§The Build System
The Play build system uses sbt, a high-performance integrated build for Scala and Java projects. Using sbt as our build tool brings certain requirements to play which are explained on this page.
§Understanding sbt
sbt functions quite differently to the way many traditional build tasks. Fundamentally, sbt is a task engine. Your build is represented as a tree of task dependencies that need to be executed, for example, the compile task depends on the sources task, which depends on the sourceDirectories task and the sourceGenerators task, and so on.
sbt breaks typical build executions up into very fine grained tasks, and any task at any point in the tree can be arbitrarily redefined in your build. This makes sbt very powerful, but also requires a shift in thinking if you’ve come from other build tools that break your build up into very coarsely grained tasks.
The documentation here describes Play’s usage of sbt at a very high level. As you start to use sbt more in your project, it is recommended that you follow the sbt tutorial to get an understanding for how sbt fits together. Another resource that many people have found useful is this series of blog posts.
§Play application directory structure
Most people get started with Play using the activator new command which produces a directory structure like this:
/: The root folder of your application/README: A text file describing your application that will get deployed with it./app: Where your application code will be stored./build.sbt: The sbt settings that describe building your application./conf: Configuration files for your application/project: Further build description information/public: Where static, public assets for your application are stored./test: Where your application’s test code will be stored.
For now, we are going to concern ourselves with the /build.sbt file and the /project directory.
§The /build.sbt file.
When you use the activator new foo command, the build description file, /build.sbt, will be generated like this:
name := "foo"
version := "1.0-SNAPSHOT"
libraryDependencies ++= Seq(
jdbc,
anorm,
cache
)
lazy val root = (project in file(".")).enablePlugins(PlayScala)The name line defines the name of your application and it will be the same as the name of your application’s root directory, /, which is derived from the argument that you gave to the activator new command.
The version line provides the version of your application which is used as part of the name for the artifacts your build will produce.
The libraryDependencies line specifies the libraries that your application depends on. More on this below.
You should use the PlayJava or PlayScala plugin to configure sbt for Java or Scala respectively.
§Using scala for building
Activator is also able to construct the build requirements from scala files inside your project’s project folder. The recommended practice is to use build.sbt but there are times when using scala directly is required. If you find yourself, perhaps because you’re migrating an older project, then here are a few useful imports:
import sbt._
import Keys._
import play.Play.autoImport._
import PlayKeys._
The line indicating autoImport is the correct means of importing an sbt plugin’s automatically declared properties. Along the same lines, if you’re importing an sbt-web plugin then you might well:
import com.typesafe.sbt.less.autoImport._
import LessKeys._
§The /project directory
Everything related to building your project is kept in the /project directory underneath your application directory. This is an sbt requirement. Inside that directory, there are two files:
/project/build.properties: This is a marker file that declares the sbt version used./project/plugins.sbt: SBT plugins used by the project build including Play itself.
§Play plugin for sbt (/project/plugins.sbt)
The Play console and all of its development features like live reloading are implemented via an sbt plugin. It is registered in the /project/plugins.sbt file:
addSbtPlugin("com.typesafe.play" % "sbt-plugin" % playVersion) // where version is the current Play version, i.e. "2.4.10"
Note that
build.propertiesandplugins.sbtmust be manually updated when you are changing the play version.
Next: About sbt settings