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| <s1 title="Transformation API For XML"> |
| <ul> |
| <li><link anchor="intro">Introduction</link></li> |
| <li><link anchor="terminology">General Terminology</link></li> |
| <li><link anchor="requirements">Requirements</link></li> |
| <li><link anchor="model">Model</link></li> |
| <li><link anchor="patterns">Patterns</link></li> |
| </ul><anchor name="intro"/> |
| <s2 title="Introduction"> |
| <p>This document describes the transformation API for XML (TrAX), the set of APIs contained in |
| <jump href="apidocs/javax/xml/transform/package-summary.html">javax.xml.transform</jump>, <jump href="apidocs/javax/xml/transform/stream/package-summary.html">javax.xml.transform.stream</jump>, <jump href="apidocs/javax/xml/transform/dom/package-summary.html">javax.xml.transform.dom</jump>, and <jump href="apidocs/javax/xml/transform/sax/package-summary.html">javax.xml.transform.sax</jump>.</p> |
| |
| <p>There is a broad need for Java applications to be able to transform XML |
| and related tree-shaped data structures. In fact, XML is not normally very |
| useful to an application without going through some sort of transformation, |
| unless the semantic structure is used directly as data. Almost all XML-related |
| applications need to perform transformations. Transformations may be described |
| by Java code, Perl code, <jump href="http://www.w3.org/TR/xslt">XSLT</jump> |
| Stylesheets, other types of script, or by proprietary formats. The inputs, one |
| or multiple, to a transformation, may be a URL, XML stream, a DOM tree, SAX |
| Events, or a proprietary format or data structure. The output types are the |
| pretty much the same types as the inputs, but different inputs may need to be |
| combined with different outputs.</p> |
| |
| <p>The great challenge of a transformation API is how to deal with all the |
| possible combinations of inputs and outputs, without becoming specialized for |
| any of the given types.</p> |
| |
| <p>The Java community will greatly benefit from a common API that will |
| allow them to understand and apply a single model, write to consistent |
| interfaces, and apply the transformations polymorphically. TrAX attempts to |
| define a model that is clean and generic, yet fills general application |
| requirements across a wide variety of uses. </p> |
| |
| </s2><anchor name="terminology"/> |
| <s2 title="General Terminology"> |
| |
| <p>This section will explain some general terminology used in this |
| document. Technical terminology will be explained in the Model section. In many |
| cases, the general terminology overlaps with the technical terminology.</p> |
| |
| <gloss> |
| <label>Tree</label> |
| <item>This term, as used within this document, describes an |
| abstract structure that consists of nodes or events that may be produced by |
| XML. A Tree physically may be a DOM tree, a series of well balanced parse |
| events (such as those coming from a SAX2 ContentHander), a series of requests |
| (the result of which can describe a tree), or a stream of marked-up |
| characters.</item> |
| |
| <label>Source Tree(s)</label> |
| <item>One or more trees that are the inputs to the |
| transformation.</item> |
| |
| <label>Result Tree(s)</label> |
| <item>One or more trees that are the output of the |
| transformation.</item> |
| |
| |
| |
| <label>Transformation</label> |
| <item>The process of consuming a stream or tree to produce |
| another stream or tree.</item> |
| |
| |
| |
| <label>Identity (or Copy) Transformation</label> |
| <item>The process of transformation from a source to a result, |
| making as few structural changes as possible and no informational changes. The |
| term is somewhat loosely used, as the process is really a copy. from one |
| "format" (such as a DOM tree, stream, or set of SAX events) to |
| another.</item> |
| |
| |
| |
| <label>Serialization</label> |
| <item>The process of taking a tree and turning it into a stream. In |
| some sense, a serialization is a specialized transformation.</item> |
| |
| |
| |
| <label>Parsing</label> |
| <item>The process of taking a stream and turning it into a tree. In |
| some sense, parsing is a specialized transformation.</item> |
| |
| |
| |
| <label>Transformer</label> |
| <item>A Transformer is the object that executes the transformation. |
| </item> |
| |
| |
| |
| <label>Transformation instructions</label> |
| <item>Describes the transformation. A form of code, script, or |
| simply a declaration or series of declarations.</item> |
| |
| |
| |
| <label>Stylesheet</label> |
| <item>The same as "transformation instructions," except it is |
| likely to be used in conjunction with <jump href="http://www.w3.org/TR/xslt">XSLT</jump>.</item> |
| |
| |
| |
| <label>Templates</label> |
| <item>Another form of "transformation instructions." In the TrAX |
| interface, this term is used to describe processed or compiled transformation |
| instructions. The Source flows through a Templates object to be formed into the |
| Result.</item> |
| |
| |
| |
| <label>Processor</label> |
| <item>A general term for the thing that may both process the |
| transformation instructions, and perform the transformation.</item> |
| |
| |
| |
| |
| <label>DOM</label> |
| <item>Document Object Model, specifically referring to the |
| <jump href="http://www.w3.org/TR/DOM-Level-2%20">Document Object Model |
| (DOM) Level 2 Specification</jump>.</item> |
| |
| |
| <label>SAX</label> |
| <item>Simple API for XML, specifically referring to the |
| <jump href="http://www.megginson.com/SAX/SAX2">SAX 2.0 |
| release</jump>.</item> |
| </gloss> |
| </s2> <anchor name="requirements"/> |
| <s2 title="Requirements"> |
| |
| <p>The following requirements have been determined from broad experience |
| with XML projects from the various members participating on the JCP.</p> |
| |
| <ul> |
| <li>TrAX must provide a clean, simple |
| interface for simple uses.</li> |
| <li>TrAX must be powerful enough to be |
| applied to a wide range of uses, such as, e-commerce, content management, |
| server content delivery, and client applications.</li> |
| <li>A processor that implements a TrAX |
| interface must be optimizeable. Performance is a critical issue for most |
| transformation use cases.</li> |
| <li>As a specialization of the above |
| requirement, a TrAX processor must be able to support a compiled model, so that |
| a single set of transformation instructions can be compiled, optimized, and |
| applied to a large set of input sources.</li> |
| <li>TrAX must not be dependent an any |
| given type of transformation instructions. For instance, it must remain |
| independent of <jump href="http://www.w3.org/TR/xslt">XSLT</jump>.</li> |
| <li>TrAX must be able to allow processors |
| to transform DOM trees.</li> |
| <li>TrAX must be able to allow processors to |
| produce DOM trees.</li> |
| <li>TrAX must allow processors to transform |
| SAX events.</li> |
| <li>TrAX must allow processors to produce SAX |
| events.</li> |
| <li>TrAX must allow processors to |
| transform streams of XML.</li> |
| <li>TrAX must allow processors to produce |
| XML, HTML, and other types of streams.</li> |
| <li>TrAX must allow processors to |
| implement the various combinations of inputs and outputs within a single |
| processor.</li> |
| <li>TrAX must allow processors |
| to implement only a limited set of inputs. For instance, it should be possible |
| to write a processor that implements the TrAX interfaces and that only |
| processes DOM trees, not streams or SAX events.</li> |
| <li>TrAX should allow a |
| processor to implement transformations of proprietary data structures. For |
| instance, it should be possible to implement a processor that provides TrAX |
| interfaces that performs transformation of JDOM trees.</li> |
| <li>TrAX must allow the setting |
| of serialization properties, without constraint as to what the details of those |
| properties are.</li> |
| <li>TrAX must allow the setting |
| of parameters to the transformation instructions.</li> |
| <li>TrAX must support the |
| setting of parameters and properties as XML Namespaced items (i.e., qualified |
| names).</li> |
| <li>TrAX must support URL |
| resolution from within the transformation, and have it return the needed data |
| structure.</li> |
| <li>TrAX must have a mechanism for |
| reporting errors and warnings to the calling application.</li> |
| </ul> |
| |
| </s2> <anchor name="model"/> |
| <s2 title="Model"> |
| |
| <p>The section defines the abstract model for TrAX, apart from the details |
| of the interfaces.</p> |
| |
| <p>A TRaX <link anchor="pattern-TransformerFactory">TransformerFactory</link> is an object |
| that processes transformation instructions, and produces |
| <link anchor="pattern-Templates">Templates</link> (in the technical |
| terminology). A <link anchor="pattern-Templates">Templates</link> |
| object provides a <link anchor="pattern-Transformer">Transformer</link>, which transforms one or |
| more <link anchor="pattern-Source">Source</link>s into one or more |
| <link anchor="pattern-Result">Result</link>s.</p> |
| |
| <p>To use the TRaX interface, you create a |
| <link anchor="pattern-TransformerFactory">TransformerFactory</link>, |
| which may directly provide a <link anchor="pattern-Transformer">Transformers</link>, or which can provide |
| <link anchor="pattern-Templates">Templates</link> from a variety of |
| <link anchor="pattern-Source">Source</link>s. The |
| <link anchor="pattern-Templates">Templates</link> object is a processed |
| or compiled representation of the transformation instructions, and provides a |
| <link anchor="pattern-Transformer">Transformer</link>. The |
| <link anchor="pattern-Transformer">Transformer</link> processes a |
| <link anchor="pattern-Transformer">Source</link> according to the |
| instructions found in the <link anchor="pattern-Templates">Templates</link>, and produces a |
| <link anchor="pattern-Result">Result</link>.</p> |
| |
| <p>The process of transformation from a tree, either in the form of an |
| object model, or in the form of parse events, into a stream, is known as |
| <ref>serialization</ref>. We believe this is the most suitable term for |
| this process, despite the overlap with Java object serialization.</p> |
| </s2><anchor name="patterns"/> |
| <s2 title="TRaX Patterns"> |
| <p>The intent, responsibilities, and thread safety of TrAX objects:</p> |
| <ul> |
| <li><link anchor="pattern-Processor">Processor</link></li> |
| <li><link anchor="pattern-TransformerFactory">TransformerFactory</link></li> |
| <li><link anchor="pattern-Templates">Templates</link></li> |
| <li><link anchor="pattern-Transformer">Transformer</link></li> |
| <li><link anchor="pattern-Source">Source</link></li> |
| <li><link anchor="pattern-Result">Result</link></li> |
| </ul> |
| <anchor name="pattern-Processor"/><s3 title="Processor"> |
| <gloss> |
| <label>Intent</label> |
| <item>Generic concept for the |
| set of objects that implement the TrAX interfaces.</item> |
| <label>Responsibilities</label> |
| <item>Create compiled transformation instructions, transform |
| sources, and manage transformation parameters and |
| properties.</item> |
| <label>Thread safety</label> |
| <item>Only the Templates object can be |
| used concurrently in multiple threads. The rest of the processor does not do |
| synchronized blocking, and so may not be used to perform multiple concurrent |
| operations.</item> |
| </gloss> |
| </s3> <anchor name="pattern-TransformerFactory"/> |
| <s3 title="TransformerFactory"> |
| <gloss> |
| <label>Intent</label> |
| <item>Serve as a vendor-neutral Processor interface for |
| <jump href="http://www.w3.org/TR/xslt">XSLT</jump> and similar |
| processors.</item> |
| <label>Responsibilities</label> |
| <item>Serve as a factory for a concrete |
| implementation of an TransformerFactory, serve as a direct factory for |
| Transformer objects, serve as a factory for Templates objects, and manage |
| processor specific features.</item> |
| <label>Thread safety</label> |
| <item>A TransformerFactory may not perform mulitple concurrent |
| operations.</item> |
| </gloss> |
| </s3><anchor name="pattern-Templates"/> |
| <s3 title="Templates"> |
| <gloss> |
| <label>Intent</label> |
| <item>The runtime representation of the transformation instructions.</item> |
| <label>Responsibilities</label> |
| <item>A data bag for transformation instructions; act as a factory |
| for Transformers.</item> |
| <label>Thread safety</label> |
| <item>Threadsafe for concurrent |
| usage over multiple threads once construction is complete.</item> |
| </gloss> |
| </s3><anchor name="pattern-Transformer"/> |
| <s3 title="Transformer"> |
| <gloss> |
| <label>Intent</label> |
| <item>Act as a per-thread |
| execution context for transformations, act as an interface for performing the |
| transformation.</item> |
| <label>Responsibilities</label> |
| <item>Perform the |
| transformation.</item> |
| <label>Thread safety</label> |
| <item>Only one instance per thread |
| is safe.</item> |
| </gloss> |
| <note>The Transformer is bound to the Templates |
| object that created it.</note> |
| </s3><anchor name="pattern-Source"/> |
| <s3 title="Source"> |
| <gloss> |
| <label>Intent</label> |
| <item>Serve as a |
| single vendor-neutral object for multiple types of input.</item> |
| <label>Responsibilities</label> |
| <item>Act as simple data holder for System IDs, DOM nodes, streams, |
| etc.</item> |
| <label>Thread safety</label> |
| <item>Threadsafe concurrently over multiple |
| threads for read-only operations; must be synchronized for edit |
| operations.</item> |
| </gloss> |
| </s3><anchor name="pattern-Result"/> |
| <s3 title="Result"> |
| <p>Alternative name: ResultTarget.</p> |
| <gloss> |
| <label>Intent</label> |
| <item>Serve |
| as a single object for multiple types of output, so there can be simple process |
| method signatures.</item> |
| <label>Responsibilities</label> |
| <item>Act as simple data holder for |
| output stream, DOM node, ContentHandler, etc.</item> |
| <label>Thread safety</label> |
| <item>Threadsafe concurrently over multiple threads for read-only, |
| must be synchronized for edit.</item> |
| </gloss> |
| </s3> |
| </s2> |
| </s1> |