these -deps libraries can be easily kept up-to-date by running sbt intellij again. This is necessary whenever the dependencies in the sbt build change, for example when the starr version is updated.

Note that this command only patches the dependency lists, all other settings in the IntelliJ project definition are unchanged.

To overwrite the project definition files by copying the .SAMPLE files again run sbt intellijFromSample.

Updating the .SAMPLE files

The command intellijToSample overwrites the .SAMPLE files using the current project definition files.