ui

The tags in this library add templating — a powerful view composition technique — to JSF. Templating is so useful that there are entire frameworks, such as Tiles and SiteMesh, that are built around the concept of templating. So what is templating, how can you benefit from it, and how does this tag library implement it?

If you've used JSP before, you've probably used jsp:include. The prototypical example for jsp:include is a header on each page in a web application. One JSP page, say header.jsp, encapsulates the header content, and the header is included by each page. You encapsulate and reuse content, so that changes to one file, header.jsp, affect the header on every page.

This tab library contains a tag —ui:include — that's analagous to jsp:include, but encapsulating and reusing content is only half the templating story, because templating also lets you encapsulate and reuse layout. You define a single template (meaning layout), and you reuse that template with multiple compositions. So now you can control the layout of many pages with a single template (layout). Let's take a look at an example.

A Templating Example

First, we define a template:

In the preceeding listing, we've defined a layout, also known as a template. That template uses the ui:insert tag to insert pieces of a page — namely, title, heading, and content — defined in a composition. Notice that on line 8, we define a default title, in case one isn't provided by the composition. Also note that on line 12 we have the ui:debug tag, which lets the user activate a popup window with debugging information by typing CTRL + Shift + d.

The title, heading, and content pieces of the page referenced in the template are defined in a separate XHTML file in a composition, like this:

At runtime, JSF synthesizes the two previous XHTML pages to create a single JSF view by inserting the pieces defined in the composition into the template (that template is layout.xhtml, which is the first listing above). JSF also disregards everything outside of the composition tag so that we don't wind up with two body elements in the view. Also, note that we use the ui:include tag on line 14 to include content (which happens to be a table) from another XHTML page, so that we can reuse that table in other views.

So why do we have two XHTML pages to define a single view? Why not simply take the pieces and manually insert them into the layout so that we have only a single XHTML page? The answer is simple: we have separated layout from the content so that we can reuse that layout among multiple compositions. For example, now we can define another composition that uses the same layout:

By encapsulating the layout, we can reuse that layout among multiple compositions. Just like ui:include lets us encapsulate and reuse conent, JSF compositions let us encapsulate and reuse layout, so that changes to a single layout can affect multiple views. Fundamentally, that's what this tag library is all about.