The Build Server Protocol has the concept of BSP server discovery, which means: A BSP client can connect to any BSP server, as long as there is a connection file under the
To install a BSP connection file
.bsp/mill-bsp.json for Mill, run:
Working known clients of the BSP server are IntelliJ IDEA and Metals.
You can fine control some BSP server options by specifying command options:
When using Metals by default Bloop will be used as your build server unless you explicitly choose Mill.
When in a Mill workspace use the "Switch Build Server" command from Metals which will allow you to switch to using Mill as your build
Metals is relying on SemanticDB to enable many of its features. Mill BSP now also supports to provide this SemanticDB data, when the BSP client (like Metals) requests it.
As of writing this, Metals 0.11.8 is the latest available version, which is not able to detect the fact, that Mill is already providing SemanticDB data. As a consequence, it reports missing SemanticDB data, although some SemanticDB-based features are already working. This may be fixed eventually in a future release of Metals.
Updating older setups
In the past, we provided SemanticDB support via
In case things go wrong, it can be sometimes hard to find the cause. BSP is just a protocol, and as such, issues on the server side (the Mill BSP server) might not well propagated to the BSP client (your IDE).
For that reason Mill BSP server is writing a log file under
.bsp/mill-bsp.stderr, where you can find various information about what’s going on.
Its content should look familiar, as it contains regular Mill output accompanied by additional BSP client-server communication details.
You can increase the verbosity of that log file, when you run Mill with
--debug at installation time (of the BSP discovery file).
mill --debug mill.bsp.BSP/install
If you wish to use Bloop as your build server (it is the default metals BSP implementation) with metals / vscode, consider using the mill bloop plugin.
This is preferred to the instructions (currently) on the bloop website.
Please be aware, that the Bloop contrib plugin is maintained by the community and is probably not on a par with the built-in BSP server.
IntelliJ IDEA also supports the BSP protocol. To use this support, just follow the directions in Build Server Protocol (BSP).
Additionally, you can generate IDEA project files directly with Mill. This is probably the preferred way if you work on polyglot projects and need support for other languages like Kotlin or AspectJ.
To generate IntelliJ IDEA project files into