Integrating Play Framework and Maven

I recently had to integrate a Play Framework project with a Maven-based build process. This wasn’t as straight-forward as I thought it would be, and so here’s how I did it.

TL;DR

Check out this repo for a complete and working example.

Requirements

The integration I was after had the following requirements:

Multi-module project (separate the play “web app” module and the “core” module)
All configuration should be via pom files, especially dependencies
Allow working with Maven’s Dependency Management which centralizes dependency information in parent poms
Allow building/testing/running thru $ mvn
Allow building/testing/running thru $ play
Access all the special play commands thru maven

Project Folder Structure

my-application-core/
 └ pom.xml
 └ src/
my-application-play/
 └ pom.xml
 └ app/
 └ conf/
 └ public/
 └ test/
pom.xml
project/
 └ Build.scala
 └ Pom.scala
 └ build.properties
 └ plugins.sbt

Maven

In order to compile scala and play in maven, we need to use three different plugins:

net.alchim31.maven.scala-maven-plugin for compiling scala in the core module
com.google.code.play2-maven-plugin for compiling/running the play module
org.apache.maven.plugins.maven-surefire-plugin for running tests in both modules

Pom file examples:

Using the play2-maven-plugin allows you to run play commands such as run, start, dist, etc. in maven by running:

$ mvn play2:dist -pl my-application-play

Play

The play command is essentially just a wrapper around sbt. We configure sbt for multiple project, with the play project set as the root.

// Build.scala

import sbt._
import sbt.Keys._

object MyBuild extends Build {

  val core = Project("my-application-core", file("my-application-core"))
    .settings(
      version := Pom.version(baseDirectory.value),
      libraryDependencies ++= Pom.dependencies(baseDirectory.value))

  val root = play.Project("my-application-play", path = file("my-application-play"))
    .dependsOn(core)
    .settings(
      version := Pom.version(baseDirectory.value),
      libraryDependencies ++= Pom.dependencies(baseDirectory.value).filterNot(d => d.name == core.id))

  override def rootProject = Some(root)
}

There are a few interesting things here. First, notice that the version and dependencies are taken from the pom file by using the Pom object (see below). Second, you’ll see that after adding the dependencies to the play project, the core project is filtered out. That’s because the play project is already configured with .dependsOn(core) and so the dependency defined in the pom file isn’t required (it also won’t work otherwise because the core module can’t be found).

// Pom.scala

import sbt._
import scala.xml.XML
import scala.language.postfixOps

object Pom {
  private val prefix = "[INFO]    "

  def version(pomDir: File): String = {
    val pom = new File(pomDir, "pom.xml")
    val xml = XML.loadFile(pom)
    val version = (xml \\ "project" \ "version").text
    version
  }

  def dependencies(pomDir: File): Seq[ModuleID] = {
    val list = (Process("mvn dependency:list", pomDir) !!)
    val lines = list.split("\n").collect {
      case line if (line.startsWith(prefix)) => line.replace(prefix, "").trim
    }
    lines.map(parse).collect { case Some(dep) => dep }
  }

  private def parse(line: String): Option[ModuleID] = {
    val parts = line.split(":")

    if (parts.size < 4) None
    else {
      val groupId = parts(0)
      val artifactId = parts(1)
      val revision = parts(parts.size - 2)
      val configuration = parts.last
      val dependency = (groupId % artifactId % revision % configuration) intransitive()

      if (parts.size > 5) {
        val cls = parts(parts.size - 3)
        Some(dependency classifier cls)
      } else Some(dependency)
    }
  }
}

This is where the most important thing happens – calculating the dependencies. Remember that one of the requirements is to have all dependencies defined in maven pom files. They can also be defined in parent pom file we’re inheriting. Its parent (our grandparent) pom can also have dependencies defined or have the version of our dependencies defined (Dependency Management). This means that simply parsing the pom.xml file and adding the depdencies won’t work – we need maven to resolve the dependencies using it’s entire pom hierarchy.

This is exactly what we do – we run a process with mvn dependency:list, which evaluates all our dependencies and the ones we inherit from our parent. After getting the list, it’s parsed line-by-line, and each is converted to an sbt ModuleID.

Every time the play (sbt) project is loaded/reloaded, the dependency list command will run and the dependencies will be added to the project. This means that reloading the project can take a little longer, depending on the number of dependencies you have.

Now we can add new dependencies in any of our pom files, run $ play> reload, and have it available in play.

Bottom Line

Structuring your project this way allows you to enjoy both worlds – having a maven-based build, with access to all the play special commands, and using play as stand-alone tool for development/debugging/testing.

Check out the complete example to see how it all fits together.