FreeMarker tags are extensions of the generic Struts Tags provided by the framework. You can jump right in just by knowing the generic structure in which the tags can be accessed: <@s.tag> ...</@s.tag>, where tag is any of the tags supported by the framework.
For example, in JSP you might create a form using Struts tags.
In FreeMarker the same form can also be built using Struts tags.
But, wait there's more!
Aside from doing everything that the JSP tags do, the FTL tags boast additional features that you can use to make your pages even easier to code. You can even invoke third-party JSP taglibs as if there were native FTL tags.
Attributes and Parameters
Unlike older versions of JSP (in which the JSP Tags are based), FreeMarker allows for dynamic attributes, much like JSP 2.0. You can supply attributes to the tags that the tag doesn't explicitedly support. Those attributes that cannot be applied directly to the tag object will be set to the tag's general-purpose parameters Map.
Suppose we wanted to build an URL in a JSP. The URL needs to take an arbitary parameter to the query string, that (being arbitary) isn't specified on the URL tag. In a JSP, we'd have to use the url and param tags together.
In FreeMarker, we can pass the arbitrary parameter directly and create the URL in one simple statement.
Using inline attributes with templates
Suppose you created a "three column" theme to replace the typical two column theme (xhtml). You might want an additional parameter to display in the third column called "description". Using FreeMarker, you can just pop the description attribute into the textfield tag, no fuss, no muss.
In the new template, the description is referenced via the parameters Map: "${parameters.description}".
Attribute Types
Remember that all tag attributes must first be set as Strings - they are then later evaluated (using OGNL) to a different type, such as List, int, or boolean. This generally works just fine, but it can be limiting when using FreeMarker which provides more advanced ways to apply attributes. Suppose the following example:
What will happen here is that each attribute will be evaluated to a String as best it can. This may involve calling the toString method on the internal FreeMarker objects. In this case, all objects will end up being exactly what you would expect. Then, when the tag runs, the list attribute will be converted from a String to a List using OGNL's advanced collection support.
But suppose you wish to use FreeMarker's list or hash support instead? You can do this:
Notice that the list attribute no longer has quotes around it. Now it will come in to the tag as an object that can't easily be converted to a String. Normally, the tag would just call toString, which would return "[1, 2, 3]" and be unable to be converted back to a List by OGNL. Rather than go through all this back and forth, the frameworks's FreeMarker tag support will recognize collections and not pass them through the normal tag attribute. Instead, the framework will set them directly in the parameters Map, ready to be consumed by the template.
In the end, everything tends to do what you would expect, but it can help to understand the difference of when OGNL is being used and when it isn't, and how attribute types get converted.
JSP Tag Support
While the framework provides native FreeMarker Tags, you might wish to use other third-party tags that are only available for JSP. Fortunately, FreeMarker has the ability to run JSP tags. To do so, you must include the JspSupportServlet in the application's web.xml, as this allows the FreeMarker integration to get access to the required objects needed to emulate a JSP taglib container.
Once you've done that, you can simply add something like this in your templates: