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§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 can be fully defined in the parent project’s build file, although here we put sub-projects’ settings in their own 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:

name := "my-first-application"

version := "1.0"

lazy val myFirstApplication = (project in file("."))
    .enablePlugins(PlayScala)
    .aggregate(myLibrary)
    .dependsOn(myLibrary)

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 myFirstApplication project declares the base project. If you don’t have any sub projects, this is already implied, however when declaring sub projects, it’s usually required to declare it so that you can ensure that it aggregates (that is, runs things like compile/test etc on the sub projects when run in the base project) and depends on (that is, adds the sub projects to the main projects classpath) the sub projects.

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
 └ build.sbt
 └ app
 └ conf
 └ public
 └ myLibrary
   └ build.sbt
   └ src
     └ main
       └ java
       └ scala

myLibrary has its own build.sbt file, this is where it can declare its own settings, dependencies etc.

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:

If you want your sub projects and root projects to share some common settings or code, then these can be placed in a Scala file in the project directory of the root project. For example, in project/Common.scala you might have:

import sbt._
import Keys._

object Common {
  val settings: Seq[Setting[_]] = Seq(
    organization := "com.example",
    version := "1.2.3-SNAPSHOT"
  )

  val fooDependency = "com.foo" %% "foo" % "2.4"
}

Then in each of your build.sbt files, you can reference anything declared in the file:

name := "my-sub-module"

Common.settings

libraryDependencies += Common.fooDependency

One thing to note is that if you have a mix of Play and non-Play projects, you may need to share Play configuration explicitly. For example, you may want to share the InjectedRoutesGenerator and specs2 for every Play project:

object Common {

  val playSettings = settings ++ Seq(
    routesGenerator := InjectedRoutesGenerator,
    libraryDependencies += specs2 % Test,
    resolvers += Resolver.ApacheMavenSnapshotsRepo // contains pekko(-http) snapshots
  )
}

And in the sub-project’s build.sbt file, you would have the following:

Common.playSettings

§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. You can make any sub module a Play application by adding the PlayJava or PlayScala plugins, depending on whether your project is a Java or Scala project, in its corresponding build.sbt file.

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. For example, controllers in the admin module should have the fully qualified package name of admin.MyController.

§Splitting the route file

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 configuration

build.sbt:

name := "myproject"

lazy val admin = (project in file("modules/admin")).enablePlugins(PlayScala)

lazy val main = (project in file("."))
    .enablePlugins(PlayScala).dependsOn(admin).aggregate(admin)

modules/admin/build.sbt

name := "myadmin"

libraryDependencies ++= Seq(
  "com.mysql" % "mysql-connector-j" % "8.0.33",
  jdbc,
  anorm
)

§Project structure

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

Note: Configuration and route file names must be unique in the whole project structure. Particularly, there must be only one application.conf file and only one routes file. To define additional routes or configuration in sub-projects, use sub-project-specific names. For instance, the route file in admin is called admin.routes. To use a specific set of settings in development mode for a sub project, it would be even better to put these settings into the build file, e.g. PlayKeys.devSettings += ("play.http.router", "admin.Routes").

conf/routes:

GET /index                  controllers.HomeController.index()

->  /admin admin.Routes

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

modules/admin/conf/admin.routes:

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

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

Note: Resources are served from a unique classloader, and thus resource path must be relative from project classpath root.
Subprojects resources are generated in target/web/public/main/lib/{module-name}, so the resources are accessible from /public/lib/{module-name} when using play.api.Application#resources(uri) method, which is what the Assets.at method does.

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

Java

package controllers.admin;

import controllers.AssetsMetadata;
import javax.inject.Inject;
import play.api.Environment;
import play.api.http.HttpErrorHandler;
import play.api.mvc.*;

public class Assets extends controllers.Assets {

  @Inject
  public Assets(HttpErrorHandler errorHandler, AssetsMetadata meta, Environment env) {
    super(errorHandler, meta, env);
  }

  public Action<AnyContent> at(String path, String file) {
    boolean aggressiveCaching = true;
    return super.at(path, file, aggressiveCaching);
  }
}

Scala

import javax.inject._

import play.api.http.HttpErrorHandler
import play.api.Environment

class Assets @Inject() (
    errorHandler: HttpErrorHandler,
    assetsMetadata: controllers.AssetsMetadata,
    environment: Environment
) extends controllers.AssetsBuilder(errorHandler, assetsMetadata, environment)

And a controller:

Java

/*
 * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
 */

package controllers.admin;

import play.mvc.Controller;
import play.mvc.Result;

public class HomeController extends Controller {
  public Result index() {
    return ok("admin");
  }
}

Scala

package controllers.admin

import javax.inject.Inject

import play.api.mvc._

class HomeController @Inject() (val controllerComponents: ControllerComponents) extends BaseController {
  def index: Action[AnyContent] = Action { implicit request => Ok("admin") }
}

§Reverse routing in `admin`

in case of a regular controller call:

controllers.admin.routes.HomeController.index

and for Assets:

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

§Through the browser

http://localhost:9000/index

triggers

controllers.HomeController.index

and

http://localhost:9000/admin/index