Skin Extension Tutorial

Last modified by Vincent Massol on 2023/10/10

This tutorial demonstrates how to write a XWiki Skin Extension.


Introduction to XWiki Skin Extensions

XWiki Skins extensions (abbreviated as SX) is a mechanism available in XWiki that allows to customize the layout of your wiki, or just some pages of your wiki, without the need of changing its skin templates and/or stylesheets. For this, the Skin Extension plugin (bundled in all XWiki versions superior to 1.5) provides the ability to send to the browser extra JavaScript and CSS files that are not part of the actual skin of the wiki. The code for these extensions is defined in wiki objects.

To illustrate usage of Skin Extension in XWiki, this tutorial will guide you through the creation of minimal JavaScript and StyleSheet working extensions. Then, will push it further to build a fully functional extension based on Natalie Downe's addSizes.js script.

A minimal JavaScript and CSS knowledge is also needed to take full advantage of XWiki Skin Extensions, although expert knowledge in those fields is not needed to follow the tutorial.

If you are interested by the Skin extension mechanism itself and its internals, you should read its plugin page, and its design page on This tutorial does not address this topic. Or, since this is an Open Source project, feel free to browse the code, and propose enhancements or improvements.

My first Skin extension

Skin extensions are defined as XWiki Objects. As a consequence, you can create them from your browser.

Two types of extensions are currently supported: 

  • JavaScript extensions (incarnated by XWiki objects of class XWiki.JavaScriptExtension), also known as JSX
  • StyleSheet extensions (incarnated by XWiki objects of class XWiki.StyleSheetExtension), also known as SSX

The very first step to create an extension is then... to create its object!

Minimal JavaScript extension

Creating an extension object

Point your wiki on the page you want to create your extension in, and edit it with the object editor. The page itself can be any XWiki page - an existing page or a new page. I use in this example the page XWiki.SkinExt. From the New Object drop-down list of the object editor choose XWiki.JavaScriptExtension. Then, click the "Add" button. 


Once the page is loaded, you should see your extension object in the object list.

Writing the extension

Now that the object is available, we can just start writing the actual extension. For this, we will fill in all the fields of the created object. The first one is the extension name. This is easy! We can just write here Hello World (this information is only descriptive, it is not actually used by the SX plugin). The next field name is code, and this is where we will write the javascript code we want our extension to execute. This extension is supposed to be minimalist, so let's write something very basic here: a traditional greeting alert 

alert("Hello World!");

Now the next field asks us if we want this extension to be used Always or On Demand. We will explore all the differences between those two modes later in the tutorial, let us for now just precise we want it On Demand, which will force us to call the extension explicitly to see it executed. 

Next thing our extension wants to know is if we want its content being parsed or not. This option allows to write velocity code, for example to dynamically generate the javascript code to be executed. We did not write any velocity, so we can just say No. We will see later on an example of an extension with parsed content.

Finally, we can precise a caching policy, to tune the HTTP headers that will be returned with the generated javascript file. Let's not go wild, and chose the default one here 

That's it ! our extension is production-ready ! It should by now look like the following:


Note: the "code" area size has been intentionally reduced for this screenshot.

Testing the actual extension

Let's now test the whole thing! Remember we chose that our extension should be used on demand ? Well, that's what we are going to do right now. For this we will make a call to the Skin Extension plugin. We can do it for instance in the wiki content of our extension page, or any other page. For this, we edit the target page in Wiki mode, and write the following:

#set ($discard = $xwiki.jsx.use('XWiki.SkinExt.WebHome'))

Of course, if you did not use this page name for your extension, you should adapt it. Click "Save and View", et voila! If everything is fine, you should see the magic:


You may have noticed that the javascript alert displays before the document is fully loaded in the browser. This is actually expected! If you look close at the generated sources, you will see that your extension has actually been added in the HTML header as any other .js files from the skin: (comments added for this tutorial)

<script type="text/javascript" src="/xwiki/skins/albatross/skin.js"></script>
<!-- [SNIP] here are all others javascript files from the skin -->
<script type="text/javascript" src="/xwiki/bin/skin/skins/albatross/scripts/shortcuts.js"></script>

<!-- And here is your JSX ! You can open its URL in a browser and see the code -->
<script type='text/javascript' src='/xwiki/bin/jsx/XWiki/SkinExt?lang=en'></script>

You may also note, that the browser is delivered a minified version of the script given in the object's text. This is good practice for the memory of the browser but may make it hard to debug. Using the extra parameter debug=true is a way to prevent it as explained in the Debugging Page.

Minimal StyleSheet extension

Good, we wrote our first javascript extension. But, we see things big and we already are looking forward to modify the graphical appearance of wiki pages using those extensions. That's what StyleSheet extensions are meant for. And the good news is that it just works the same as javascript extensions, the only difference being that the code written is CSS code

Create a new page named XWiki.MyFirstStylesheetExtension. From the New Object drop-down list of the object editor choose XWiki.StyleSheetExtension. Then, click the "Add" button. We will name it Blue Background, give it a default cache policy, ask it not to parse the content, and write the following code:

#xwikicontent {
  background-color: lightBlue;

If you want to use the colors of your active ColorTheme you can check how to call those variables.

Put all together

Now let's try something new with this extension. Instead of loading it "On Demand", we can ask to have it used "Always on this wiki". For this to happen however, you need to save the extension document with programming rights.

Your StyleSheet extension should now look like the following:


Note: the "code" area size has been intentionally reduced for this Screenshot.

It's time to test it. No need to call the SkinExtension plugin this time, this is the power of Use Always extensions, just click "Save and View" and see the SSX Magic. You can browse your wiki, all pages will be affected by your extension, for example the Main.WebHome page:


Note: if you want to use StyleSheet extension on demand, the principle is the same as for javascript, except that the plugin name is ssx, not jsx. Just make your call like this, and you are done:

#set ($discard = $xwiki.ssx.use('XWiki.MyFirstStylesheetExtension.WebHome'))

A document can have as many ssx or jsx object as it needs, but a skin extension is identified by the name of the document, so in the end an extension is a document. The content of a skin extension is the concatenation of the objects in that document, so it's impossible to write two different extensions in a single document, only different parts of the same extension.

Real-world extension with addSizes.js

This tutorial is kept here for documentation purpose but beware it uses an external JSON-P API service previously hosted on Google App Engine but not available anymore.

Let's now go further with this idea, and build a complete extension that will dynamically add the file type and size next to certain links that are present in a wiki document. This extension will make usage of the addSizes.js script published by Natalie Downe. This Javascript snippet itself relies on json-head, a Google App Engine application by Simon Willison which "provides a JSON-P API for running HEAD requests against an arbitrary URL". addSizes.js consumes this service to dynamically add the file type and size next to links in HTML documents. And this is what we will do in our new extension, using the aforementioned script and service.

Our new skin extension will be composed of a javascript and a stylesheet extension. We will hold the two objects in the same wiki page, namely XWiki.AddSizesExtension.

Once the document is loaded the javascript extension will be in charge of finding all the interesting links we want to decorate with sizes and file type icons, actually query for their size on the cloud, and finally inject this information next to each concerned link.

The stylesheet extension will just define the style we want for the extra size information that is injected next to the links.

The implementation below looks for the following file formats:

  • Writer, Calc, and Impress (.odt, .ods, .odp)
  • The most well known proprietary equivalents of the formats above
  • Zip archives (.zip)
  • PDFs (.pdf)

Of course, this can be adapted to look for other formats that are relevant for your business 

Requesting and injecting files size with JSX

Our javascript extension will be composed of two code snippets. The second one will be the actual addSizes.js code, ported to work with Prototype instead of jQuery. The first one is a function needed by this portage.

AddSizes.js relies on the JSON with padding technique to query the json-head service, which is located on a different domain than the wiki, in a transparent manner. An alternative to this would be to have a similar service on the wiki itself (for example, in the groovy language), and query it using a traditional AJAX request. Prototype.js, the javascript framework bundled with XWiki, does not yet provide support for JSON-P requests. To do JSON-P requests, we will use a code snippet by Yuriy Zaytsev written for this purpose. Let's first paste his code in a new JSX object, in XWiki.AddSizesExtension :

// getJSON function by Juriy Zaytsev
var id = 0, head = $$('head')[0], global = this;
global.getJSON = function(url, callback) {
var script = document.createElement('script'), token = '__jsonp' + id;
// callback should be a global function
global[token] = callback;
// url should have "?" parameter which is to be replaced with a global callback name
script.src = url.replace(/\?(&|$)/, '__jsonp' + id + '$1');
// clean up on load: remove script tag, null script variable and delete global callback function
script.onload = function() {
script = null;
delete global[token];
// callback name should be unique

With this we can now have a prototype version of addSizes.js. We can just paste this second snippet under the first one in the code area of our extension object, or add a new JavaScriptExtension object to the page (as SX combines all the objects of the same page into a single response):

// addSizes was written by Natalie Downe
// ported to prototype.js by Jerome Velociter, and adapted to XWiki for this tutorial
// Copyright (c) 2008, Natalie Downe under the BSD license

Event.observe(window, 'load', function(event) {
 'a[href$=".pdf"], a[href$=".doc"], a[href$=".zip"], a[href$=".xls"], a[href$=".odt"], a[href$=".ods"], a[href$=".odp"], a[href$=".ppt"]')
   var bits = link.href.split('.');
   var type = bits[bits.length -1];

   var url = ""+encodeURIComponent (link.href)+"&callback=?";

   getJSON(url, function(json){
     var content_length = json.headers['Content-Length'];
     if(!content_length) {
       content_length = json.headers['content-length'];
     if(json.ok && content_length) {
       var length = parseInt(content_length, 10);
       // divide the length into its largest unit
       var units = [
          [1024 * 1024 * 1024, 'GB'],
          [1024 * 1024, 'MB'],
          [1024, 'KB'],
          [1, 'bytes']
       for(var i = 0; i < units.length; i++){
         var unitSize = units[i][0];
         var unitText = units[i][1];
         if (length >= unitSize) {
           length = length / unitSize;
           // 1 decimal place
           length = Math.ceil(length * 10) / 10;
           var lengthUnits = unitText;
       // insert the text in a span directly after the link and add a class to the link
       Element.insert(link, {'after':
            ' <span class="filesize">(' + length + ' ' + lengthUnits + ')</span>'});
  }); // each matched link

This is it! At this point, the extension should be already functional. If you test it now, you should be able to see the size of links getting injected next to matching each link in the content of a wiki document.

We will now make this information look nicer, and add an icon to represent the file type of the link, thanks to a stylesheet extension.

Making it look nice with SSX

This time, we will take advantage of the Parse attribute of extensions that has been evoked earlier in this tutorial. This way, we can be lazy and generate the CSS code using the velocity templating language, instead of writing a rule for each file format manually. Thanks to velocity and to the XWiki api, we will also be able to reference images attached to the extension document.

The class name that is added to each matching link by the JSX is actually the matching file extension itself (doc, pdf, etc.). Thus, we can then iterate over the extensions that we target and generate a rule for each one of them. And more, if we name our icons with the convention of using the file extension, we can also reference the image within the same iteration.

An archive with the set of icons used for this tutorial can be downloaded here. The icons for MS products, for zip and pdf files are from the Silk Icons Set by Mark James, available under the Creative Commons Attribution 2.5 License. To add the icons to your extension, just unzip the archive and attach them manually to your XWiki.AddSizesExtension document. Of course, you can also use your own set of icons. If you change the name of the files however, keep in mind that you will have to adapt the stylesheet extension below.

Once you have the icons attached, create the stylesheet extension, set its parse attribute to Yes, and paste this code in the "Code" section:

/* A little padding on the right of links for the icons to fit */
#foreach($ext in ['odt', 'ods', 'odp', 'doc', 'xls', 'ppt', 'pdf', 'zip'])
#xwikicontent a.${ext} #if($velocityCount < 8), #end
#end {

/* File icons as background for the links */
#foreach($ext in ['odt', 'ods', 'odp', 'doc', 'xls', 'ppt', 'pdf', 'zip'])
 #xwikicontent a.${ext} {
  background:transparent url($doc.getAttachmentURL("${ext}.png")) no-repeat scroll right center;

/* Nice spans for file size information */
#xwikicontent span.filesize {
  font-size: 0.6em;

When asked to serve the CSS file, XWiki will evaluate this code using its Velocity Rendering engine, and will return a file that contains pure CSS code!

Testing the final extension

Ok, it's time for us to see the whole thing in action! The snippet below is intended to showcase the extension on its own wiki page. It requests the jsx and ssx plugins to load javascript and css objects we created and showcases the links.

#set($discard = $xwiki.jsx.use($doc.fullName))
#set($discard = $xwiki.ssx.use($doc.fullName))

* [[An Writer document>>]]
* [[A MS Word document>>]]
* [[An OOo Spreadshet>>]]
* [[Link to a MS Excel document>>]]
* [[An OOo Presentation>>]]
* [[Link to a MS Powerpoint file>>]]
* [[A great archive>>]]
* [[A PDF file>>]]

Now there are two things to keep in mind:

  • The browser must be able to reach the Internet, since the extension does need the help of the json-head service hosted on Google App Engine.
  • As Natalie Downe wrote, "this may not be 100% reliable due to App Engine being occasionally and unavoidably flakey". You may for example experience a long loading time (but since the extension triggers only once the whole wiki document is loaded, this will not penalize the wiki users).

In a future extension of this tutorial, we will address these two issues. We will write our own version of the json-head service on the wiki itself, using the groovy programming language.

Enough talk, let's see the result!


Bonus: links to activate/deactivate the extension


You can add this snippet in the content section of the extension document, and users with the programming right granted will be provided a link to activate or not the extension on all pages of the wiki:

#if($xwiki.hasAccessLevel('programming', $context.user)) ## Only programmers should be able to change the loading type
                                                        ## otherwise, Always-used extensions will not work

 #if($doc.getObject('XWiki.JavaScriptExtension').getProperty('use').value == 'always')
  #info('This extension is configured to be loaded on all the pages of this wiki.')

 <span class=buttonwrapper>
   <a href="$doc.getURL('save','XWiki.JavaScriptExtension_0_use=onDemand&XWiki.StyleSheetExtension_0_use=onDemand')">
   De-activate loading for all pages.
  #info('This extension is configured to be loaded only on pages that request it.')

 <span class=buttonwrapper>
   <a href="$doc.getURL('save','XWiki.JavaScriptExtension_0_use=always&XWiki.StyleSheetExtension_0_use=always')">
   Activate loading for all pages.


Additional Details

How to use Velocity in parsed content

Example for XWiki.JavaScriptExtension code:

#if (!$xcontext.userReference)
alert("Hello guest!");
alert("Hello user!");

will show different alerts for different users on page refresh.

Example for XWiki.StyleSheetExtension code:

#if (!$xcontext.userReference)
#mainContentArea {
background-color: grey;
#mainContentArea {
background-color: blue;

will show different background colors for authenticated and anonymous users.

Velocity doesn't know about CSS. Everything that looks like a Velocity macro or variable is evaluated. So, undefined macros and
variables are printed as is. E.g. #xwikicontent will be used as CSS ID field, unless xwikicontent Velocity macro is defined.

Velocity is a template language, so when the Velocity code is evaluated the Velocity variables are simply substituted in the template by their values. So: 


works because false* is a literal boolean value in JavaScript.



will be evaluated e.g. as alert(XWiki.User); which will throw an exception unless User is a JavaScript variable previously defined, therefore you need to wrap the value in quotes, e.g.:


Or even more, because the value coming from Velocity can contain characters that are not allowed in a JavaScript string literal. So safest is to write:


You can use XWiki global velocity variables $doc, $request and $xcontext in parsed content.

How to communicate data to JSX or SSX?

It's often useful to communicate data to JSX or SSX in order to have them serve some customized content depending on the context, the user, data from the page etc.

How to communicate data to JSX

There are three main ways to communicate data to JSX:

  • Via the JSX plugin, using the map parameter in a Velocity script: $xwiki.jsx.use('Document.Name', {'minify' : false, 'language': $context.language, 'myParameter': 'value'}). The parameters are then available as query parameters, so it can be used in a Javasript Object by calling $request.myParameter - this is the recommended practice.
  • By reading data directly in the HTML page from the JSX (jQuery or Prototype can be helpful to do this).
  • By setting an attribute in the user session in Velocity, and reading this attribute in Velocity from the JSX. However, this approach has several downsides: the variable will be cached by the server (except if you disable the JSX cache, but this can raise performance issues), and it can bloat the session.

How to communicate data to SSX

Data can also be communicated to SSX in the same ways as to JSX, except the second option (accessing HTML elements from the page), since CSS cannot read data from the HTML page.


Since XWiki 6.4M2, you can use LESS in your Skin Extensions. Get a look to Skin Extension Plugin for more information.

When you add styles to your application, there are several languages you can use: CSS, LESS, Velocity. What language to use depends on if you need advanced functions, access to the XWiki API, if you need to reuse existing variables, etc. 

For example, depending on where your code is (in an SSX, in a skin or in a LESS file), you can use the old Color Themes variables (needs Velocity to be parsed) or the Flamingo Theme variables (needs LESS to be compiled).

If you are not sure what to use and where, here is a small summary: 

          CSS                                              LESS                                                    Velocity
CSS       SSX-CSS; Skin style.css; Web Resources           SSX-LESS; ColorTheme Advanced; Skin LESS files; Bootstrap/XWiki variables; SSX-CSS-Parse; Skin style.css; $theme variables
LESS      SSX-LESS; ColorTheme Advanced; Skin LESS files; Bootstrap/XWiki variables;  SSX-LESS; ColorTheme Advanced; Skin LESS files; Bootstrap/XWiki variables;         SSX-LESS-Parse; style.less.vm; $theme variables; Bootstrap/XWiki variables;
Velocity  SSX-CSS-Parse; Skin style.css; $theme variables  SSX-LESS-Parse; style.less.vm; $theme variables; Bootstrap/XWiki variables; { {Velocity} } macro; Generic Templates; Skin Templates


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