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<title>Transformation API For XML (TrAX)</title>
<pubdate>November 12, 2000</pubdate>
<orgname>IBM Research</orgname>
<para>This overview describes the set of APIs contained in
<ulink url="">javax.xml.transform</ulink>, <ulink url=""></ulink>, <ulink url="">javax.xml.transform.dom</ulink>, and <ulink url="">javax.xml.transform.sax</ulink>. For the sake of brevity, these interfaces are referred to
as TrAX (Transformation API for XML). </para>
<para>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, <ulink url="">XSLT</ulink>
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.</para>
<para>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.</para>
<para>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. </para>
<title>General Terminology</title>
<para>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.</para>
<listitem>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
<term>Source Tree(s)</term>
<listitem>One or more trees that are the inputs to the
<term>Result Tree(s)</term>
<listitem>One or more trees that are the output of the
<listitem>The process of consuming a stream or tree to produce
another stream or tree.</listitem>
<term>Identity (or Copy) Transformation</term>
<listitem>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
<listitem>The process of taking a tree and turning it into a stream. In
some sense, a serialization is a specialized transformation.</listitem>
<listitem>The process of taking a stream and turning it into a tree. In
some sense, parsing is a specialized transformation.</listitem>
<listitem>A Transformer is the object that executes the transformation.
<term>Transformation instructions</term>
<listitem>Describes the transformation. A form of code, script, or
simply a declaration or series of declarations.</listitem>
<listitem>The same as "transformation instructions," except it is
likely to be used in conjunction with <ulink
<listitem>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
<listitem>A general term for the thing that may both process the
transformation instructions, and perform the transformation.</listitem>
<listitem>Document Object Model, specifically referring to the
<termref link-url=" ">Document Object Model
(DOM) Level 2 Specification</termref>.</listitem>
<listitem>Simple API for XML, specifically referring to the
<termref link-url="">SAX 2.0
<para>The following requirements have been determined from broad experience
with XML projects from the various members participating on the JCP.</para>
<listitem id="requirement-simple">TrAX must provide a clean, simple
interface for simple uses.</listitem>
<listitem id="requirement-general">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.</listitem>
<listitem id="requirement-optimizeable">A processor that implements a TrAX
interface must be optimizeable. Performance is a critical issue for most
transformation use cases.</listitem>
<listitem id="requirement-compiled-model">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.</listitem>
<listitem id="requirement-independence">TrAX must not be dependent an any
given type of transformation instructions. For instance, it must remain
independent of <ulink url="">XSLT</ulink>.</listitem>
<listitem id="requirement-from-dom">TrAX must be able to allow processors
to transform DOM trees.</listitem>
<listitem id="requirement-to-dom">TrAX must be able to allow processors to
produce DOM trees.</listitem>
<listitem id="requirement-from-sax">TrAX must allow processors to transform
SAX events.</listitem>
<listitem id="requirement-to-sax">TrAX must allow processors to produce SAX
<listitem id="requirement-from-stream">TrAX must allow processors to
transform streams of XML.</listitem>
<listitem id="requirement-to-stream">TrAX must allow processors to produce
XML, HTML, and other types of streams.</listitem>
<listitem id="requirement-combo-input-output">TrAX must allow processors to
implement the various combinations of inputs and outputs within a single
<listitem id="requirement-limited-input-output">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.</listitem>
<listitem id="requirement-proprietary-data-structures">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.</listitem>
<listitem id="requirement-serialization-props">TrAX must allow the setting
of serialization properties, without constraint as to what the details of those
properties are.</listitem>
<listitem id="requirement-setting-parameters">TrAX must allow the setting
of parameters to the transformation instructions.</listitem>
<listitem id="requirement-namespaced-properties">TrAX must support the
setting of parameters and properties as XML Namespaced items (i.e., qualified
<listitem id="requirement-relative-url-resolution">TrAX must support URL
resolution from within the transformation, and have it return the needed data
<listitem id="requirement-error-reporting">TrAX must have a mechanism for
reporting errors and warnings to the calling application.</listitem>
</orderedlist> </requirements>
<para>The section defines the abstract model for TrAX, apart from the details
of the interfaces.</para>
<para>A TRaX <termref
link-url="pattern-TransformerFactory">TransformerFactory</termref> is an object
that processes transformation instructions, and produces
<termref link-url="pattern-Templates">Templates</termref> (in the technical
terminology). A <termref link-url="pattern-Templates">Templates</termref>
object provides a <termref
link-url="pattern-Transformer">Transformer</termref>, which transforms one or
more <termref link-url="pattern-Source">Source</termref>s into one or more
<termref link-url="pattern-Result">Result</termref>s.</para>
<para>To use the TRaX interface, you create a
<termref link-url="pattern-TransformerFactory">TransformerFactory</termref>,
which may directly provide a <termref
link-url="pattern-Transformers">Transformers</termref>, or which can provide
<termref link-url="pattern-Templates">Templates</termref> from a variety of
<termref link-url="pattern-Source">Source</termref>s. The
<termref link-url="pattern-Templates">Templates</termref> object is a processed
or compiled representation of the transformation instructions, and provides a
<termref link-url="pattern-Transformer">Transformer</termref>. The
<termref link-url="pattern-Transformer">Transformer</termref> processes a
<termref link-url="pattern-Transformer">Source</termref> according to the
instructions found in the <termref
link-url="pattern-Templates">Templates</termref>, and produces a
<termref link-url="pattern-Result">Result</termref>.</para>
<para>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
<termref>serialization</termref>. We believe this is the most suitable term for
this process, despite the overlap with Java object serialization.</para>
<patterns module="TRaX"> <pattern><pattern-name
id="pattern-Processor">Processor</pattern-name><intent>Generic concept for the
set of objects that implement the TrAX interfaces.</intent>
<responsibilities>Create compiled transformation instructions, transform
sources, and manage transformation parameters and
properties.</responsibilities><thread-safety>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
<pattern-name id="pattern-TransformerFactory">TransformerFactory</pattern-name>
<intent>Serve as a vendor-neutral Processor interface for
<ulink url="">XSLT</ulink> and similar
processors.</intent> <responsibilities>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.</responsibilities> <thread-safety>A
TransformerFactory may not perform mulitple concurrent
operations.</thread-safety> </pattern> <pattern>
<pattern-name id="pattern-Templates">Templates</pattern-name> <intent>The
runtime representation of the transformation instructions.</intent>
<responsibilities>A data bag for transformation instructions; act as a factory
for Transformers.</responsibilities> <thread-safety>Threadsafe for concurrent
usage over multiple threads once construction is complete.</thread-safety>
</pattern> <pattern> <pattern-name
id="pattern-Transformer">Transformer</pattern-name> <intent>Act as a per-thread
execution context for transformations, act as an interface for performing the
transformation.</intent><responsibilities>Perform the
transformation.</responsibilities> <thread-safety>Only one instance per thread
is safe.</thread-safety> <notes>The Transformer is bound to the Templates
object that created it.</notes> </pattern> <pattern>
<pattern-name id="pattern-Source">Source</pattern-name> <intent>Serve as a
single vendor-neutral object for multiple types of input.</intent>
<responsibilities>Act as simple data holder for System IDs, DOM nodes, streams,
etc.</responsibilities> <thread-safety>Threadsafe concurrently over multiple
threads for read-only operations; must be synchronized for edit
operations.</thread-safety> </pattern><pattern>
<pattern-name id="pattern-Result">Result</pattern-name>
<potential-alternate-name>ResultTarget</potential-alternate-name> <intent>Serve
as a single object for multiple types of output, so there can be simple process
method signatures.</intent> <responsibilities>Act as simple data holder for
output stream, DOM node, ContentHandler, etc.</responsibilities>
<thread-safety>Threadsafe concurrently over multiple threads for read-only,
must be synchronized for edit.</thread-safety> </pattern> </patterns>