HOWTO FIX: "Warning:scalac: Cannot connect to compile server at localhost/127.0.0.1:3200 Trying to compile without it"

scala-compile-server-options.md

Applicable for:

• IntelliJ IDEA CE 2019.1 (2018.* – ?)
• macOS Mojave
• may also work for other IDEA/macOS versions

Steps to fix:

1. Go to directory ~/Library/Preferences/IdeaIC${IDEA_VERSION}/
   • for IntelliJ IDEA CE 2019.1: ~/Library/Preferences/IdeaIC2019.1/.
2. There're two files in this directory:
   • ./settingsRepository/repository/scala.xml;
   • ./options/scala.xml.
3. Ensure both files have the same value for the COMPILE_SERVER_PORT option (if some file lacks this option, add it):
   • for example: <option name="COMPILE_SERVER_PORT" value="3200" />;
   • COMPILE_SERVER_PORT can take any valid free port number;
   • perhaps other options in these two files should have same values as well.
4. Restart your IntelliJ IDEA (not sure if it's necessary, but I'd suggest to).

Confirmed for macOS Catalina and IntelliJ IDEA 2020.3 👍

For IntelliJ IDEA, the path is ~/Library/Application Support/JetBrains/IntelliJIdea2021.1/settingsRepository/repository/scala.xml.

To debug, you can run lsof -PiTCP -sTCP:LISTEN to list open ports. You should see one spawned by java on localhost:3200.

On my current macOS Big Sur and IntelliJ IDEA 2021.2 Community Edition there's only
~/Library/Application Support/JetBrains/IdeaIC2021.2/options/scala.xml
and no settingsRepository directory at all.
But the compile server seems to be working correctly nevertheless.
Hopefully, they fixed it up eventually.

On my current macOS Big Sur and IntelliJ IDEA 2021.2 Community Edition there's only ~/Library/Application Support/JetBrains/IdeaIC2021.2/options/scala.xml and no settingsRepository directory at all. But the compile server seems to be working correctly nevertheless. Hopefully, they fixed it up eventually.

Set the project SDK to java 8 fixed it for me.