Introduction General Terminology Requirements Model Patterns
| Introduction |
| This document describes the transformation API for XML (TrAX), the set of APIs contained in javax.xml.transform, javax.xml.transform.stream, javax.xml.transform.dom, and javax.xml.transform.sax. 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 process 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 release.
| Requirements |
| The following requirements have been determined from broad experience with XML projects from the various members participating on the JCP. TrAX must provide a clean, simple interface for simple uses. 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. A processor that implements a TrAX interface must be optimizeable. Performance is a critical issue for most transformation use cases. 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. TrAX must not be dependent an any given type of transformation instructions. For instance, it must remain independent of XSLT. TrAX must be able to allow processors to transform DOM trees. TrAX must be able to allow processors to produce DOM trees. TrAX must allow processors to transform SAX events. TrAX must allow processors to produce SAX events. TrAX must allow processors to transform streams of XML. TrAX must allow processors to produce XML, HTML, and other types of streams. TrAX must allow processors to implement the various combinations of inputs and outputs within a single processor. 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. 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. TrAX must allow the setting of serialization properties, without constraint as to what the details of those properties are. TrAX must allow the setting of parameters to the transformation instructions. TrAX must support the setting of parameters and properties as XML Namespaced items (i.e., qualified names). TrAX must support URL resolution from within the transformation, and have it return the needed data structure. TrAX must have a mechanism for reporting errors and warnings to the calling application.
| 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 |
| The intent, responsibilities, and thread safety of TrAX objects: Processor TransformerFactory Templates Transformer Source Result
| 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.
| 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.
| 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 |
| Alternative 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.

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