Use <o:deferredScript>
Yes, it is possible with the <o:deferredScript> component which is new since OmniFaces 1.8.1. For the technically interested, here's the involved source code:
Basically, the component will during the postAddToView event (thus, during the view build time) via UIViewRoot#addComponentResource() add itself as a new script resource in end of <body> and via Hacks#setScriptResourceRendered() notify JSF that the script resource is already rendered (using Hacks class as there's no standard JSF API approach for that (yet?)), so that JSF won't forcibly auto-include/render the script resource anymore. In case of Mojarra and PrimeFaces, a context attribute with key of name+library and a value of true has to be set in order to disable auto-inclusion of the resource.
The renderer will write a <script> element with OmniFaces.DeferredScript.add() whereby the JSF-generated resource URL is passed. This JS helper will in turn collect the resource URLs and dynamically create new <script> elements for each of them during the onload event.
The usage is fairly simple, just use <o:deferredScript> the same way as <h:outputScript>, with a library and name. It doesn't matter where you place the component, but most self-documenting would be in the end of the <h:head> like this:
<h:head>
...
<o:deferredScript library="libraryname" name="resourcename.js" />
</h:head>
You can have multiple of them and they will ultimately be loaded in the same order as they're declared.
How to use <o:deferredScript> with PrimeFaces?
This is a little tricky, indeed because of all those inline scripts generated by PrimeFaces, but still doable with a helper script and accepting that jquery.js won't be deferred (it can however be served via a CDN, see later). In order to cover those inline PrimeFaces.xxx() calls to primefaces.js file which is almost 220KiB large, a helper script needs to be created which is less than 0.5KiB minified:
DeferredPrimeFaces = function() {
var deferredPrimeFaces = {};
var calls = [];
var settings = {};
var primeFacesLoaded = !!window.PrimeFaces;
function defer(name, args) {
calls.push({ name: name, args: args });
}
deferredPrimeFaces.begin = function() {
if (!primeFacesLoaded) {
settings = window.PrimeFaces.settings;
delete window.PrimeFaces;
}
};
deferredPrimeFaces.apply = function() {
if (window.PrimeFaces) {
for (var i = 0; i < calls.length; i++) {
window.PrimeFaces[calls[i].name].apply(window.PrimeFaces, calls[i].args);
}
window.PrimeFaces.settings = settings;
}
delete window.DeferredPrimeFaces;
};
if (!primeFacesLoaded) {
window.PrimeFaces = {
ab: function() { defer("ab", arguments); },
cw: function() { defer("cw", arguments); },
focus: function() { defer("focus", arguments); },
settings: {}
};
}
return deferredPrimeFaces;
}();
Save it as /resources/yourapp/scripts/primefaces.deferred.js. Basically, all what it does is capturing the PrimeFaces.ab(), cw() and focus() calls (as you can find in the bottom of the script) and deferring them to the DeferredPrimeFaces.apply() call (as you can find halfway the script). Note that there are possibly more PrimeFaces.xxx() functions which need to be deferred, if that is the case in your app, then you can add them yourself inside window.PrimeFaces = {} (no, it's in JavaScript not possible to have a "catch-all" method to cover the undetermined functions).
Before using this script and <o:deferredScript>, we first need to determine the auto-included scripts in the generated HTML output. For the test page as shown in the question, the following scripts are auto-included in generated HTML <head> (you can find this by rightclicking the page in webbrowser and choosing View Source):
<script type="text/javascript" src="/playground/javax.faces.resource/jquery/jquery.js.xhtml?ln=primefaces&v=4.0"></script>
<script type="text/javascript" src="/playground/javax.faces.resource/jquery/jquery-plugins.js.xhtml?ln=primefaces&v=4.0"></script>
<script type="text/javascript" src="/playground/javax.faces.resource/primefaces.js.xhtml?ln=primefaces&v=4.0"></script>
<script type="text/javascript" src="/playground/javax.faces.resource/layout/layout.js.xhtml?ln=primefaces&v=4.0"></script>
<script type="text/javascript" src="/playground/javax.faces.resource/watermark/watermark.js.xhtml?ln=primefaces&v=4.0"></script>
<script type="text/javascript" src="/playground/javax.faces.resource/fileupload/fileupload.js.xhtml?ln=primefaces&v=4.0"></script>
You need to skip the jquery.js file and create <o:deferredScripts> in exactly the same order for the remaining scripts. The resource name is the part after /javax.faces.resource/ excluding the JSF mapping (.xhtml in my case). The library name is represented by ln request parameter.
Thus, this should do:
<h:head>
...
<h:outputScript library="yourapp" name="scripts/primefaces.deferred.js" target="head" />
<o:deferredScript library="primefaces" name="jquery/jquery-plugins.js" />
<o:deferredScript library="primefaces" name="primefaces.js" onbegin="DeferredPrimeFaces.begin()" />
<o:deferredScript library="primefaces" name="layout/layout.js" />
<o:deferredScript library="primefaces" name="watermark/watermark.js" />
<o:deferredScript library="primefaces" name="fileupload/fileupload.js" onsuccess="DeferredPrimeFaces.apply()" />
</h:head>
Now all those scripts with a total size of about 516KiB are deferred to onload event. Note that DeferredPrimeFaces.begin() must be called in onbegin of <o:deferredScript name="primefaces.js"> and that DeferredPrimeFaces.apply() must be called in onsuccess of the last <o:deferredScript library="primefaces">.
In case you're using PrimeFaces 6.0 or newer, where the primefaces.js has been replaced by core.js and components.js, use the below instead:
<h:head>
...
<h:outputScript library="yourapp" name="scripts/primefaces.deferred.js" target="head" />
<o:deferredScript library="primefaces" name="jquery/jquery-plugins.js" />
<o:deferredScript library="primefaces" name="core.js" onbegin="DeferredPrimeFaces.begin()" />
<o:deferredScript library="primefaces" name="components.js" />
<o:deferredScript library="primefaces" name="layout/layout.js" />
<o:deferredScript library="primefaces" name="watermark/watermark.js" />
<o:deferredScript library="primefaces" name="fileupload/fileupload.js" onsuccess="DeferredPrimeFaces.apply()" />
</h:head>
As to performance improvement, important measuring point is the DOMContentLoaded time as you can find in bottom of Network tab of Chrome's developer tools. With the test page as shown in the question served by Tomcat on a 3 year old laptop, it decreased from ~500ms to ~270ms. This is relatively huge (almost the half!) and makes the most difference on mobiles/tablets as they render HTML relatively slow and touch events are fully blocked until the DOM content is loaded.
Noted should be that you're in case of (custom) component libraries dependent on whether they obey the JSF resource management rules/guidelines or not. RichFaces for example didn't and homebrewed another custom layer over it, making it impossible to use <o:deferredScript> on it. See also what is the resource library and how should it be used?
Warning: if you're adding new PrimeFaces components on the same view afterwards and are facing JavaScript undefined errors, then the chance is big that the new component also comes with its own JS file which should also be deferred, because it's depending on primefaces.js. A quick way to figure the right script would be to check the generated HTML <head> for the new script and then add another <o:deferredScript> for it based on the above instructions.
Bonus: CombinedResourceHandler recognizes <o:deferredScript>
If you happen to use OmniFaces CombinedResourceHandler, then it's good to know that it transparently recognizes <o:deferredScript> and combines all deferred scripts with the same group attribute into a single deferred resource. E.g. this ...
<o:deferredScript group="essential" ... />
<o:deferredScript group="essential" ... />
<o:deferredScript group="essential" ... />
...
<o:deferredScript group="non-essential" ... />
<o:deferredScript group="non-essential" ... />
... wil
