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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Copyright (c) 2000, 2005, Oracle and/or its affiliates. All rights reserved.
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DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<title>Transformation API For XML</title>
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<meta name="CVS"
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content="$Id: overview.html,v 1.2 2005/06/10 03:50:39 jeffsuttor Exp $" />
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<meta name="AUTHOR"
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content="Jeff.Suttor@Sun.com" />
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</head>
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<body>
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<h2>Transformation API For XML</h2>
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<h3>Introduction</h3>
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<p>This overview describes the set of APIs contained in
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javax.xml.transform. For the sake of brevity, these interfaces are referred to
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as TrAX (Transformations for XML). </p>
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<p>There is a broad need for Java applications to be able to transform XML
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and related tree-shaped data structures. In fact, XML is not normally very
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useful to an application without going through some sort of transformation,
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unless the semantic structure is used directly as data. Almost all XML-related
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applications need to perform transformations. Transformations may be described
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by Java code, Perl code, <A href="http://www.w3.org/TR/xslt">XSLT</A>
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Stylesheets, other types of script, or by proprietary formats. The inputs, one
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or multiple, to a transformation, may be a URL, XML stream, a DOM tree, SAX
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Events, or a proprietary format or data structure. The output types are the
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pretty much the same types as the inputs, but different inputs may need to be
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combined with different outputs.</p>
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<p>The great challenge of a transformation API is how to deal with all the
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possible combinations of inputs and outputs, without becoming specialized for
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any of the given types.</p>
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<p>The Java community will greatly benefit from a common API that will
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allow them to understand and apply a single model, write to consistent
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interfaces, and apply the transformations polymorphically. TrAX attempts to
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define a model that is clean and generic, yet fills general application
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requirements across a wide variety of uses. </p>
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<h3>General Terminology</h3>
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<p>This section will explain some general terminology used in this
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document. Technical terminology will be explained in the Model section. In many
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cases, the general terminology overlaps with the technical terminology.</p>
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<ul>
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<li>
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<p>
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<b>Tree</b>
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<br>This term, as used within this document, describes an
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abstract structure that consists of nodes or events that may be produced by
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XML. A Tree physically may be a DOM tree, a series of well balanced parse
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events (such as those coming from a SAX2 ContentHander), a series of requests
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(the result of which can describe a tree), or a stream of marked-up
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characters.</p>
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</li>
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<li>
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<p>
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<b>Source Tree(s)</b>
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<br>One or more trees that are the inputs to the
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transformation.</p>
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</li>
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<li>
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<p>
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<b>Result Tree(s)</b>
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<br>One or more trees that are the output of the
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transformation.</p>
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</li>
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<li>
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<p>
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<b>Transformation</b>
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<br>The processor of consuming a stream or tree to produce
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another stream or tree.</p>
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</li>
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<li>
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<p>
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<b>Identity (or Copy) Transformation</b>
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<br>The process of transformation from a source to a result,
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making as few structural changes as possible and no informational changes. The
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term is somewhat loosely used, as the process is really a copy. from one
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"format" (such as a DOM tree, stream, or set of SAX events) to
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another.</p>
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</li>
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<li>
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<p>
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<b>Serialization</b>
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<br>The process of taking a tree and turning it into a stream. In
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some sense, a serialization is a specialized transformation.</p>
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</li>
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<li>
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<p>
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<b>Parsing</b>
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<br>The process of taking a stream and turning it into a tree. In
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some sense, parsing is a specialized transformation.</p>
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</li>
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<li>
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<p>
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<b>Transformer</b>
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<br>A Transformer is the object that executes the transformation.
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</p>
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</li>
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<li>
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<p>
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<b>Transformation instructions</b>
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<br>Describes the transformation. A form of code, script, or
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simply a declaration or series of declarations.</p>
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</li>
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<li>
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<p>
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<b>Stylesheet</b>
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<br>The same as "transformation instructions," except it is
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likely to be used in conjunction with <A href="http://www.w3.org/TR/xslt">XSLT</A>.</p>
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</li>
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<li>
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<p>
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<b>Templates</b>
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<br>Another form of "transformation instructions." In the TrAX
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interface, this term is used to describe processed or compiled transformation
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instructions. The Source flows through a Templates object to be formed into the
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Result.</p>
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</li>
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<li>
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<p>
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<b>Processor</b>
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<br>A general term for the thing that may both process the
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transformation instructions, and perform the transformation.</p>
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</li>
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<li>
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<p>
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<b>DOM</b>
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<br>Document Object Model, specifically referring to the
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<A href="#http://www.w3.org/TR/DOM-Level-2%20">Document Object Model
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(DOM) Level 2 Specification</A>.</p>
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</li>
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<li>
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<p>
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<b>SAX</b><br>
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Simple API for XML, specifically referring to the <a href="http://sax.sourceforge.net/">SAX 2.0.2 release</a>.
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</p>
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</li>
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</ul>
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<h3>Model</h3>
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<p>The section defines the abstract model for TrAX, apart from the details
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of the interfaces.</p>
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<p>A TRaX <A href="#pattern-TransformerFactory">TransformerFactory</A> is an object
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that processes transformation instructions, and produces
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<A href="#pattern-Templates">Templates</A> (in the technical
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terminology). A <A href="#pattern-Templates">Templates</A>
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object provides a <A href="#pattern-Transformer">Transformer</A>, which transforms one or
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more <A href="#pattern-Source">Source</A>s into one or more
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<A href="#pattern-Result">Result</A>s.</p>
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<p>To use the TRaX interface, you create a
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<A href="#pattern-TransformerFactory">TransformerFactory</A>,
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which may directly provide a <A href="#pattern-Transformers">Transformers</A>, or which can provide
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<A href="#pattern-Templates">Templates</A> from a variety of
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<A href="#pattern-Source">Source</A>s. The
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<A href="#pattern-Templates">Templates</A> object is a processed
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or compiled representation of the transformation instructions, and provides a
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<A href="#pattern-Transformer">Transformer</A>. The
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<A href="#pattern-Transformer">Transformer</A> processes a
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<A href="#pattern-Transformer">Source</A> according to the
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instructions found in the <A href="#pattern-Templates">Templates</A>, and produces a
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<A href="#pattern-Result">Result</A>.</p>
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<p>The process of transformation from a tree, either in the form of an
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object model, or in the form of parse events, into a stream, is known as
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<code>serialization</code>. We believe this is the most suitable term for
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this process, despite the overlap with Java object serialization.</p>
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<H3>TRaX Patterns</H3>
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<ul>
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<p>
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<b><a name="pattern-Processor">Processor</a></b>
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<br>
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<br>
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<i>Intent: </i>Generic concept for the
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set of objects that implement the TrAX interfaces.<br>
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<i>Responsibilities: </i>Create compiled transformation instructions, transform
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sources, and manage transformation parameters and
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properties.<br>
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<i>Thread safety: </i>Only the Templates object can be
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used concurrently in multiple threads. The rest of the processor does not do
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synchronized blocking, and so may not be used to perform multiple concurrent
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operations. Different Processors can be used concurrently by different
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threads.</p>
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<p>
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<b><a name="pattern-TransformerFactory">TransformerFactory</a></b>
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<br>
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<br>
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<i>Intent: </i>Serve as a vendor-neutral Processor interface for
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<A href="http://www.w3.org/TR/xslt">XSLT</A> and similar
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processors.<br>
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<i>Responsibilities: </i>Serve as a factory for a concrete
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implementation of an TransformerFactory, serve as a direct factory for
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Transformer objects, serve as a factory for Templates objects, and manage
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processor specific features.<br>
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<i>Thread safety: </i>A
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TransformerFactory may not perform mulitple concurrent
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operations.</p>
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<p>
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<b><a name="pattern-Templates">Templates</a></b>
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<br>
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<br>
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<i>Intent: </i>The
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runtime representation of the transformation instructions.<br>
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<i>Responsibilities: </i>A data bag for transformation instructions; act as a factory
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for Transformers.<br>
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<i>Thread safety: </i>Threadsafe for concurrent
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usage over multiple threads once construction is complete.</p>
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<p>
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<b><a name="pattern-Transformer">Transformer</a></b>
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<br>
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<br>
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<i>Intent: </i>Act as a per-thread
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execution context for transformations, act as an interface for performing the
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transformation.<br>
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<i>Responsibilities: </i>Perform the
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transformation.<br>
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<i>Thread safety: </i>Only one instance per thread
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is safe.<br>
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<i>Notes: </i>The Transformer is bound to the Templates
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object that created it.</p>
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<p>
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<b><a name="pattern-Source">Source</a></b>
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<br>
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<br>
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<i>Intent: </i>Serve as a
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single vendor-neutral object for multiple types of input.<br>
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<i>Responsibilities: </i>Act as simple data holder for System IDs, DOM nodes, streams,
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etc.<br>
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<i>Thread safety: </i>Threadsafe concurrently over multiple
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threads for read-only operations; must be synchronized for edit
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operations.</p>
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<p>
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<b><a name="pattern-Result">Result</a></b>
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<br>
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<br>
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<i>Potential alternate name: </i>ResultTarget<br>
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<i>Intent: </i>Serve
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as a single object for multiple types of output, so there can be simple process
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method signatures.<br>
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<i>Responsibilities: </i>Act as simple data holder for
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output stream, DOM node, ContentHandler, etc.<br>
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<i>Thread safety: </i>Threadsafe concurrently over multiple threads for read-only,
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must be synchronized for edit.</p>
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</ul>
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</body>
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</html>
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