Scala Dependency Injection / Eager bindings:

If you want to create them once and also create them eagerly when the application starts up, rather than lazily when they are needed, then you can Guice’s eager singleton binding.
...
Eager singletons can be used to start up a service when an application starts.

However, there is no mention that dev mode does not create eager bindings until the first request. It's mentioned in a 2016 thread here:

https://groups.google.com/g/play-framework/c/21G3eEU-3Y8

I think the eager bindings chapter would be a good spot to mention this.

Play Version

2.8.2

API

Scala