Debugging Scala code with simple-build-tool (sbt) and IntelliJ

Question

There's a very convenient -jvm-debug flag in the official SBT packages for Mac, Linux & Windows. You can use the flag to specify the debug port:

sbt -jvm-debug 5005

Under the covers, this starts the JVM for SBT with the typical verbose debugging incantation:

-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005

You now can run your code as normal, for example with the sbt run command.

Configuring IntelliJ to connect to the running code...

Now you connect IntelliJ to your running process using a Remote Debug configuration. Note that the upper 3 fields in this form, while scary, are just for you to copy text out of, rather than into (they're giving the verbose debugging incantation specified above, which -jvm-debug already takes care of for you) - the only configuration you can change is in theSettings section halfway down:

For ordinary debugging in IntelliJ, you can use an Application run/debug configuration in the usual way, regardless of whether you're using sbt to compile your code.

To connect to your application running in Jetty, you'll need to create a Remote debug configuration. When you do so, IntelliJ will give you a set of command line arguments for running the remote JVM -- something like

-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005

Launch sbt with these arguments and then execute jetty-run. Finally, launch your remote debug configuration in IntelliJ. This thread might be useful.

I had some trouble with this too, so at the risk of being overly detailed, here's what I did:

SETUP

1. Create a run configuration for sbt jetty-run

   • Go to Run > Edit Configurations
   • Click the [+] icon and choose Scala Compilation Server
   • Enter whatever name you want, and click the "Run SBT Action" checkbox and select the SBT Action jetty-run from the [...]

2. Create a debug configuration for remote debugging

   • Go to Run > Edit Configurations
   • Click the [+] icon and choose Remote
   • Enter whatever name you want and copy the line -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005 (make sure to click OK to actually create the configuration)

3. Set up sbt plugin to run the vm options above

   • Go to File > Settings > SBT
   • Paste the line -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005 in the VM parameters box, after the ones that are already there

DEBUGGING

1. Set breakpoints as desired
2. Start the jetty web server by choosing the sbt jetty-run configuration you created above and choosing Run > Run or by clicking the green arrow
3. Start the remote debugger by choosing the remote debugging configuration you created above and choosing Run > Debug or by clicking the bug icon

This one works for me every time, and the only thing you need to set up is remote debugging in IntelliJ; I start up SBT with JVM parameters from the terminal in IntelliJ:

sbt -J-Xdebug -J-Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005

After that I can start remote debugging on localhost:5005