§Working with public assets
§The public/ folder
By convention public assets are stored in the
public folder of your application. This folder can be organized the way that you prefer. We recommend the following organization:
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.
WebJars provide a convenient and conventional packaging mechanism that is a part of Activator and sbt. For example you can declare that you will be using the popular Bootstrap library simply by adding the following dependency in your build file:
libraryDependencies += "org.webjars" % "bootstrap" % "3.3.4"
WebJars are automatically extracted into a
lib folder relative to your public assets for convenience. For example, if you declared a dependency on RequireJs then you can reference it from a view using a line like:
lib/requirejs/require.js path. The
lib folder denotes the extract WebJar assets, the
requirejs folder corresponds to the WebJar artifactId, and the
require.js refers to the required asset at the root of the WebJar.
§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, all assets for the application, including all sub projects, are aggregated into a single jar, in
target/my-first-app-1.0.0-assets.jar. This jar is included in the distribution so that your Play application can serve them. This jar can also be used to deploy the assets to a CDN or reverse proxy.
§The Assets controller
Play comes with a built-in controller to serve public assets. By default, this controller provides caching, ETag, gzip and compression 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)
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
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:
The router will invoke the
Assets.at action with the following parameters:
This action will look-up and serve the file and if it exists.
§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:
This will produce the following result:
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.
However, if you define two mappings for the
Assets.at action, like this:
GET /images/*file controllers.Assets.at(path="/public/images", file)
You will then need to specify both parameters when using the reverse router:
<img src="@routes.Assets.at("/public/images", "logo.png")" />
§Reverse routing and fingerprinting for public assets
sbt-web brings the notion of a highly configurable asset pipeline to Play e.g. in your build file:
pipelineStages := Seq(rjs, digest, gzip)
The above will order the RequireJs optimizer (
sbt-rjs), the digester (
sbt-digest) and then compression (
sbt-gzip). Unlike many sbt tasks, these tasks will execute in the order declared, one after the other.
In essence asset fingerprinting permits your static assets to be served with aggressive caching instructions to a browser. This will result in an improved experience for your users given that subsequent visits to your site will result in less assets requiring to be downloaded. Rails also describes the benefits of asset fingerprinting.
The above declaration of
pipelineStages and the requisite
addSbtPlugin declarations in your
plugins.sbt for the plugins you require are your start point. You must then declare to Play what assets are to be versioned. The following routes file entry declares that all assets are to be versioned:
GET /assets/*file controllers.Assets.versioned(path="/public", file: Asset)
Make sure you indicate that
fileis an asset by writing
You then use the reverse router, for example within a
<link rel="stylesheet" href="@routes.Assets.versioned("assets/css/app.css")">
We highly encourage the use of asset fingerprinting.
Assets controller automatically manages ETag HTTP Headers. The ETag value is generated from the digest (if
sbt-digest is being used in the asset pipeline) or otherwise 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 then the server can respond with 304 NotModified.
If a resource with the same name but using a
.gz suffix is found then the
Assets controller will also serve the latter and add the following HTTP header:
sbt-gzip plugin in your build and declaring its position in the
pipelineStages is all that is required to generate gzip files.
Using Etag is usually enough for the purposes of 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
Many plugins use sbt-web’s js-engine plugin. js-engine is able to execute plugins written to the Node API either within the JVM via the excellent Trireme project, or directly on Node.js for superior performance. Note that these tools are used during the development cycle only and have no involvement during the runtime execution of your Play application. If you have Node.js installed then you are encouraged to declare the following environment variable. For Unix, if
SBT_OPTS has been defined elsewhere then you can:
export SBT_OPTS="$SBT_OPTS -Dsbt.jse.engineType=Node"
The above declaration ensures that Node.js is used when executing any sbt-web plugin.
Next: Using CoffeeScript
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