§Working with sub-projects

A complex project is not necessarily composed of a single Play application. You may want to split a large project into several smaller applications, or even extract some logic into a standard Java or Scala library that has nothing to do with a Play application.

It will be helpful to read the SBT documentation on multi-project builds. Sub-projects do not have their own build file, but share the parent project’s build file.

§Adding a simple library sub-project

You can make your application depend on a simple library project. Just add another sbt project definition in your build.sbt file:

import play.Project._

import play.Project._

name := "my-first-application"

version := "1.0"

playScalaSettings

lazy val myLibrary = project

The lowercased project on the last line is a Scala Macro which will use the name of the val it is being assigned to in order to determine the project’s name and folder.

The above example defines a sub-project in the application’s myLibrary folder. This sub-project is a standard sbt project, using the default layout:

myProject
 └ app
 └ conf
 └ public
myLibrary
 └ src
    └ main
       └ java
       └ scala
project
 └ Build.scala

When you have a sub-project enabled in your build, you can focus on this project and compile, test or run it individually. Just use the projects command in the Play console prompt to display all projects:

[my-first-application] $ projects
[info] In file:/Volumes/Data/gbo/myFirstApp/
[info] 	 * my-first-application
[info] 	   my-library

The default project is the one whose variable name comes first alphabetically. You may make your main project by making its variable name aaaMain. To change the current project use the project command:

[my-first-application] $ project my-library
[info] Set current project to my-library
>

When you run your Play application in dev mode, the dependent projects are automatically recompiled, and if something cannot compile you will see the result in your browser:

§Splitting your web application into several parts

As a Play application is just a standard sbt project with a default configuration, it can depend on another Play application.

The following example uses a build.scala file to declare a play.Project. This approach was the way Play applications were defined prior to version 2.2. The approach is retained in order to support backward compatibility. We recommend that you convert to the build.sbt based approach or, if using a build.scala, you use sbt’s Project type and project macro.

Configure your sub-project as a play.Project:

import sbt._
import Keys._
import play.Project._

object ApplicationBuild extends Build {

  val appName = "zenexity.com"
  val appVersion = "1.2"

  val common = play.Project(
    appName + "-common", appVersion, path = file("common")
  )
  
  val website = play.Project(
    appName + "-website", appVersion, path = file("website")
  ).dependsOn(common)
  
  val adminArea = play.Project(
    appName + "-admin", appVersion, path = file("admin")
  ).dependsOn(common)
  
  val main = play.Project(
    appName, appVersion, path = file("main")
  ).dependsOn(
    website, adminArea
  )
}

Here we define a complete project split in two main parts: the website and the admin area. Moreover these two parts depend themselves on a common module.

If you would like the dependent projects to be recompiled and tested when you recompile and test the main project then you will need to add an “aggregate” clause.

val main = play.Project(
  appName, appVersion, appDependencies
).dependsOn(
  website, adminArea
).aggregate(
  website, adminArea
)

Note: in order to avoid naming collision, make sure your controllers, including the Assets controller in your subprojects are using a different name space than the main project

§Splitting the route file

As of play 2.1 it’s also possible to split the route file into smaller pieces. This is a very handy feature if you want to create a robust, reusable multi-module play application

§Consider the following build file

project/Build.scala:

import sbt._
import Keys._
import play.Project._

object ApplicationBuild extends Build {

    val appName         = "myproject"
    val appVersion      = "1.0-SNAPSHOT"

    val adminDeps = Seq(
      // Add your project dependencies here,
       "mysql" % "mysql-connector-java" % "5.1.18",
      jdbc,
      anorm
    )

    val mainDeps = Seq()
  
    lazy val admin = play.Project(appName + "-admin", appVersion, adminDeps, path = file("modules/admin"))


    lazy  val main = play.Project(appName, appVersion, mainDeps).settings(
      // Add your own project settings here      
    ).dependsOn(admin).aggregate(admin)

}

§project structure

app
  └ controllers
  └ models
  └ views
conf
  └ application.conf
  └ routes
modules
  └ admin
    └ conf/admin.routes
    └ app/controllers
    └ app/models
    └ app/views     
project
 └ build.properties
 └ Build.scala
 └ plugins.sbt

Note: there is only a single instance of application.conf. Also, the route file in admin is called admin.routes

conf/routes:

GET /index                  controllers.Application.index()

->  /admin admin.Routes

GET     /assets/*file       controllers.Assets.at(path="/public", file)

modules/admin/conf/admin.routes:

GET /index                  controllers.admin.Application.index()

GET /assets/*file           controllers.admin.Assets.at(path="/public", file)

§Assets and controller classes should be all defined in the controllers.admin package

modules/admin/controllers/Assets.scala:

package controllers.admin
object Assets extends controllers.AssetsBuilder

Note: Java users can do something very similar i.e.

// Assets.java
package controllers.admin;
import play.api.mvc.*;

public class Assets {
  public static Action<AnyContent> at(String path, String file) {
    return controllers.Assets.at(path, file);
  }
}

and a controller:

modules/admin/controllers/Application.scala:

package controllers.admin

import play.api._
import play.api.mvc._
import views.html._

object Application extends Controller {

  def index = Action { implicit request =>
    Ok("admin")
  }
}

§Reverse routing in admin

in case of a regular controller call:

controllers.admin.routes.Application.index

and for Assets:

controllers.admin.routes.Assets.at("...")

§Through the browser

http://localhost:9000/index

triggers

controllers.Application.index

and

http://localhost:9000/admin/index

triggers

controllers.admin.Application.index