Default Templates

Have you ever accidentally (or deliberately) applied an empty stylesheet to a document? For example, if we apply empty.xsl:

 

 

to shortcatalog.xml, the result is:

 

 

Although the stylesheet was empty, the output is not. The reason for this is that XSLT, as we saw briefly in Chapter 2, contains some default templates to match elements, attributes, and text nodes that are not matched elsewhere.

 

The main templates in this category are:

 

The first of these matches the document root and all elements. In both cases, it causes processing to continue by applying an <xsl:apply-templates> instruction to all child nodes. This means that, if you have a template matching an element buried deep in your source tree, but no template for one or more of its ancestors, the default template will cause processing to continue until that element is reached.

 

The second template matches text nodes and all attributes, causing the values of these to be output.

 

The combination of the first template, ensuring that the document root and all elements are processed, and the second, ensuring that all the text is copied to the result tree, is what generated our output.

 

There are also default templates for comments, processing instructions and namespace nodes, each saying "do nothing". Finally, in Chapter 4, we will meet another default template covering the mode attribute, which we have not met in detail yet.

 

Remember that these are only defaults. If you define your own templates, they will over-ride these. It could be that you are only processing a few elements in a document and don't want others processed. In this case, including the following line in your stylesheet will ensure that this happens:

 

Template Match Conflicts

When several templates match the same source node, the XSLT processor needs rules for choosing which template to use. In fact, because of the existence of the default templates, every stylesheet will contain multiple rules matching any node. Luckily, XSLT provides a clear set of rules for deciding which template to use.

 

We will see in Chapter 5 that one stylesheet can import another. If there is a conflict over which rule to apply when one belongs to the importing stylesheet and one to an imported stylesheet, the rule in the importing stylesheet is said to have to higher precedence, and will be the rule applied. If the conflict is between rules in imported stylesheets, then stylesheets imported later have higher precedence than those imported earlier. Of course, imported stylesheets can import other stylesheets. If you get into this situation, the recommendation handles all cases using the concept of an import tree. Refer to the XSLT Recommendation itself to understand this.

 

The default rules are treated as though they were imported before the stylesheet itself, and so have the lowest possible import precedence.

 

Template rules can also have a priority assigned. We saw how to do this earlier, when discussing the <xsl:template> element. Where a priority is not assigned, a default priority value is used. These default priorities are in the range -0.5 to 0.5. In general, the more specific the match expression, the higher the priority.

 

If there is still a conflict after applying the precedence rules, the rule with the highest priority is applied.

 

It is possible that there will still be a conflict after the priority rules have been applied. This is treated as an error. The stylesheet processor will either indicate this error in some way or choose the last rule in the stylesheet that matches the pattern.