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Developing Scala in IntelliJ IDEA

Use the latest IntelliJ release and install the Scala plugin from within the IDE.

Initial Setup

To create the IntelliJ project files:

  • Run sbt intellij
  • Open src/intellij/scala.ipr in IntelliJ
  • In File → Project Structure → Project → Project SDK, create an SDK entry named "1.8" containing the Java 1.8 SDK (1.6 if you're on the Scala the 2.11.x branch)

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.

Dependencies

For every module in the IntelliJ project there is a corresponding -deps library, for example compiler-deps provides ant.jar 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.

Switching Branches

The 2.12.x branch contains IntelliJ module files for actors and forkjoin even though these modules only exist in 2.11.x. This allows using the same IntelliJ project files when switching to the 2.11.x branch (without causing any issues while working on 2.12.x).

When switching between 2.11.x and 2.12.x, make sure to run sbt intellij. Note that the Project SDK is not updated in this process. If you want to use the Java 1.6 SDK while working on 2.11.x you need to change it manually (File → Project Structure → Project → Project SDK).

If you switch between 2.11.x and 2.12.x often, it makes sense to have a separate clone of the repository for each branch.

Incremental Compilation

Run Build → Make Project to build all modules of the Scala repository (library, compiler, etc). 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 and (deprecated) ant builds. This allows building incrementally in IntelliJ and directly use 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

Updating the .SAMPLE files

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