<xsl:stylesheet 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xalan="http://xml.apache.org/xslt"
xmlns:java="http://xml.apache.org/xslt/java">
   
<xalan:component prefix="Math" functions="sin cos tan atan">
 <xalan:script lang="javaclass" src="http://xml.apache.org/xslt/java"/>
</xalan:component>
   
<xsl:variable name="PI" select="4.0 * java:java.lang.Math:atan(1.0)"/>
   
<!-- ... -->
   
</xsl:stylesheet>

Use this form if you want to access a wide variety of Java-based extensions with a single namespace declaration. The disadvantage is that each invocation becomes more verbose.