Explain Build.scala in play framework - scala

Question

Can somebody explain the syntax for Build.scala in detail? For example, I have the following Build.scala:
import sbt._
import Keys._
import play.Project._
object Build extends sbt.Build {
val appName = "myapp"
val appVersion = "1.0"
val appDependencies = Seq(
"postgresql" % "postgresql" % "9.1-901-1.jdbc4",
javaCore,
javaJdbc,
javaEbean,
"org.json" %"org.json" % "chargebee-1.0",
"org.reflections" % "reflections" % "0.9.8",
"org.mockito" % "mockito-all" % "1.9.5" % "test"
)
val main = play.Project(appName, appVersion, appDependencies).settings(
libraryDependencies += "com.jolbox" % "bonecp" % "0.8.0-rc2-SNAPSHOT",
resolvers += Resolver.url("sbt-plugin-snapshots", new URL("http://repo.scala-sbt.org/scalasbt/sbt-plugin-snapshots/"))(Resolver.ivyStylePatterns),
resolvers += "Sonatype OSS Snapshots" at "https://oss.sonatype.org/content/repositories/snapshots/"
)
}
How do I match the appDependencies with the resolvers? Or how do I know what resolvers I need to add for what appDependencies? Where do I look in the resolver's repo? What is libraryDependencies? How are they different from appDependencies?
Thanks.

Answer 1

There are a few things we need to disentangle here.
Let's first distinguish between an sbt build file and Build.scala. While the build file is exactly like a Maven pom or a Gradle build file, think of Build.scala as a build capability with the full power and expressiveness of Scala because it is a Scala class like any other. I believe the gap between the two has narrowed though with the latest version of sbt.
Now in both sbt and Build.scala, you have the notion of library dependencies, which are the jar libraries containing code you can use for your projects. These libraries can be found in lots of places--Maven repositories, the local file system, etc. You use resolvers to specify those locations.
So you tell both which jars you need and where to find them.
As for appDependencies, that's not actually a thing. As I said, Build.scala is a class like any other, and appDependencies is just a variable name. It just makes sense to use that name because that Seq is what you will pass to the Project constructor.