Template Conflict Resolution
Template Conflict Resolution
Another important aspect of template use is conflict resolution . If two templates match the same node or node set, XSLT relies on the priority of the two templates to determine which template to apply.
Each template has a default priority based on the select attribute value. Generally, the more specific the match or expression (such as "PLANET" versus "*" ) is, the higher its priority. Chapter 4 looks at how the processor determines priorities and how the processor deals with templates that have the same priority.
You can also set the priority of a template with the priority attribute. Heres an example; in this case, a rule created with the element <xsl:template priority="1"/> has lower priority than one created with the <xsl:template priority="2"/> element:
Listing 3.8 Setting Template Priority
<?xml version="1.0"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:template match="/PLANETS"> <HTML> <HEAD> <TITLE> The Planets Table </TITLE> </HEAD> <BODY> <H1> The Planets Table </H1> <TABLE BORDER="2"> <TR> <TD>Name</TD> <TD>Mass</TD> <TD>Radius</TD> <TD>Day</TD> </TR> <xsl:apply-templates/> </TABLE> </BODY> </HTML> </xsl:template> <xsl:template match="PLANET"> <TR> <TD><xsl:value-of select="NAME"/></TD> <TD><xsl:apply-templates select="MASS"/></TD> <TD><xsl:apply-templates select="RADIUS"/></TD> <TD><xsl:apply-templates select="DAY"/></TD> </TR> </xsl:template> <xsl:template match="MASS" priority="2"> <xsl:value-of select="."/> (<I>Very</I> heavy) </xsl:template> <xsl:template match="MASS" priority="1"> <xsl:value-of select="."/> </xsl:template> <xsl:template match="RADIUS"> <xsl:value-of select="."/> <xsl:text> </xsl:text> <xsl:value-of select="@UNITS"/> </xsl:template> <xsl:template match="DAY"> <xsl:value-of select="."/> <xsl:text> </xsl:text> <xsl:value-of select="@UNITS"/> </xsl:template> </xsl:stylesheet>
The XSLT processor selects the template with the higher priority, and that template adds the text "(<I>Very</I> heavy)" after each mass measurement. Heres the result, where you can see the template with the higher priority was used:
<HTML> <HEAD> <TITLE> The Planets Table </TITLE> </HEAD> <BODY> <H1> The Planets Table </H1> <TABLE BORDER="2"> <TR> <TD>Name</TD> <TD>Mass</TD> <TD>Radius</TD> <TD>Day</TD> </TR> <TR> <TD>Mercury</TD> <TD>.0553(<I>Very</I> heavy)</TD> <TD>1516 miles</TD> <TD>58.65 days</TD> </TR> <TR> <TD>Venus</TD> <TD>.815(<I>Very</I> heavy)</TD> <TD>3716 miles</TD> <TD>116.75 days</TD> </TR> <TR> <TD>Earth</TD> <TD>1(<I>Very</I> heavy)</TD> <TD>2107 miles</TD> <TD>1 days</TD> </TR> </TABLE> </BODY> </HTML>
Upcoming in XSLT 2.0The question of template priority is one that XSLT 2.0 is supposed to address. In particular, W3C is considering adding a new element, hypothetically named <xsl: next -match/> , that will enable you to select the second best match to a template. |
Heres another useful thing to know about priority: If two templates match the same node and no priority has been assigned these templates, the XSLT processor will choose the template that is a more specific match. For example, a match to "PLANET" is preferred over a match to the generic "*" .
