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<HTML><HEAD><SCRIPT language="JavaScript" src="resources/script.js" type="text/javascript"></SCRIPT><TITLE>XSLTC Compiler Design</TITLE></HEAD><BODY alink="#ff0000" bgcolor="#ffffff" leftmargin="4" link="#0000ff" marginheight="4" marginwidth="4" text="#000000" topmargin="4" vlink="#0000aa"><TABLE border="0" cellpadding="0" cellspacing="0" width="620"><TR><TD align="left" height="60" rowspan="3" valign="top" width="135"><IMG alt="logo" border="0" height="60" hspace="0" src="resources/logo.gif" vspace="0" width="135"></TD><TD align="left" colspan="4" height="5" valign="top" width="456"><IMG alt="line" border="0" height="5" hspace="0" src="resources/line.gif" vspace="0" width="456"></TD><TD align="left" height="60" rowspan="3" valign="top" width="29"><IMG alt="right" border="0" height="60" hspace="0" src="resources/right.gif" vspace="0" width="29"></TD></TR><TR><TD align="left" bgcolor="#0086b2" colspan="4" height="35" valign="top" width="456"><IMG alt="" border="0" height="35" hspace="0" src="graphics/xsltc_compiler-header.jpg" vspace="0" width="456"></TD></TR><TR><TD align="left" height="20" valign="top" width="168"><IMG alt="bottom" border="0" height="20" hspace="0" src="resources/bottom.gif" vspace="0" width="168"></TD><TD align="left" height="20" valign="top" width="96"><A href="http://xml.apache.org/" onMouseOut="rolloverOff('xml');" onMouseOver="rolloverOn('xml');" target="new"><IMG alt="http://xml.apache.org/" border="0" height="20" hspace="0" name="xml" onLoad="rolloverLoad('xml','resources/button-xml-hi.gif','resources/button-xml-lo.gif');" src="resources/button-xml-lo.gif" vspace="0" width="96"></A></TD><TD align="left" height="20" valign="top" width="96"><A href="http://www.apache.org/" onMouseOut="rolloverOff('asf');" onMouseOver="rolloverOn('asf');" target="new"><IMG alt="http://www.apache.org/" border="0" height="20" hspace="0" name="asf" onLoad="rolloverLoad('asf','resources/button-asf-hi.gif','resources/button-asf-lo.gif');" src="resources/button-asf-lo.gif" vspace="0" width="96"></A></TD><TD align="left" height="20" valign="top" width="96"><A href="http://www.w3.org/" onMouseOut="rolloverOff('w3c');" onMouseOver="rolloverOn('w3c');" target="new"><IMG alt="http://www.w3.org/" border="0" height="20" hspace="0" name="w3c" onLoad="rolloverLoad('w3c','resources/button-w3c-hi.gif','resources/button-w3c-lo.gif');" src="resources/button-w3c-lo.gif" vspace="0" width="96"></A></TD></TR></TABLE><TABLE border="0" cellpadding="0" cellspacing="0" width="620"><TR><TD align="left" valign="top" width="120"><IMG alt="join" border="0" height="14" hspace="0" src="resources/join.gif" vspace="0" width="120"><BR>
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<UL>
<LI><A href="#overview">Compiler Overview</A></LI>
<LI><A href="#ast">Building the Abstract Syntax Tree</A></LI>
<LI><A href="#typecheck">Type-check and Cast Expressions</A></LI>
<LI><A href="#compile">JVM byte-code generation</A></LI>
</UL>
<A name="overview"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="666699" colspan="2" width="494"><TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="492"><FONT color="#ffffff" face="arial,helvetica,sanserif" size="+1"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Compiler overview</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="484"><FONT color="#000000" face="arial,helvetica,sanserif">
<P>The main component of the XSLTC compiler is the class</P>
<UL>
<LI><CODE><FONT face="courier, monospaced">org.apache.xalan.xsltc.compiler.XSLTC</FONT></CODE></LI>
</UL>
<P>This class uses three parsers to consume the input stylesheet(s):</P>
<UL>
<LI><CODE><FONT face="courier, monospaced">javax.xml.parsers.SAXParser</FONT></CODE></LI>
</UL>
<P>is used to parse the stylesheet document and pass its contents to
the compiler as basic SAX2 events.</P>
<UL>
<LI><CODE><FONT face="courier, monospaced">com.sun.xslt.compiler.XPathParser</FONT></CODE></LI>
</UL>
<P> is a parser used to parse XPath expressions and patterns. This parser
is generated using JavaCUP and JavaLEX from Princeton University.</P>
<UL>
<LI><CODE><FONT face="courier, monospaced">com.sun.xslt.compiler.Parser</FONT></CODE></LI>
</UL>
<P>is a wrapper for the other two parsers. This parser is responsible for
using the other two parsers to build the compiler's abstract syntax tree
(which is described in more detail in the next section of this document).
</P>
</FONT></TD></TR></TABLE><BR>
<A name="ast"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="666699" colspan="2" width="494"><TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="492"><FONT color="#ffffff" face="arial,helvetica,sanserif" size="+1"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Building an Abstract Syntax Tree</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="484"><FONT color="#000000" face="arial,helvetica,sanserif">
<P>An abstract syntax tree (AST) is a data-structure commonly used by
compilers to separate the parse-phase from the later phases of the
compilation. The AST has one node for each parsed token from the stylesheet
and can easily be parsed at the stages of type-checking and bytecode
generation.</P>
<UL>
<LI>
<A href="#mapping">Mapping stylesheet elements to AST nodes</A>
</LI>
<LI>
<A href="#domxsl">Building the AST from AST nodes</A>
</LI>
<LI>
<A href="#mapping">Mapping XPath expressions and patterns to additional AST nodes</A>
</LI>
</UL>
<P>The SAX parser passes the contents of the stylesheet to XSLTC's main
parser. The SAX events represent a decomposition of the XML document that
contains the stylesheet. The main parser needs to create one AST node from
each node that it receives from the SAX parser. It also needs to use the
XPath parser to decompose attributes that contain XPath expressions and
patterns. Remember that XSLT is in effect two languages: XML and XPath,
and one parser is needed for each of these languages. The SAX parser breaks
down the stylesheet document, the XPath parser breaks down XPath expressions
and patterns, and the main parser maps the decomposed elements into nodes
in the abstract syntax tree.</P>
<A name="mapping"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Mapping stylesheets elements to AST nodes</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>Every element that is defined in the XSLT 1.0 spec is represented by a
a class in the <CODE><FONT face="courier, monospaced">org.apache.xalan.xsltc.compiler</FONT></CODE> package. The
main parser class contains a <CODE><FONT face="courier, monospaced">Hashtable</FONT></CODE> that that maps XSL
elements into Java classes that make up the nodes in the AST. These Java
classes all reside in the <CODE><FONT face="courier, monospaced">org.apache.xalan.xsltc.compiler</FONT></CODE>
package and extend either the <CODE><FONT face="courier, monospaced">TopLevelElement</FONT></CODE> or the
<CODE><FONT face="courier, monospaced">Instruction</FONT></CODE> classes. (Both these classes extend the
<CODE><FONT face="courier, monospaced">SyntaxTreeNode</FONT></CODE> class.)</P>
<P>The mapping from XSL element names to Java classes/AST nodes is set up
in the <CODE><FONT face="courier, monospaced">initClasses()</FONT></CODE> method of the main parser:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
private void initStdClasses() {
try {
initStdClass(&quot;template&quot;, &quot;Template&quot;);
initStdClass(&quot;param&quot;, &quot;Param&quot;);
initStdClass(&quot;with-param&quot;, &quot;WithParam&quot;);
initStdClass(&quot;variable&quot;, &quot;Variable&quot;);
initStdClass(&quot;output&quot;, &quot;Output&quot;);
:
:
:
}
}
private void initClass(String elementName, String className)
throws ClassNotFoundException {
_classes.put(elementName,
Class.forName(COMPILER_PACKAGE + '.' + className));
}</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
</FONT></TD></TR></TABLE><BR>
<A name="domxsl"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Building the AST from AST nodes</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>The parser builds an AST from the various syntax tree nodes. Each node
contains a reference to its parent node, a vector containing references
to all child nodes and a structure containing all attribute nodes:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
protected SyntaxTreeNode _parent; // Parent node
private Vector _contents; // Child nodes
protected Attributes _attributes; // Attributes of this element</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
<P>These variables should be accessed using these methods:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
protected final SyntaxTreeNode getParent();
protected final Vector getContents();
protected String getAttribute(String qname);
protected Attributes getAttributes();</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
<P>At this time the AST only contains nodes that represent the XSL elements
from the stylesheet. A SAX parse is generic and can only handle XML files
and will not break up and identify XPath patterns/expressions (these are
stored as attributes to the various nodes in the tree). Each XSL instruction
gets its own node in the AST, and the XPath patterns/expressions are stored
as attributes of these nodes. A stylesheet looking like this:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
&lt;xsl:stylesheet .......&gt;
&lt;xsl:template match=&quot;chapter&quot;&gt;
&lt;xsl:text&gt;Chapter&lt;/xsl:text&gt;
&lt;xsl:value-of select=&quot;.&quot;&gt;
&lt;/xsl:template&gt;
&lt;/xsl&gt;stylesheet&gt;</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
<P>will be stored in the AST as indicated in the following picture:</P>
<P><IMG align="right" alt="ast_stage1.gif" border="0" hspace="4" src="images/ast_stage1.gif" vspace="4"><BR clear="all"></P>
<P><I>Figure 1: The AST in its first stage</I></P>
<P>All objects that make up the nodes in the initial AST have a
<CODE><FONT face="courier, monospaced">parseContents()</FONT></CODE> method. This method is responsible for:</P>
<UL>
<LI>parsing the values of those attributes that contain XPath expressions
or patterns, breaking each expression/pattern into AST nodes and inserting
them into the tree.</LI>
<LI>reading/checking all other required attributes</LI>
<LI>propagate the <CODE><FONT face="courier, monospaced">parseContents()</FONT></CODE> call down the tree</LI>
</UL>
</FONT></TD></TR></TABLE><BR>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Mapping XPath expressions and patterns to additional AST nodes</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>The nodes that represent the XPath expressions and patterns extend
either the <CODE><FONT face="courier, monospaced">Expression</FONT></CODE> or <CODE><FONT face="courier, monospaced">Pattern</FONT></CODE> class
respectively. These nodes are not appended to the <CODE><FONT face="courier, monospaced">_contents</FONT></CODE>
vectory of each node, but rather stored as individual references in each
AST element node. One example is the <CODE><FONT face="courier, monospaced">ForEach</FONT></CODE> class that
represents the <CODE><FONT face="courier, monospaced">&lt;xsl:for-each&gt;</FONT></CODE> element. This class has
a variable that contains a reference to the AST sub-tree that represents
its <CODE><FONT face="courier, monospaced">select</FONT></CODE> attribute:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
private Expression _select;</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
<P>There is no standard way of storing these XPath expressions and each
AST node that contains one or more XPath expression/pattern must handle
that itself. This handling basically involves passing the attribute's
value to the XPath parser and receiving back an AST sub-tree.</P>
<P>With all XPath expressions/patterns expanded, the AST will look somewhat
like this:</P>
<P><IMG align="right" alt="ast_stage2.gif" border="0" hspace="4" src="images/ast_stage2.gif" vspace="4"><BR clear="all"></P>
<P><I>Fiugre 2: The AST in its second stage</I></P>
</FONT></TD></TR></TABLE><BR>
</FONT></TD></TR></TABLE><BR>
<A name="typecheck"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="666699" colspan="2" width="494"><TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="492"><FONT color="#ffffff" face="arial,helvetica,sanserif" size="+1"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Type-check and Cast Expressions</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="484"><FONT color="#000000" face="arial,helvetica,sanserif">
<P>In many cases we will need to typecast the top node in the expression
sub-tree to suit the expected result-type of the expression, or to typecast
child nodes to suit the allowed types for the various operators in the
expression. This is done by calling 'typeCheck()' on the root-node in the
XSL tree. Each SyntaxTreeNode node is responsible for inserting type-cast
nodes between itself and its child nodes or XPath nodes. These type-cast
nodes will convert the output-types of the child/XPath nodes to the expected
input-type of the parent node. Let look at our AST again and the node that
represents the <CODE><FONT face="courier, monospaced">&lt;xsl:value-of&gt;</FONT></CODE> element. This element
expects to receive a string from its <CODE><FONT face="courier, monospaced">select</FONT></CODE> XPath expression,
but the <CODE><FONT face="courier, monospaced">Step</FONT></CODE> expression will return either a node-set or a
single node. An extra node is inserted into the AST to perform the
necessary type conversions:</P>
<P><IMG align="right" alt="ast_stage3.gif" border="0" hspace="4" src="images/ast_stage3.gif" vspace="4"><BR clear="all"></P>
<P><I>Figure 3: XPath expression type cast</I></P>
<P>The <CODE><FONT face="courier, monospaced">typeCheck()</FONT></CODE> method of each SyntaxTreeNode object will
call <CODE><FONT face="courier, monospaced">typeCheck()</FONT></CODE> on each of its XPath expressions. This method
will return the native type returned by the expression. The AST node will
insert an additional type-conversion node if the return-type does not match
the expected data-type. Each possible return type is represented by a class
in the <CODE><FONT face="courier, monospaced">org.apache.xalan.xsltc.compiler.util</FONT></CODE> package. These
classes all contain methods that will generate bytecodes needed to perform
the actual type conversions (at runtime). The type-cast nodes in the AST
mainly consist of calls to these methods.</P>
</FONT></TD></TR></TABLE><BR>
<A name="compile"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="666699" colspan="2" width="494"><TABLE border="0" cellpadding="0" cellspacing="0" width="494"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="492"><FONT color="#ffffff" face="arial,helvetica,sanserif" size="+1"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>JVM byte-code generation</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="492"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="492"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="484"><FONT color="#000000" face="arial,helvetica,sanserif">
<UL>
<LI><A href="#stylesheet">Compiling the stylesheet</A></LI>
<LI><A href="#toplevel">Compiling top-level elements</A></LI>
<LI><A href="#templates">Compiling template code</A></LI>
<LI><A href="#instructions">Compiling instructions, functions expressions and patterns</A></LI>
</UL>
<P>Evey node in the AST extends the <CODE><FONT face="courier, monospaced">SyntaxTreeNode</FONT></CODE> base class
and implements the <CODE><FONT face="courier, monospaced">translate()</FONT></CODE> method. This method is
responsible for outputting the actual bytecodes that make up the
functionality required for each element, function, expression or pattern.
</P>
<A name="stylesheet"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Compiling the stylesheet</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>Some nodes in the AST require more complex code than others. The best
example is the <CODE><FONT face="courier, monospaced">&lt;xsl:stylesheet&gt;</FONT></CODE> element. The code that
represents this element has to tie together the code that is generated by
all the other elements and generate the actual class definition for the main
translet class. The <CODE><FONT face="courier, monospaced">Stylesheet</FONT></CODE> class generates the translet's
constructor and methods that handle all top-level elements.</P>
</FONT></TD></TR></TABLE><BR>
<A name="toplevel"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Compiling top-level elements</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>The bytecode that handles top-level elements must be generated before any
other code. The '<CODE><FONT face="courier, monospaced">translate()</FONT></CODE>' method in these classes are
mainly called from these methods in the Stylesheet class:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
private String compileBuildKeys(ClassGenerator);
private String compileTopLevel(ClassGenerator, Enumeration);
private void compileConstructor(ClassGenerator, Output);</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
<P>These methods handle most top-level elements, such as global variables
and parameters, <CODE><FONT face="courier, monospaced">&lt;xsl:output&gt;</FONT></CODE> and
<CODE><FONT face="courier, monospaced">&lt;xsl:decimal-format&gt;</FONT></CODE> instructions.</P>
</FONT></TD></TR></TABLE><BR>
<A name="templates"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Compiling template code</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>All XPath patterns in <CODE><FONT face="courier, monospaced">&lt;xsl:apply-template&gt;</FONT></CODE>
instructions are converted into numeric values (known as the pattern's
kernel 'type'). All templates with identical pattern kernel types are
grouped together and inserted into a table known as a test sequence.
(The table of test sequences is found in the Mode class in the compiler
package. There will be one such table for each mode that is used in the
stylesheet). This table is used to build a big <CODE><FONT face="courier, monospaced">switch()</FONT></CODE>
statement in the translet's <CODE><FONT face="courier, monospaced">applyTemplates()</FONT></CODE> method. This
method is initially called with the root node of the input document.</P>
<P>The <CODE><FONT face="courier, monospaced">applyTemplates()</FONT></CODE> method determines the node's type and
passes this type to the <CODE><FONT face="courier, monospaced">switch()</FONT></CODE> statement to look up the
matching template. The test sequence code (the <CODE><FONT face="courier, monospaced">TestSeq</FONT></CODE> class)
is responsible for inserting bytecodes to find one matching template
in cases where more than one template matches the current node type.</P>
<P>There may be several templates that share the same pattern kernel type.
Here are a few examples of templates with patterns that all have the same
kernel type:</P><DIV align="right"><TABLE border="0" cellpadding="0" cellspacing="4" width="464"><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#ffffff" width="462"><FONT size="-1"><PRE>
&lt;xsl:template match=&quot;A/C&quot;&gt;
&lt;xsl:template match=&quot;A/B/C&quot;&gt;
&lt;xsl:template match=&quot;A | C&quot;&gt;</PRE></FONT></TD><TD bgcolor="#0086b2" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" height="1" width="462"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="462"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></DIV>
<P>All these templates will be grouped under the type for
<CODE><FONT face="courier, monospaced">&lt;C&gt;</FONT></CODE> and will all get the same kernel type (the type for
<CODE><FONT face="courier, monospaced">&quot;C&quot;</FONT></CODE>). The last template will be grouped both under
<CODE><FONT face="courier, monospaced">&quot;C&quot;</FONT></CODE> and <CODE><FONT face="courier, monospaced">&quot;A&quot;</FONT></CODE>, since it matches either element.
If the type identifier for <CODE><FONT face="courier, monospaced">&quot;C&quot;</FONT></CODE> in this case is 8, all these
templates will be put under <CODE><FONT face="courier, monospaced">case 8:</FONT></CODE> in
<CODE><FONT face="courier, monospaced">applyTemplates()</FONT></CODE>'s big <CODE><FONT face="courier, monospaced">switch()</FONT></CODE> statement. The
<CODE><FONT face="courier, monospaced">TestSeq</FONT></CODE> class will insert some code under the
<CODE><FONT face="courier, monospaced">case 8:</FONT></CODE> statement (similar to if's and then's) in order to
determine which of the three templates to trigger.</P>
</FONT></TD></TR></TABLE><BR>
<A name="instructions"><!--anchor--></A>
<TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="666699" colspan="2" width="484"><TABLE border="0" cellpadding="0" cellspacing="0" width="484"><TR><TD bgcolor="#039acc" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#039acc" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#039acc" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#0086b2" width="482"><FONT color="#ffffff" face="arial,helvetica,sanserif"><IMG alt="" border="0" height="2" hspace="0" src="resources/void.gif" vspace="0" width="2"><B>Compiling instructions, functions, expressions and patterns</B></FONT></TD><TD bgcolor="#017299" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR><TR><TD bgcolor="#0086b2" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD><TD bgcolor="#017299" height="1" width="482"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="482"></TD><TD bgcolor="#017299" height="1" width="1"><IMG alt="" border="0" height="1" hspace="0" src="resources/void.gif" vspace="0" width="1"></TD></TR></TABLE></TD></TR><TR><TD width="10">&nbsp;</TD><TD width="474"><FONT color="#000000" face="arial,helvetica,sanserif" size="-1">
<P>The template code is generated by calling <CODE><FONT face="courier, monospaced">translate()</FONT></CODE> on
each <CODE><FONT face="courier, monospaced">Template</FONT></CODE> object in the abstract syntax tree. This call
will be propagated down the abstract syntax tree and every element will
output the bytecodes necessary to complete its task.</P>
<P>The Java Virtual Machine is stack-based, which goes hand-in-hand with
the tree structure of a stylesheet and the AST. A node in the AST will
call <CODE><FONT face="courier, monospaced">translate()</FONT></CODE> on its child nodes and any XPath nodes before
it generates its own bytecodes. In that way the correct sequence of JVM
instructions is generated. Each one of the child nodes is responsible of
creating code that leaves the node's output value (if any) on the stack.
The typical procedure for the parent node is to create JVM code that
consumes these values off the stack and then leave its own output on the
stack (for its parent).</P>
<P>The tree-structure of the stylesheet is in this way closely tied with
the stack-based JVM. The design does not offer any obvious way of extending
the compiler to output code for other non-stack-based VMs or processors.</P>
</FONT></TD></TR></TABLE><BR>
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Copyright &copy; 2004 The Apache Software Foundation.
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