Improve Spring-Boot integration, support for Spring-Boot Assemblies

[lefou]

Being a Java framework, Spring-Boot not necessarily requires dedicated support. Yet, The Spring-Boot framework provides features and tooling which could use a better Mill integration, namely Spring-Boot Assemblies.

Mill already comes with Assembly support. Spring-Boot provides an alternative implementation with different trade-offs. For example, in Spring-Boot Assemblies, library dependencies are not unpacked but embedded as-is, which has various benefits:

• no re-packaging
• no special setup for file collisions needed
• no licensing issues due to repackaging
• works for Non-Spring applications as well
• potentially more docker-friendly

This comes at the price, that a Spring-Boot application uses it's own main class as an entry point and a specialized classloader.

It would be nice to have a SpringBootAssemblyModule, that supports those features out of the box. There is a prior-art external plugin mill-spring-boot which could be integrated for that.

A Spring-Boot specific tutorial should be added and discuss the features and its pros and cons. Also, other tutorials using Spring-Boot should be reviewed if they can benefit from the new module.

[lihaoyi]

I think what we can aim for is 4-5 example projects showing off how to use Mill with Spring-Boot to support the most common use cases, that we can include on a dedicated page in the documentation under Building Java with Mill.

We should also try to quantify and state the value prop of using Mill for Spring-Boot projects: are compile times better? is --watch more convenient? Easier integrations with non-JVM languages in the build? I'm not actually sure what concrete comparisons would make the most sense here, but I feel we should be able to find some benefits of using Mill rather than Maven or Gradle