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Log:
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<?xml version="1.0"?>
<!DOCTYPE s1 SYSTEM
"file:///C:\x\xml-stylebook\styles\apachexml\dtd\document.dtd">
<s1 title="Xalan-J 2.0 Design">
<p><link>Xalan-J 2.0 Design</link><img src="xmllogo.gif"
alt="xmllogo.gif"/></p>
<ul>
<li>Author: Scott Boag</li>
<li>State: In Progress</li>
<li><jump href="http://xml.apache.org/xalan-j/apidocs/index.html";>Xalan-J
2.0 Javadoc</jump></li>
</ul>
<s2 title="Introduction">
<p><link idref="intro">Introduction</link></p>
<p>This document presents the basic design for Xalan-J 2.0, which is a
<jump
href="http://www.awl.com/cseng/titles/0-201-89542-0/techniques/refactoring.htm";>refactoring</jump>
and redesign of the Xalan-J 1.x proces
sor. The main goals of this redesign are
to: </p>
<ol>
<li>Make the design and code more understandable by Open Source
people.</li>
<li>Reduce code size and complexity.</li>
<li>By simplifying the code, make optimization easier.</li>
<li>Make modules generally more localized, and less tangled
with other
modules.</li>
<li>Begin the adoption of the TrAX (Transformations for XML)
interfaces.</li>
<li>Increase the streamability of transformations.</li></ol>
<p>The techniques used toward these goals are to:</p>
<ol>
<li>In general, flatten the hierarchy of packages, in order to
make the
structure more apparent from the top-level view.</li>
<li>Break the construction and the validation of the XSLT
stylesheet from
the stylesheet objects themselves.</li>
<li>Drive the construction of the stylesheet through a table,
so that it
is less prone to error.</li>
<li>Break the transformation process into a separate package,
away from
the stylesheet objects.</li>
<li>Create this design document, as a start-point for people
wanting to
approach the code.</li>
</ol>
<p>The goals are not:</p>
<ol>
<li>To add more features in the progress of this refactoring.
This is
design and code clean-up, to meet the above-named goals. In
the course of the
refactoring, it is expected that it will be <em>much</em>
easier to add
features once this work is completed.</li>
<li>To optimize code for the sake of optimization. However, it
is
expected that the code will be faster once the work is
complete.</li>
</ol>
<p>How well we've achieved the goals will be measured by feedback from
the
<link
anchor="http://xml-archive.webweaving.org/xml-archive-xalan";>Xalan-dev</link>
list, and by software metrics tools.</p>
<p>Please note that the diagrams in this design document are meant to
be
useful abstractions, and may not always be exact.</p>
</s2>
<s2 title="Overview of Architecture">
<p><link idref="overview">Overview of Architecture</link></p>
<p>Xalan 2.0 is divided into four major modules, and various smaller
modules. The main modules are:</p>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/processor/package-summary.html";>org.apache.xalan.process</link></code></label>
<item>The module that processes the stylesheet, and provides
the main
entry point into Xalan.</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/templates/package-summary.html";>org.apache.xalan.templates</link></code></label>
<item>The module that defines the stylesheet structures,
including the
Stylesheet object, template element instructions, and
Attribute Value
Templates. </item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/transformer/package-summary.html";>org.apache.xalan.transformer</link></code></label>
<item>The module that applies the source tree to the Templates,
and
produces a result tree.</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xpath/package-summary.html";>org.apache.xpath</link></code></label>
<item>The module that processes both XPath expressions, and
XSLT Match
patterns.</item>
</gloss>
<p>In addition to the above modules, Xalan implements the
<link anchor="http://trax.openxml.org/";>TrAX</link> interfaces,
and depends on the
<link
anchor="http://www.megginson.com/SAX/Java/index.html";>SAX2</link> and <link
anchor="http://www.w3.org/TR/DOM-Level-2/";>DOM</link> packages.
</p><p><img src="trax.gif" alt="trax.gif"/></p><p>There is also a general
utilities package that contains both XML utility
classes such as QName, but generally useful classes such as
StringToIntTable.</p>
<p>In the diagram below, the dashed lines denote visibility. All
packages
access the SAX2 and DOM packages.</p>
<p><img src="xalan1_1x1.gif" alt="xalan1_1x1.gif"/></p>
<p>In addition to the above packages, there are the following
additional
packages:</p>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/client/package-summary.html";>org.apache.xalan.client</link></code></label>
<item>This package has a client applet. I suspect this should
be moved
into the samples directory.</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/extensions/package-summary.html";>org.apache.xalan.extensions</link></code></label>
<item>This holds classes belonging to the Xalan extensions
mechanism,
which allows Java code and script to be called from within a
stylesheet.</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/lib/package-summary.html";>org.apache.xalan.lib</link></code></label>
<item>This is the built-in Xalan extensions library, which holds
extensions such as Redirect (which allows a stylesheet to
produce multiple
output files).</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/res/package-summary.html";>org.apache.xalan.res</link></code></label>
<item>This holds resource files needed by Xalan, such as error
message
resources.</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/trace/package-summary.html";>org.apache.xalan.trace</link></code></label>
<item>This package contains classes and interfaces that allow a
caller to
add trace listeners to the transformation, allowing an
interface to XSLT
debuggers and similar tools.</item>
</gloss>
<gloss>
<label><code><link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/xslt/package-summary.html";>org.apache.xalan.xslt</link></code></label>
<item>This package is for backwards compatibility with
applications that
depend on Xalan 1.x interfaces.</item>
</gloss>
<p>A more conceptual view of this architecture is as follows:</p><p><img
src="conceptual.gif" alt="Picture of conceptual
architecture."/></p></s2><anchor name="process"/>
<s2 title="Process Module">
<p><link idref="process">Process Module</link></p>
<p>The <code>org.apache.xalan.process</code> module implements the
<code>org.apache.xalan.trax.Processor</code> interface, which
provides a
factory method for creating a concrete Processor instance, and
provides methods
for creating a <code>org.apache.xalan.trax.Templates</code>
instance, which, in
Xalan and XSLT terms, is the Stylesheet. Thus the task of the
process module is
to read the XSLT input in the form of a file, stream, SAX
events, or a DOM
tree, and produce a Templates/Stylesheet object.</p>
<p>The overall strategy is to define a schema that dictates the legal
structure for XSLT elements and attributes, and to associate
with those
elements construction-time processors that can fill in the
appropriate fields
in the top-level Stylesheet object, and also associate classes
in the templates
module that can be created in a generalized fashion. This makes
the validation
object-to-class associations centralized and declarative.</p>
<p>The schema's root class is
<code>org.apache.xalan.processor.XSLTSchema</code>, and it is
here that the
XSLT schema structure is defined. XSLTSchema uses
<code>org.apache.xalan.processor.XSLTElementDef</code> to
define elements, and
<code>org.apache.xalan.processor.XSLTAttributeDef</code> to
define attributes.
Both classes hold the allowed namespace, local name, and type
of element or
attribute. The XSLTElementDef also holds a reference to a
<code>org.apache.xalan.processor.XSLTElementProcessor</code>,
and a sometimes a
<code>Class</code> object, with which it can create objects
that derive from
<code>org.apache.xalan.templates.ElemTemplateElement</code>. In
addition, the
XSLTElementDef instance holds a list of XSLTElementDef
instances that define
legal elements or character events that are allowed as children
of the given
element.</p>
<p>The implementation of the
<code>org.apache.xalan.trax.Processor</code>
interface is in
<code>org.apache.xalan.processor.StylesheetProcessor</code>,
which creates a
<code>org.apache.xalan.processor.StylesheetHandler</code>
instance. This instance acts as the ContentHandler for the
parse events, and is
handed to the <code>org.xml.sax.XMLReader</code>, which the
StylesheetProcessor
uses to parse the XSLT document. The StylesheetHandler then
receives the parse
events, which maintains the state of the construction, and
passes the events on
to the appropriate XSLTElementProcessor for the given event, as
dictated by the
XSLTElementDef that is associated with the given event.</p>
<p><img src="process.gif" alt="process.gif"/></p>
</s2><anchor name="templates"/>
<s2 title="Templates Module">
<p><link idref="templates">Templates Module</link></p>
<p>The <code>org.apache.xalan.templates</code> module implements the
<code>org.apache.xalan.trax.Templates</code> interface, and
defines a set of
classes that represent a Stylesheet. The primary purpose of
this module is to
hold stylesheet data, not to perform procedural tasks
associated with the
construction of the data, nor tasks associated with the
transformation itself.
</p>
<p>A <code>StylesheetRoot</code>, which implements the
<code>Templates</code> interface, is a type of
<code>StylesheetComposed</code>,
which is a <code>Stylesheet</code> composed of itself and all
included
<code>Stylesheet</code> objects. A <code>StylesheetRoot</code>
has a global
imports list, which is a list of all imported
<code>StylesheetComposed</code>
instances. From each <code>StylesheetComposed</code> object,
one can iterate
through the list of directly or indirectly included
<code>Stylesheet</code>
objects, and one call also iterate through the list of all
<code>StylesheetComposed</code> objects of lesser import
precedence.
<code>StylesheetRoot</code> is a
<code>StylesheetComposed</code>, which is a
<code>Stylesheet</code>.</p>
<p>Each stylesheet has a set of properties, which can be set by various
means, usually either via an attribute on xsl:stylesheet, or
via a top-level
xsl instruction (for instance, xsl:attribute-set). The get
methods for these
properties only access the declaration within the given
<code>Stylesheet</code>
object, and never takes into account included or imported
stylesheets. The
<code>StylesheetComposed</code> derivative object, if it is a
root
<code>Stylesheet</code> or imported <code>Stylesheet</code>,
has "composed"
getter methods that do take into account imported and included
stylesheets, for
some of these properties. The table of Stylesheet properties,
with composed
methods, is as follows. Note that the names of the attributes
are according to
a formula for translating the xsl names to the Java get/set
method names.</p>
<table>
<tr>
<th>Property</th>
<th>Type</th>
<th>XSL Origin</th>
<th>Composed Methods</th>
<th>Note</th>
</tr>
<tr>
<td>XmlnsXsl</td>
<td>String</td>
<td>xmlns:xsl</td>
<td>(none, applies to stylesheet only)</td>
<td></td>
</tr>
<tr>
<td>ExtensionElementPrefixes</td>
<td>StringVector</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#extension-element";>extension-element-prefixes</jump></code>
attribute</td>
<td>(none, applies to stylesheet only)</td>
<td></td>
</tr>
<tr>
<td>ExcludeResultPrefixes</td>
<td>StringVector</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#literal-result-element";>exclude-result-prefixes
or xsl:exclude-result-prefixes</jump></code>
attributes</td>
<td>(not sure about this... only from root?)</td>
<td>I think this should be a root method, and a single list
should be
made, like with xsl:output.</td>
</tr>
<tr>
<td>Id</td>
<td>String</td>
<td>The <code><jump
href="http://www.w3.org/TR/xslt#section-Embedding-Stylesheets";>id</jump></code>
attribute</td>
<td>(none, applies to stylesheet only)</td>
<td></td>
</tr>
<tr>
<td>Version</td>
<td>String</td>
<td>The <code><jump
href="http://www.w3.org/TR/xslt#forwards";>version</jump></code> attribute</td>
<td>(none, applies to stylesheet only)</td>
<td></td>
</tr>
<tr>
<td>XmlSpace</td>
<td>boolean</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#strip";>xml:space</jump></code> attribute</td>
<td>(none, applies to stylesheet only)</td>
<td></td>
</tr>
<tr>
<td>Import</td>
<td>Vector (list of StylesheetComposed objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#import";>xsl:import</jump></code> element</td>
<td>getImportComposed(int i) / getImportCountComposed()</td>
<td>Composed list contains all imported sheets, not the
importing sheet
itself.</td>
</tr>
<tr>
<td>Include</td>
<td>Vector (list of Stylesheet objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#include";>xsl:include</jump></code> element</td>
<td>getIncludeComposed(int i) /
getIncludeCountComposed()</td>
<td>Composed list contains all directly or indirectly included
stylesheets.</td>
</tr>
<tr>
<td>DecimalFormat</td>
<td>Stack (list of DecimalFormatProperties objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#format-number";>xsl:decimal-format</jump></code>
element</td>
<td>getDecimalFormatComposed(QName name)</td>
<td></td>
</tr>
<tr>
<td>StripSpaces</td>
<td>Stack (list of XPath match pattern objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#strip";>xsl:strip-space</jump></code>
element</td>
<td>getWhiteSpaceInfo(TransformerImpl transformContext, Node
sourceTree, Element targetElement)</td>
<td></td>
</tr>
<tr>
<td>PreserveSpaces</td>
<td>Stack (list of XPath match pattern objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#strip";>xsl:preserve-space</jump></code>
element</td>
<td>getWhiteSpaceInfo(TransformerImpl transformContext, Node
sourceTree, Element targetElement)</td>
<td></td>
</tr>
<tr>
<td>Output</td>
<td>OutputFormatExtended</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#output";>xsl:output</jump></code> element</td>
<td>getOutputComposed() on StylesheetRoot only</td>
<td></td>
</tr>
<tr>
<td>Key</td>
<td>Vector (list of KeyDeclaration objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#key";>xsl:key</jump></code> element</td>
<td>getKeysComposed()</td>
<td></td>
</tr>
<tr>
<td>AttributeSet</td>
<td>Vector (list of ElemAttributeSet objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#attribute-sets";>xsl:attribute-set</jump></code>
element</td>
<td>On StylesheetRoot only?</td>
<td></td>
</tr>
<tr>
<td>Variable</td>
<td>Vector (list of ElemVariable objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#top-level-variables";>xsl:variable</jump></code>
element</td>
<td></td>
<td></td>
</tr>
<tr>
<td>Param</td>
<td>Vector (list of ElemParam objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#top-level-variables";>xsl:param</jump></code>
element</td>
<td></td>
<td></td>
</tr>
<tr>
<td>Template</td>
<td>Vector (list of ElemTemplate objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#section-Defining-Template-Rules";>xsl:template</jump></code>
element</td>
<td>getTemplateComposed(TransformerImpl transformContext, Node
sourceTree, Node targetNode, QName mode) and
getTemplateComposed(QName
qname)</td>
<td></td>
</tr>
<tr>
<td>NamespaceAlias</td>
<td>Vector (list of ElemTemplate objects)</td>
<td><code><jump
href="http://www.w3.org/TR/xslt#literal-result-element";>xsl:namespace-alias</jump></code>
element</td>
<td>On StylesheetRoot only?</td>
<td></td>
</tr>
<tr>
<td>NonXslTopLevel</td>
<td>Hashtable (table of opaque objects keyed by QName)</td>
<td>Any top-level non-xslt element.</td>
<td>none.</td>
<td></td>
</tr>
<tr>
<td>Href</td>
<td>URL</td>
<td>The location of the stylesheet, possibly set by
xsl:include or
xsl:import.</td>
<td>none.</td>
<td></td>
</tr>
<tr>
<td>StylesheetRoot</td>
<td>StylesheetRoot</td>
<td>The root of the stylesheet tree, for quick access.</td>
<td>none.</td>
<td></td>
</tr>
<tr>
<td>StylesheetParent</td>
<td>Stylesheet</td>
<td>The importing or including stylesheet.</td>
<td>none.</td>
<td></td>
</tr>
<tr>
<td>StylesheetComposed</td>
<td>StylesheetComposed</td>
<td>The closest importing stylesheet.</td>
<td>none.</td>
<td></td>
</tr>
<tr>
<td>NamespaceDecls</td>
<td>Linked list of NameSpace elements</td>
<td>xmlns:foo attribute map</td>
<td>(none, applies to stylesheet only)</td>
<td></td>
</tr>
</table>
</s2><anchor name="transformer"/>
<s2 title="Transformer Module">
<p><link idref="transformer">Transformer Module</link></p>
<p>The <link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/transformer/package-summary.html";>Transformer</link>
module is in charge of run-time transformations. The <link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/transformer/TransformerImpl.html";>TransformerImpl</link>
object, which implements the TrAX <link
anchor="http://trax.openxml.org/javadoc/trax/Transformer.html";>Transformer</link>
interface, and has an association with a <link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/templates/StylesheetRoot.html";>StylesheetRoot</link>
object, begins the processing of the source tree (or provides a <link
anchor="http://www.megginson.com/SAX/Java/javadoc/org/xml/sax/ContentHandler.html";>ContentHandler</link>
reference), and performs the transformation. The Transformer package does as
much of the transformation as it can, but element level operations are
generally performed in the <link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/templ
ates/ElemTemplateElement.html#execute(org.apache.xalan.transformer.TransformerImpl,
org.w3c.dom.Node,
org.apache.xalan.utils.QName)">ElemTemplateElement.execute(...)</link>
methods.</p><p>Result Tree events are fed into a <link
anchor="http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/transformer/ResultTreeHandler.html";>ResultTreeHandler</link>
object, which acts as a layer between the direct calls to the result
tree content handler (often a Serializer), and the Transformer. For one
thing,
we have to delay the call to
startElement(name, atts) because of the
xsl:attribute and xsl:copy calls. In other words,
the attributes have to be fully collected before you
can call startElement.</p><p>Other important classes in this package
are:</p><gloss><label>CountersTable and Counter</label><item>The Counter class
does incremental counting for support of xsl:number.
This class stores a cache of counted nodes (m_countNodes).
It tries to cache the counted nodes in document order...
the node count is based on its position in the cache list. The
CountersTable class is a table of counters, keyed by ElemNumber objects, each
of which has a list of Counter
objects.</item></gloss><gloss><label>KeyIterator, KeyManager, and
KeyTable</label><item>These classes handle mapping of keys declared with the
xsl:key element.</item></gloss><gloss><label>TransformState</label><item>This
interface is meant to be used by a consumer of SAX2 events produced by Xalan,
and enables the consumer
to get information about the state of the transform. It
is primarily intended as a tooling interface.</item></gloss><p>Even though
the following modules are defined in the org.apache.xalan package, instead of
the transformer package, they are defined in this section as they are mostly
related to runtime transformation.</p>
<s3 title="Stree Module"><p><link idref="stree">Stree Module [And
discussions about streaming]</link></p><p>The Stree module implements the
default <link anchor="http://www.w3.org/TR/xpath#data-model";>Source Tree
</link> for Xalan, that is to be transformed. It implements read-only <link
anchor="http://www.w3.org/TR/DOM-Level-2/";>DOM2</link> interfaces, and provides
some information needed for fast transforms, such as document order indexes.
It also attempts to allow a streaming transform by launching the transform on a
secondary thread as soon as the SAX2 <link
anchor="http://www.megginson.com/SAX/Java/javadoc/org/xml/sax/ContentHandler.html#startDocument()">StartDocument</link>
event has occured. When the transform requests a node, and node is not
present, the getFirstChild and GetNextSibling methods will wait until the child
node has arrived, or an <link
anchor="http://www.megginson.com/SAX/Java/javadoc/org/xml/sax/ContentHandler.html#endElement(java.lang.String,%20java.lang.String,%20java.lang.Str
ing)">endElement</link> event has occured.</p><p>Note that the secondary thread
is an issue. It would be better to do the same thing as described above on a
single thread, but using the parser in 'pull' mode, or simply with a parseNext
method so the parse would occur in blocks.</p><p>This kind of streaming is not
perfect because it still requires an entire source tree to be concretely built.
There have been a lot of good discussions on the xalan-dev list about how to
do static analysis of a stylesheet, and be able to allocate only the nodes
needed by the transform, while they are needed (or not allocate source objects
at all).</p><p>Vincent-Olivier Arsenault <[EMAIL PROTECTED]> has proposed
the following design:</p><p>By looking at the stylesheet you know how
streamable it is (of course this
needs strict adherence to the xslt recommendation). since there's a root
template and no <xsl:apply-templates/> you can build your context list
containing only absolute x-path (which means nodes get out of context
faster).</p>
<p>The paths of the relevant nodes, for this stylesheet, are (ok this is an
example, so I may be missing some):</p>
<ol>
<li>path: "/address" context: "address" (at </address>, you get rid of
the
whole "person/address" stuff);</li>
<li>path: "/adn" context: "adn";</li>
<li>path: "/medicalrecord" context: "/" (for possibly repetitive nodes, the
context is always the parent node).</li>
</ol>
<p>And all the rest goes to trash!!!!</p>
<p>Let me refine:</p>
<p>you analyze the whole stylesheet like that (would be good if optimization
and x-path list could be done simultaneously) and you end up with a list of
expanded paths mapped to all the templates.</p>
<p>An entry in the list (i would call this list the transformation stack)
would
consist of 4 things:</p>
<ol>
<li>the relevance context xpath (on which the input nodes will be tested for
pertinence: do we keep it of not);</li>
<li>the transformation rule to apply to the matching nodes (this can just be a
forwarder to another template transformation stack);</li>
<li>a result buffer (in which the nodes that can't be streamed are temporarily
stored);</li>
<li>the streaming context xpath (triggers streaming of the buffer to the
output).</li>
</ol>
</s3><s3 title="Extensions Module"><p><link idref="extensions">Extensions
Module</link></p><p>This package contains an implementation of Xalan Extension
Mechanism, which uses the <link
anchor="http://oss.software.ibm.com/developerworks/opensource/bsf/";>Bean
Scripting Framework</link>.
The Bean Scripting Framework (BSF) is an architecture for incorporating
scripting into Java applications and applets. Scripting languages such as
Netscape Rhino (Javascript), VBScript, Perl, Tcl, Python, NetRexx and Rexx can
be used to augment XSLT's functionality. In addition, the Xalan extension
mechanism allows use of Java classes. See the <link
anchor="http://xml.apache.org/xalan/extensions.html";>XalanJ 1 extension
documentation</link> for a description of using extensions in a stylesheet.
Please note that the W3C XSL Working Group is working on a specification for
standard extension bindings, and this module will change to follow that
specification. </p><p>[More needed... -sb]</p></s3></s2><anchor name="xpath"/>
<s2 title="XPath Module">
<p><link idref="xpath">XPath Module</link></p>
<p>This module is pulled out of the Xalan package, and put in the
org.apache package, to emphasize that the intention is that this package can be
used independently of the XSLT engine, even though it has dependencies on the
Xalan utils module.</p><p><img src="org_apache.gif" alt="xalan --->
xpath"/></p>
<p>The XPath module first compiles the XPath strings into expression trees,
and then executes these expressions via a call to the XPath execute(...)
function. </p> <p>Major classes
are:</p><gloss><label>XPath</label><item>Represents a compiled XPath. Major
function is <code>XObject execute(XPathContext xctxt, Node contextNode,
PrefixResolver
namespaceContext).</code></item></gloss><gloss><label>XPathAPI</label><item>The
methods in this class are convenience methods into the
low-level XPath
API.</item></gloss><gloss><label>XPathContext</label><item>Used as the runtime
execution context for
XPath.</item></gloss><gloss><label>DOMHelper</label><item>Used as a helper for
handling DOM issues. May be subclassed to take advantage
of specific DOM
implementations.</item></gloss><gloss><label>SourceTreeManager</label><item>bottlenecks
all management of source trees. The methods
in this class should allow easy garbage collection of source
trees, and should centralize parsing for those source
trees.</item></gloss><gloss><label>Expression</label><item>The base-class of
all expression objects, allowing polymorphic behaviors.</item></gloss><p>The
general architecture of the XPath module is diveded into the compiler, and
categories of expression objects.</p><p><img src="xpath.gif" alt="xpath
modules"/></p><p>The most important module is the axes module. This module
implements the DOM2 <link
anchor="http://www.w3.org/TR/DOM-Level-2/traversal.html#Iterator-overview";>NodeIterator</link>
interface, and is meant to allow XPath clients to either override the default
behavior or to replace this behavior.</p><p>The LocPathIterator and
UnionPathIterator classes implement the <link
anchor="http://www.w3.org/TR/DOM-Level-2/java-binding.html#org.w3c.dom.traversal.NodeIterator";>NodeIterator</link>
interface, and polymorphically use AxesWalker derived objects to execute each
step in the path. The whole trick is to execute the LocationPath in
depth-first do
cument order so that nodes can be found without necessarily looking ahead or
performing a bredth-first search.</p><s3 title="XPath Database
Connection"><p><link idref="xpath-database">XPath Direct Database
Connections</link></p><p>An important part of the XPath design in both Xalan 1
and Xalan 2, is to enable database connections to be used as drivers directly
to the XPath <link
anchor="http://www.w3.org/TR/xpath#location-paths";>LocationPath</link>
handling. This allows databases to be directly connected to the transform, and
be able to take advantage of internal indexing and the like. While in Xalan 1
this was done via the <link
anchor="http://xml.apache.org/xalan/apidocs/org/apache/xalan/xpath/XLocator.html";>XLocator</link>
interface, in Xalan 2 this interface is no longer used, and has been replaced
by the DOM2 <link
anchor="http://www.w3.org/TR/DOM-Level-2/traversal.html#Iterator-overview";>NodeIterator</link>
interface. An application or extension should be able to install their own
NodeIterator for a
given document.</p><p><img src="data.gif" alt="data.gif"/></p><p>[More to
do]</p></s3></s2>
<s2 title="Utils Package">
<p><link idref="utils">Utils Package</link></p>
<p>This package contains general utilities for use by both the xalan and
xpath packages. It is the intention that many of these utility classes (or
their equivelents) be eventually brought into the org.apache.xml package for
general use. The list of major utilities are as
follows:</p><gloss><label>AttList</label><item>Wraps a DOM attribute list in a
SAX Attributes.</item></gloss><gloss><label>BoolStack, IntStack, IntVector,
etc.</label><item>Simple stacks and vectors for primative
values.</item></gloss><gloss><label>DefaultErrorHandler</label><item>Implements
SAX error handler for default
reporting.</item></gloss><gloss><label>DOMBuilder</label><item>Takes SAX events
(in addition to some extra events
that SAX doesn't handle yet) and adds the result to a document
or document fragment.</item></gloss><gloss><label>Heap</label><item>Classic
heap
implementation.</item></gloss><gloss><label>MutableAttrListImpl</label><item>Mutable
version of
AttributesImpl.</item></gloss><gloss><label>NameSpace</label><item>A
representation of a
namespace.</item></gloss><gloss><label>NodeVector</label><item>A very simple
table that stores a list of
Nodes.</item></gloss><gloss><label>ObjectPool</label><item>Used for reuse of
objects.</item></gloss><gloss><label>PrefixResolver</label><item>The class that
implements this interface can resolve prefixes
to
namespaces.</item></gloss><gloss><label>PrefixResolverDefault</label><item>This
class implements a generic PrefixResolver for a DOM, that
can be used to perform prefix-to-namespace lookup
for an XPath.</item></gloss><gloss><label>QName</label><item>Class to
represent a qualified XML
name.</item></gloss><gloss><label>StringToStringTable</label><item>A very
simple lookup table that stores a list of strings for lookup. Used when a
hashtable is too much
overhead.</item></gloss><gloss><label>SystemIDResolver</label><item>Able to
take a SystemID string and try and turn it into a good absolute
URL.</item></gloss><gloss><label>TreeWalker</label><item>Implements a Visitor
design pattern, doing a pre-order walk of the DOM tree, calling a
ContentHandler interface as it goes. Used for DOM-to-SAX
conversion.</item></gloss><gloss><label>Trie</label><item>A digital search trie
for 7-bit ASCII text.</item></gloss><gloss><label>UnImplNode</label><item>To be
subclassed by classes that wish to act as DOM nodes, without having to
implement all the methods. Widely used.</item></gloss></s2>
<s2 title="Other Packages">
<p><link idref="other">Other Packages</link></p>
<gloss><label>client</label><item>Implementation of Xalan Applet
[should we keep this?].
</item></gloss>
<gloss><label>dtm</label><item>Implementation of the Document
Table Model (DTM) [Should we keep this?].</item></gloss>
<gloss><label>extensions</label><item>Implementation of Xalan
Extension Mechanism, which uses the Bean Scripting Framework.</item></gloss>
<gloss><label>lib</label><item>Implementation of Xalan-specific
extensions [I want to add lots more extensions to this
package!].</item></gloss><gloss><label>res</label><item>Contains strings that
require internationalization.</item></gloss></s2>
<s2 title="Coding Conventions">
<p><link idref="coding-conventions">Coding Conventions</link></p>
<p>This section documents the coding conventions used in the Xalan
source.</p>
<ol>
<li>Class files are arranged with constructors and possibly an
init()
function first, public API methods second, package specific,
protected, and
private methods following (arranged based on related
functionality), member
variables with their getter/setter access methods last.</li>
<li>Non-static member variables are prefixed with "m_".</li>
<li>static final member variables should always be upper case,
without
the "m_" prefix. They need not have accessors.</li>
<li>Private member variables that are not accessed outside the
class need
not have getter/setter methods declared.</li>
<li>Private member variables that are accessed outside the
class should
have either package specific or public getter/setter methods
declared. All
accessors should follow the bean design patterns.</li>
<li>Package-scoped member variables, public member variables,
and
protected member variables should not be declared.</li>
</ol>
</s2>
<s2 title="Open Issues">
<p><link idref="open-issues">Open Issues</link></p>
<p>This section documents architectural and design issues that I still
consider to be open or unsolved. (This list is ongoing, and
will change over
time... it's simply a place for me to note problems that are
ongoing and need
to be solved.)</p>
<gloss>
<label>Space stripping</label>
<item>In Xalan 1.x, it is clear that space stripping was a major
performance issue. This needs to be solved in Xalan 2.0 by
stripping the
space nodes as the document is being parsed. This is a major
problem though for
DOM trees. This can be perhaps be solved by preprocessing the
DOM tree and
creating a table of space-stripping parent elements, when the
nodes can't be
pre-stripped.</item>
</gloss>
</s2>
</s1>
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