§Working with public assets

This section covers serving your application’s static resources such as JavaScript, CSS and images.

Serving a public resource in Play is the same as serving any other HTTP request. It uses the same routing as regular resources: using the controller/action path to distribute CSS, JavaScript or image files to the client.

§The public/ folder

By convention, public assets are stored in the public folder of your application. This folder is organized as follows:

public
 └ javascripts
 └ stylesheets
 └ images

If you follow this structure it will be simpler to get started, but nothing stops you to modifying it once you understand how it works.

§How are public assets packaged?

During the build process, the contents of the public folder are processed and added to the application classpath. When you package your application, these files are packaged into the application JAR file (under the public/ path).

§The Assets controller

Play comes with a built-in controller to serve public assets. By default, this controller provides caching, ETag, gzip compression and JavaScript minification support.

The controller is available in the default Play JAR as controllers.Assets, and defines a single at action with two parameters:

Assets.at(path: String, file: String)

The path parameter must be fixed and defines the directory managed by the action. The file parameter is usually dynamically extracted from the request path.

Here is the typical mapping of the Assets controller in your conf/routes file:

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

Note that we define the *file dynamic part that will match the .* regular expression. So for example, if you send this request to the server:

GET /assets/javascripts/jquery.js

The router will invoke the Assets.at action with the following parameters:

controllers.Assets.at("/public", "javascripts/jquery.js")

This action will look-up the file and serve it, if it exists.

Note, if you define asset mappings outside “public,” you’ll need to tell
sbt about it, e.g. if you want:

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

you should add this to the project settings in build.sbt:

playAssetsDirectories <+= baseDirectory / "foo"

§Reverse routing for public assets

As for any controller mapped in the routes file, a reverse controller is created in controllers.routes.Assets. You use this to reverse the URL needed to fetch a public resource. For example, from a template:

<script src="@routes.Assets.at("javascripts/jquery.js")"></script>

This will produce the following result:

<script src="/assets/javascripts/jquery.js"></script>

Note that we don’t specify the first folder parameter when we reverse the route. This is because our routes file defines a single mapping for the Assets.at action, where the folder parameter is fixed. So it doesn’t need to be specified explicitly.

However, if you define two mappings for the Assets.at action, like this:

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

Then you will need to specify both parameters when using the reverse router:

<script src="@routes.Assets.at("/public/javascripts", "jquery.js")"></script>
<image src="@routes.Assets.at("/public/images", "logo.png")">

§Etag support

The Assets controller automatically manages ETag HTTP Headers. The ETag value is generated from the resource name and the file’s last modification date. (If the resource file is embedded into a file, the JAR file’s last modification date is used.)

When a web browser makes a request specifying this Etag, the server can respond with 304 NotModified.

§Gzip support

But if a resource with the same name but using a .gz suffix is found, the Assets controller will serve this one by adding the proper HTTP header:

Content-Encoding: gzip

§Additional Cache-Control directive

Usually, using Etag is enough to have proper caching. However if you want to specify a custom Cache-Control header for a particular resource, you can specify it in your application.conf file. For example:

# Assets configuration
# ~~~~~
"assets.cache./public/stylesheets/bootstrap.min.css"="max-age=3600"

§Managed assets

By default Play compiles all managed assets that are kept in the app/assets folder. The compilation process will clean and recompile all managed assets regardless of the change. This is the safest strategy since tracking dependencies can be very tricky with front end technologies.

You will learn more about managed assets on the next few pages.

Next: Using CoffeeScript