This plugin integrates Google's Closure Compiler. It is used to optimize and compress your Javascript files. Closure Compiler is a well-supported library that can be used even with modern ECMAScript features!


You inject and use the SpincastJsClosureCompilerManager component where you need to optimize Javascript files.

It is highly suggested that you only optimize a given Javascript file one time and then serve a cached version of it after that! The optimization done by the Closure Compiler can indeed take a couple of seconds.

One good way to optimize and then cache a Javascript file is by using a dynamic resource:


In the "dynJs(...)" handler, you would get the raw content of the .js file and optimize it using the SpincastJsClosureCompilerManager component before sending the result:

File jsFile = //... get the .js file

String rawJsContent = FileUtils.readFileToString(jsFile, "UTF-8");

String jsOptimized = getSpincastJsClosureCompilerManager()
                                 "--compilation_level ", "WHITESPACE_ONLY");

context.response().sendCharacters(jsOptimized, "application/javascript");

Explanation :

  • 1 : We get the .js file using the "relativePath" path parameter taken from the request. We make sure we validate this user input properly!
  • 3 : We get the raw Javascript content.
  • 5-6 : We call the compile(...) method.
  • 7 : In this example, we specify one option (using two arguments). You will learn more about options in the next section
  • 9 : We send the optimized Javascript, using the proper content-type. Since this is a dynamic resource route, the result will be cached and it is this cached version that will be served on next requests!

Flags and Options

Read the official Flags and Options documentation of Closure Compiler to learn how to tweak the optimization of your Javascript files.

There are two ways you can specify an option such as "--compilation_level" with a value of "WHITESPACE_ONLY":

1. Using two separate arguments

String jsOptimized = 
        .compile(jsContent, "--compilation_level", "WHITESPACE_ONLY");

2. Or using one argument only with a "=" between the name and the value

String jsOptimized = 
        .compile(jsContent, "--compilation_level=WHITESPACE_ONLY");

Some options take only one argument without any value at all (ex: --strict_mode_input).

You cannot use the "--js_output_file" option! This option is managed by the plugin. If you specify it, your version will be ignored.

Note that, when bundling Javascript files for Spincast itself, we had to use "--language_out ECMASCRIPT_2017" or errors were generated! You may try this option if you get errors.

SpincastJsClosureCompilerManager methods
The main methods provided by SpincastJsClosureCompilerManager are:
  • String compile(String jsContent, String... args))

    Run the Closure Compiler on the specified Javascript content, as a String. Return the optimized Javascript.

    The command generated by this method will be equivalent to:

    java -jar closure-compiler.jar --js_output_file=internalSpincastFile fileCreatedFromYourJsContent.js [YOUR ARGS]

  • String compile(File jsFile, String... args))

    Run the Closure Compiler on the content of the specified .js file. Return the optimized Javascript.

    The command generated by this method will be equivalent to:

    java -jar closure-compiler.jar --js_output_file=internalSpincastFile yourFile.js [YOUR ARGS]

  • String compile(List<File> jsFiles, String... args))

    Run the Closure Compiler on the concatenated content of multiple .js files. Return the concatenated and optimized Javascript.

    You can read more about using multiple input files here.

    The command generated by this method will be equivalent to:

    java -jar closure-compiler.jar --js_output_file=internalSpincastFile file1.js file2.js file3.js [YOUR ARGS]

  • String compileCustom(String... args))

    Run the Closure Compiler using only custom arguments. Return the optimized Javascript.

    When you use this method, you are responsible to specify all the arguments that are going to be passed to the Closure Compiler (except, again, "--js_output_file"). This means you are responsible to specify the input files as arguments too!

    To specify the input files as arguments, you can use globs patterns if you want.

    The command generated by this method will be equivalent to:

    java -jar closure-compiler.jar --js_output_file=internalSpincastFile [YOUR ARGS]

Pebble function to easily bundle multiple Javascript files
If you are using Pebble, the default Templating Engine, a function is provided by this plugin to bundle multiple Javascript files together directly from an HTML template.

Let's say your HTML template includes those .js files:

<script src="/public/js/jquery.js"></script>
<script src="/public/js/anotherLibrary.js"></script>
<script src="/publicdyn/js/{{spincast.cacheBuster}}main.js"></script>

You could convert those separate files to a single compressed and optimized bundle, so an unique and fast request is required from the client!

To do so, you use the jsBundle(...) Pebble function which is provided when this plugin is installed:

{{ jsBundle('/public/js/jquery.js',
            '/publicdyn/js/main.js') }}

This function will concatenate all the specified Javascript files, will optimize them using the Closure Compiler and will output a resulting "<script>" element pointing to the bundle:

<script src="/spincast/plugins/jsclosurecompiler/jsbundles/resultingBundle.js"></script>

The parameters accepted by the jsBundle(...) function are:

  • First, the paths to the Javascript files to bundle. Those paths must be absolute (they must start with a "/").
  • Then, the options to pass to the Closure Compiler. Those must be specified after the paths to the Javascript files. When an argument starting with "-" is found, this argument and all the following ones are considered as options for the Closure Compiler!

  • There is a single option specific to Spincast that you can use: "--spincast-no-cache-busting". When this flag is specified, no cache busting code will be added to the resulting bundle path.

Here is an example of using jsBundle(...) with options:

{{ jsBundle('/public/js/jquery.js',
            '--compilation_level', 'WHITESPACE_ONLY', 
            '--formatting', 'PRETTY_PRINT',
            '--spincast-no-cache-busting') }}

Behind the scene, calling jsBundle(...) will not only generate the bundle, but will also automatically add the associated route to your router!

The resulting bundle will be cached (it will only be generated on the very first request), will be served with caching HTTP headers and will contain a cache busting code (except if the "--spincast-no-cache-busting" flag is used).

During development, you probably want to disable the bundling so changes made to the JS files are available immediately (no cache). There is a configuration for that!

Don't forget to remove any "{{spincast.cacheBuster}}" tag when you move a regular .js file to a jsBundle(...) function!

Note that the content of the Javascript files to bundle together will be retrieved using HTTP requests! This allows you to use dynamic resources to generate this content, if required.

Bundling multiple files may take a couple of seconds... If you want to prevent the first request made to this bundle to be slow, you can pre-generate the bundle when your application starts. Here is an example where we do this, for this very website.

If, for some reason, you want to change the output made by jsBundle(...), you can extend SpincastJsClosureCompilerPebbleExtensionDefault, override the "bundlingOutput(String path)" function and bind your implementation to the SpincastJsClosureCompilerPebbleExtension interface in the Guice context.


You can bind a custom implementation of SpincastJsClosureCompilerConfig to tweak global configurations. This implementation can extend SpincastJsClosureCompilerConfigDefault as a base class.

  • String getJsBundlePebbleFunctionName()

    The name to give to the Pebble function provided by this plugin to bundle multiple Javascript files together. It is the name you are going to use in your HTML file to call the function.

    Defaults to "jsBundle".

  • String getJsBundlesUrlPath()

    The base URL path where the Javascript bundles will be served from.

    Defaults to "/spincast/plugins/jsclosurecompiler/jsbundles".

  • File getJsBundlesDir()

    The directory where the jsBundle() Pebble function will save the generated Javascript bundles.

    Defaults to "[WRITABLE_DIR]/spincast/plugins/jsclosurecompiler/jsBundles".

  • boolean isJsBundlesIgnoreSslCertificateErrors()

    When the jsBundle() Pebble function retrieves the files to be bundled together, should SSL certificate errors be ignored? Setting this to true is useful during testing, on a development environment where a self-signed certificate may be used.

    Defaults to true if isDevelopmentMode() or isTestingMode() is true .

  • boolean isJsBundlesDisabled()

    When this is true, the jsBundle() Pebble function will not bundle the specified files together. The files will be served as usual, each with their own "<script>" tag.

    This is useful during development, when you don't want any cache and want changes to the files to be immediately available!

    Defaults to true if isDevelopmentMode() is true .


This plugin depends on the Spincast HTTP Client plugin which is not provided by default by the spincast-default artifact.

It also uses Spincast Pebble plugin which is included in the spincast-default artifact.

Those plugins will be automatically installed. Note that it is always a good idea to read the documentation of the automatically installed plugins though!


1. Add this Maven artifact to your project:


2. Add an instance of the SpincastJsClosureCompilerPlugin plugin to your Spincast Bootstrapper:

        .plugin(new SpincastJsClosureCompilerPlugin())
        // ...