Basics

Note:

This section uses the DOM tree and the variable made in a previous chapter.

With the imperative approach of XML processing -- this was shown in the previous chapter -- you write an FTL program that walks the tree to find the different kind of nodes. With the declarative approach, you rather define how to handle the different kind of nodes, and then let FreeMarker walk the tree an call the handlers you have defined. This approach is useful for complex XML schemas, where the same element can occur as the child of many other elements. Examples of such schemas are XHTML and XDocBook.

The directive you most often use with the declarative approach is the recurse directive. This directive gets a node variable as parameter, and "visits" all its children nodes, one after the other, starting with the first child. "Visiting" a node means that it calls a user-defined directive (like a macro) that has the same name as the name of the child node (?node_name). We say on this, that the user-defined directive handles the node. The node that the user-defined directive just handles is available as special variable .node. For example, this FTL:

<#recurse doc>

<#macro book>
  I'm the book element handler, and the title is: ${.node.title}
</#macro>

will print (I have removed some disturbing white-space form the output):

I'm the book element handler, and the title is: Test Book

If you call recurse without parameter, then it uses .node, that is, it visits all children nodes of the node currently handled. So this FTL:

<#recurse doc>

<#macro book>
  Book element with title ${.node.title}
    <#recurse>
  End book
</#macro>

<#macro title>
  Title element
</#macro>

<#macro chapter>
  Chapter element with title: ${.node.title}
</#macro>

will print (I have removed disturbing white-space form the output):

Book element with title Test Book
Title element
Chapter element with title: Ch1
Chapter element with title: Ch2
End book

You have seen how to define handlers for element nodes, but not how to define handler for the text nodes. Since the name of the handler is the same as the node-name of nodes it handles, and as the node-name of all text nodes is @text (see the table), you define handler for the text nodes like this:

<#macro @text>${.node?html}</#macro>

Note the ?html. You have to HTML-escape the text, since you generate output of HTML format.

Here it is the template that transforms the XML to complete HTML:

<#recurse doc>

<#macro book>
  <html>
    <head>
      <title><#recurse .node.title></title>
    </head>
    <body>
      <h1><#recurse .node.title></h1>
      <#recurse>
    </body>
  </html>
</#macro>

<#macro chapter>
  <h2><#recurse .node.title></h2>
  <#recurse>
</#macro>

<#macro para>
  <p><#recurse>
</#macro>

<#macro title>
  <#--
    We have handled this element imperatively,
    so we do nothing here.
  -->
</#macro>

<#macro @text>${.node?html}</#macro>

and the output will be (now I will honestly include the annoying white-space...):

  <html>
    <head>
      <title>Test Book</title>
    </head>
    <body>
      <h1>Test Book</h1>


    <h2>Ch1</h2>


      <p>p1.1

      <p>p1.2

      <p>p1.3


    <h2>Ch2</h2>


      <p>p2.1

      <p>p2.2


    </body>
  </html>

  

Note that you can reduce substantially the amount of superfluous whitespace in the output by using the trim directives, as <#t>. See also: Template Author's Guide/Miscellaneous/White-space handling

You may say that the FTL that did it with imperative approach was much shorter. That's true, but the example XML uses a very simple schema, and as I said, the declarative approach brings its form with XML schemas that are not that firm about what element can occur where. Say, introduce element mark, that should color text to red, does not mater where do you use it; in a title, or in a para. For this, with the declarative approach, you just add a macro:

<#macro mark><font color=red><#recurse></font></#macro>

And then <mark>...</mark> will automatically work everywhere. So for certain XML schemas, declarative XML processing will actually result in shorter, and what is even more important, much clearer FTL-s, than imperative XML processing. It's up to you to decide which approach to use when; don't forget that you can mix the two approaches freely. Say, in an element handler, you can use imperative approach to process the contents of that element.