How to reference JSF image resource as CSS background image url

I often, change the images of my buttons using the image attribute, but someone told me that it is a good practice to do it using .css I tried but i cant, what i am doing wrong? This is what i did:

1-The resources of my project are stored like this:

2-This is how i created the style.css for accessing the image

.c2 {
    background: url(/resources/images/smiley.jpg);  
}

3-This is how i access the css from the body of my page(Im sure this is correct because other classes in the same document works in other tags in this page)

<h:outputStylesheet library="css" name="style.css"  />

4-This is how create a sample commandButton that uses the appropiated css class

<h:commandButton styleClass="c2"/>

I think the problem is in the .css, i tried a few combinations but none worked:

background-image: url(/resources/images/smiley.jpg);    
background: url(resources/images/smiley.jpg);   
background: url(smiley.jpg);    
background: url(../smiley.jpg); 

Where is the mistake?

update I managed to make it work by the following code:

.c2 {   
     background: url("#{resource['images:smiley.jpg']}");               
    }

Notice the difference when i use css(right) and when i use image attribute(left)

How could i solve this so the hold image is shown?

When importing CSS stylesheets by <h:outputStylesheet>, the stylesheet is imported and processed by the FacesServlet through /javax.faces.resource/*. Look at the generated <link> element pointing to the stylesheet in question and you'll understand.

You have to change all url() dependencies to use #{resource['library:location']} instead. JSF will then auto-substitute it with the right path. Given your folder structure, you need to replace

.c2 {
    background: url("/resources/images/smiley.jpg");  
}

by

.c2 {
    background: url("#{resource['images/smiley.jpg']}");  
}

Assuming that your webapp context name is playground and that your FacesServlet is mapped on *.xhtml, then the above should end up in the returned CSS file as follows

.c2 {
    background: url("/playground/javax.faces.resource/images/smiley.jpg.xhtml");
}

Noted should be that the JSF implementation will for determine only during the first request on the CSS file if it contains EL expressions. If it doesn't then it will for efficiency not anymore attempt to EL-evaluate the CSS file content. So if you add an EL expression to a CSS file for the first time, then you'd need to restart the whole application in order to get JSF to re-check the CSS file.

In case you wanted to reference a resource from a component library such as PrimeFaces, then prefix the library name, separated with :. E.g. when you're using PrimeFaces "Start" theme which is identified by primefaces-start

.c2 {
    background: url("#{resource['primefaces-start:images/ui-bg_gloss-wave_50_6eac2c_500x100.png']}");  
}

This will be generated as

.c2 {
    background: url("/playground/javax.faces.resource/images/ui-bg_gloss-wave_50_6eac2c_500x100.png.xhtml?ln=primefaces-start");
}

See also:

• How to reference CSS / JS / image resource in Facelets template?
• Changing JSF prefix to suffix mapping forces me to reapply the mapping on CSS background images

Unrelated to the concrete problem, the way how you use the library is not entirely right. It's meant to be the common identifier/subfolder of all related CSS/JS/image resources. The key idea is to be able to change the entire look'n'feel by just changing the library (which can be done by EL). You seem however to be relying on the default library. In that case, you could just omit the library from your <h:outputStylesheet> and #{resource}.

<h:outputStylesheet name="css/style.css"  />

See also:

• What is the JSF resource library for and how should it be used?

Since I struggled with this a little bit and while BalusC has already answered the question but might be able to comment as to why this is happening. I have 5 EAR projects consisting of a bundled WAR and EJB projects. I then have one standalone WAR project deployed on its own. The following code worked perfect with all the EAR's:

<h:head>
    <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
    <title>Super FIPS Calculator PRO</title>
    <style>              
        .Bimage{background-image:url(#{request.contextPath}/img/phonetoolsBackground.png);}
    </style>
</h:head>
<h:body styleClass="Bimage">
.
.
.

Where the "img" folder was within the WEB-INF folder but for the EAR project, it would not work and it wouldnt even load the picture in the browser by manually typing in the URL. I verified the resulting html was 100% accurate. So all the talk of "resources" got me thinking that maybe it was a ?security? issue of some sort which doesnt seem to make sense between the WAR and EAR deployments so I created a "resources" folderin the root of the web application, e.g. in Eclipse its parent would be WebContent, then added a subfolder to resources called "img", placed my image in there.

The code now looks like this:

<h:head>
    <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
    <title>Super FIPS Calculator PRO</title>
    <style>              
        .Bimage{background-image:url(#{request.contextPath}/resources/img/phonetoolsBackground.png);}
    </style>
</h:head>
<h:body styleClass="Bimage">
.
.
.

And now the image is displayed. Again not trying to hijack balusc's thorough answer, I just wanted to bring it up in case anyone ran into a similar issue. If someone wants me to open a separate Q and A I will!

Ahh yes, this was on JBoss EAP 7, Servlet API 3.1, Facelets 2.2, Rich Faces 4.5.17 Java 1.8.

Edit @Basil-Bourque answer What is WEB-INF used for in a Java EE web application seems fairly relevant

But its still a little confusing in that how can a WAR within an EAR access that location but not a standalone WAR?