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Transformation API For XML
Introduction
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).
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, XSLT
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.
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.
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.
General Terminology
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.
-
Tree
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.
-
Source Tree(s)
One or more trees that are the inputs to the
transformation.
-
Result Tree(s)
One or more trees that are the output of the
transformation.
-
Transformation
The processor of consuming a stream or tree to produce
another stream or tree.
-
Identity (or Copy) Transformation
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.
-
Serialization
The process of taking a tree and turning it into a stream. In
some sense, a serialization is a specialized transformation.
-
Parsing
The process of taking a stream and turning it into a tree. In
some sense, parsing is a specialized transformation.
-
Transformer
A Transformer is the object that executes the transformation.
-
Transformation instructions
Describes the transformation. A form of code, script, or
simply a declaration or series of declarations.
-
Stylesheet
The same as "transformation instructions," except it is
likely to be used in conjunction with XSLT.
-
Templates
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.
-
Processor
A general term for the thing that may both process the
transformation instructions, and perform the transformation.
-
DOM
Document Object Model, specifically referring to the
Document Object Model
(DOM) Level 2 Specification.
-
SAX
Simple API for XML, specifically referring to the SAX 2.0.2 release.
Model
The section defines the abstract model for TrAX, apart from the details
of the interfaces.
A TRaX TransformerFactory is an object
that processes transformation instructions, and produces
Templates (in the technical
terminology). A Templates
object provides a Transformer, which transforms one or
more Sources into one or more
Results.
To use the TRaX interface, you create a
TransformerFactory,
which may directly provide a Transformers, or which can provide
Templates from a variety of
Sources. The
Templates object is a processed
or compiled representation of the transformation instructions, and provides a
Transformer. The
Transformer processes a
Source according to the
instructions found in the Templates, and produces a
Result.
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
serialization. We believe this is the most suitable term for
this process, despite the overlap with Java object serialization.
TRaX Patterns
Processor
Intent: Generic concept for the
set of objects that implement the TrAX interfaces.
Responsibilities: Create compiled transformation instructions, transform
sources, and manage transformation parameters and
properties.
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
operations. Different Processors can be used concurrently by different
threads.
TransformerFactory
Intent: Serve as a vendor-neutral Processor interface for
XSLT and similar
processors.
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.
Thread safety: A
TransformerFactory may not perform mulitple concurrent
operations.
Templates
Intent: The
runtime representation of the transformation instructions.
Responsibilities: A data bag for transformation instructions; act as a factory
for Transformers.
Thread safety: Threadsafe for concurrent
usage over multiple threads once construction is complete.
Transformer
Intent: Act as a per-thread
execution context for transformations, act as an interface for performing the
transformation.
Responsibilities: Perform the
transformation.
Thread safety: Only one instance per thread
is safe.
Notes: The Transformer is bound to the Templates
object that created it.
Source
Intent: Serve as a
single vendor-neutral object for multiple types of input.
Responsibilities: Act as simple data holder for System IDs, DOM nodes, streams,
etc.
Thread safety: Threadsafe concurrently over multiple
threads for read-only operations; must be synchronized for edit
operations.
Result
Potential alternate name: ResultTarget
Intent: Serve
as a single object for multiple types of output, so there can be simple process
method signatures.
Responsibilities: Act as simple data holder for
output stream, DOM node, ContentHandler, etc.
Thread safety: Threadsafe concurrently over multiple threads for read-only,
must be synchronized for edit.