learning some sbt fu by build the bfg tool excerpt: sbt 1 2 3

I want to build the bfg tool

In my previous blog I talked about bfg the bfg-repo-cleaner. It is a fast repo maintain tool written in scala. The developer didn't provide any prebuilt binary downloads.

So, clone and built by myself.

SBT do not work well the maven way

After some simple git clone. Backed with some maven knowledge and knowing that sbt is kind of same tool.

I thought like a fool that a sdk install sbt followed by sbt build command will built a beautiful jar in target directory as maven package do.

Sbt version mismatch

Quickly, I learn the first lesson from sbt: sbt will not build the old project with latest sbt version, it will always try to build the project with declared version. If the version is not available locally, SBT will try to download it until successes with the downloading.

1. sbt will download with project declared sbt version if not already installed.
2. sbt will download the project declared scala version if not already installed.
3. sbt is big! And the download is realy slow.
4. scala is big, too, alas!!!

Sbt likes REPL

After several bindly sbt foobar runs without looking into the README a single sight. I learned my second lesson from bfg project: project README and BUILD document is important!!!

Just as maven works, the mode java runs, fire and wait for the result. sbt works the way scala runs, start a REPL and interact with it.

The power of scala runtime

The third lesson I learn from sbt is that: a pure sbt built jar will depend on scala runtime, to make an Uber jar one must use sbt assembly plugin.

With assembly plugin applied, the KB sized pure jar will quickly grow to 1x MB sized fat jar with power of scala runtime embedded. Thereafter, a separated installation of scala is not necessary anymore.