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<h2>Transformation API For XML (TrAX)</h2>
<p>
<b>Edit Date: </b>November 12, 2000</p>
<h3>Introduction</h3>
<p>This overview describes the set of APIs contained in
javax.xml.transform. For the sake of brevity, these interfaces are
referred to
as TrAX (Transformations for XML). </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, <A href="http://www.w3.org/TR/xslt";>XSLT</A>
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>
<h3>General Terminology</h3>
<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>
<ul>
<li>
<p>
<b>Tree</b>
<br>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.</p>
</li>
<li>
<p>
<b>Source Tree(s)</b>
<br>One or more trees that are the inputs to the
transformation.</p>
</li>
<li>
<p>
<b>Result Tree(s)</b>
<br>One or more trees that are the output of the
transformation.</p>
</li>
<li>
<p>
<b>Transformation</b>
<br>The processor of consuming a stream or tree to produce
another stream or tree.</p>
</li>
<li>
<p>
<b>Identity (or Copy) Transformation</b>
<br>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.</p>
</li>
<li>
<p>
<b>Serialization</b>
<br>The process of taking a tree and turning it into a stream. In
some sense, a serialization is a specialized
transformation.</p>
</li>
<li>
<p>
<b>Parsing</b>
<br>The process of taking a stream and turning it into a tree. In
some sense, parsing is a specialized
transformation.</p>
</li>
<li>
<p>
<b>Transformer</b>
<br>A Transformer is the object that executes the transformation.
</p>
</li>
<li>
<p>
<b>Transformation instructions</b>
<br>Describes the transformation. A form of code, script, or
simply a declaration or series of declarations.</p>
</li>
<li>
<p>
<b>Stylesheet</b>
<br>The same as "transformation instructions," except it is
likely to be used in conjunction with <A
href="http://www.w3.org/TR/xslt";>XSLT</A>.</p>
</li>
<li>
<p>
<b>Templates</b>
<br>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.</p>
</li>
<li>
<p>
<b>Processor</b>
<br>A general term for the thing that may both process the
transformation instructions, and perform the
transformation.</p>
</li>
<li>
<p>
<b>DOM</b>
<br>Document Object Model, specifically referring to the
<A href="#http://www.w3.org/TR/DOM-Level-2 ">Document
Object Model
(DOM) Level 2 Specification</A>.</p>
</li>
<li>
<p>
<b>SAX</b>
<br>Simple API for XML, specifically referring to the
<A href="#http://www.megginson.com/SAX/SAX2";>SAX 2.0
release</A>.</p>
</li>
</ul>
<h3>Requirements</h3>
<p>The following requirements have been determined from broad experience
with XML projects from the various members participating on the
JCP.</p>
<ol>
<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 <A
href="http://www.w3.org/TR/xslt";>XSLT</A>.</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>
</ol>
<h3>Model</h3>
<p>The section defines the abstract model for TrAX, apart from the details
of the interfaces.</p>
<p>A TRaX <A href="#pattern-TransformerFactory">TransformerFactory</A> is
an object
that processes transformation instructions, and produces
<A href="#pattern-Templates">Templates</A> (in the technical
terminology). A <A href="#pattern-Templates">Templates</A>
object provides a <A href="#pattern-Transformer">Transformer</A>,
which transforms one or
more <A href="#pattern-Source">Source</A>s into one or more
<A href="#pattern-Result">Result</A>s.</p>
<p>To use the TRaX interface, you create a
<A href="#pattern-TransformerFactory">TransformerFactory</A>,
which may directly provide a <A
href="#pattern-Transformers">Transformers</A>, or which can provide
<A href="#pattern-Templates">Templates</A> from a variety of
<A href="#pattern-Source">Source</A>s. The
<A href="#pattern-Templates">Templates</A> object is a processed
or compiled representation of the transformation instructions, and
provides a
<A href="#pattern-Transformer">Transformer</A>. The
<A href="#pattern-Transformer">Transformer</A> processes a
<A href="#pattern-Transformer">Source</A> according to the
instructions found in the <A href="#pattern-Templates">Templates</A>,
and produces a
<A href="#pattern-Result">Result</A>.</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
<U>serialization</U>. We believe this is the most suitable term for
this process, despite the overlap with Java object serialization.</p>
<H3>TRaX Patterns</H3>
<ul>
<p>
<b><a name="pattern-Processor">Processor</a></b>
<br>
<br>
<i>Intent: </i>Generic concept for the
set of objects that implement the TrAX interfaces.<br>
<i>Responsibilities: </i>Create compiled transformation instructions,
transform
sources, and manage transformation parameters and
properties.<br>
<i>Thread safety: </i>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.</p>
<p>
<b><a name="pattern-TransformerFactory">TransformerFactory</a></b>
<br>
<br>
<i>Intent: </i>Serve as a vendor-neutral Processor interface for
<A href="http://www.w3.org/TR/xslt";>XSLT</A> and similar
processors.<br>
<i>Responsibilities: </i>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.<br>
<i>Thread safety: </i>A
TransformerFactory may not perform mulitple concurrent
operations.</p>
<p>
<b><a name="pattern-Templates">Templates</a></b>
<br>
<br>
<i>Intent: </i>The
runtime representation of the transformation instructions.<br>
<i>Responsibilities: </i>A data bag for transformation instructions; act as a
factory
for Transformers.<br>
<i>Thread safety: </i>Threadsafe for concurrent
usage over multiple threads once construction is complete.</p>
<p>
<b><a name="pattern-Transformer">Transformer</a></b>
<br>
<br>
<i>Intent: </i>Act as a per-thread
execution context for transformations, act as an interface for performing
the
transformation.<br>
<i>Responsibilities: </i>Perform the
transformation.<br>
<i>Thread safety: </i>Only one instance per thread
is safe.<br>
<i>Notes: </i>The Transformer is bound to the Templates
object that created it.</p>
<p>
<b><a name="pattern-Source">Source</a></b>
<br>
<br>
<i>Intent: </i>Serve as a
single vendor-neutral object for multiple types of input.<br>
<i>Responsibilities: </i>Act as simple data holder for System IDs, DOM nodes,
streams,
etc.<br>
<i>Thread safety: </i>Threadsafe concurrently over multiple
threads for read-only operations; must be synchronized for edit
operations.</p>
<p>
<b><a name="pattern-Result">Result</a></b>
<br>
<br>
<i>Potential alternate name: </i>ResultTarget<br>
<i>Intent: </i>Serve
as a single object for multiple types of output, so there can be simple
process
method signatures.<br>
<i>Responsibilities: </i>Act as simple data holder for
output stream, DOM node, ContentHandler, etc.<br>
<i>Thread safety: </i>Threadsafe concurrently over multiple threads for
read-only,
must be synchronized for edit.</p>
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