Package gnu.xml.pipeline

This package exposes a kind of XML processing pipeline, based on sending SAX events, which can be used as components of application architectures.


Interface Summary
EventConsumer Collects the event consumption apparatus of a SAX pipeline stage.

Class Summary
CallFilter Input is sent as an XML request to given URI, and the output of this filter is the parsed response to that request.
DomConsumer This consumer builds a DOM Document from its input, acting either as a pipeline terminus or as an intermediate buffer.
DomConsumer.Handler Class used to intercept various parsing events and use them to populate a DOM document.
EventFilter A customizable event consumer, used to assemble various kinds of filters using SAX handlers and an optional second consumer.
LinkFilter Pipeline filter to remember (X)HTML links found in an (X)HTML document, so they can later be crawled.
NSFilter This filter ensures that element and attribute names are properly prefixed, and that such prefixes are declared.
PipelineFactory This provides static factory methods for creating simple event pipelines.
TeeConsumer Fans its events out to two other consumers, a "tee" filter stage in an event pipeline.
TextConsumer Terminates a pipeline, consuming events to print them as well formed XML (or XHTML) text.
ValidationConsumer This class checks SAX2 events to report validity errors; it works as both a filter and a terminus on an event pipeline.
WellFormednessFilter This filter reports fatal exceptions in the case of event streams that are not well formed.

Package gnu.xml.pipeline Description

This package exposes a kind of XML processing pipeline, based on sending SAX events, which can be used as components of application architectures. Pipelines are used to convey streams of processing events from a producer to one or more consumers, and to let each consumer control the data seen by later consumers.

There is a PipelineFactory class which accepts a syntax describing how to construct some simple pipelines. Strings describing such pipelines can be used in command line tools (see the DoParse class) and in other places that it is useful to let processing be easily reconfigured. Pipelines can of course be constructed programmatically, providing access to options that the factory won't.

Web applications are supported by making it easy for servlets (or non-Java web application components) to be part of a pipeline. They can originate XML (or XHTML) data through an InputSource or in response to XML messages sent from clients using CallFilter pipeline stages. Such facilities are available using the simple syntax for pipeline construction.

Programming Models

Pipelines should be simple to understand.

Producers: XMLReader or Custom

Many producers will be SAX2 XMLReader objects, and will read (pull) data which is then written (pushed) as events. Typically these will parse XML text (acquired from org.xml.sax.helpers.XMLReaderFactory) or a DOM tree (using a DomParser) These may be bound to event consumer using a convenience routine, EventFilter.bind(). Once bound, these producers may be given additional documents to sent through its pipeline.

In other cases, you will write producers yourself. For example, some data structures might know how to write themselves out using one or more XML models, expressed as sequences of SAX2 event callbacks. An application module might itself be a producer, issuing startDocument and endDocument events and then asking those data structures to write themselves out to a given EventConsumer, or walking data structures (such as JDBC query results) and applying its own conversion rules. WAP format XML (WBMXL) can be directly converted to producer output.

SAX2 introduced an "XMLFilter" interface, which is a kind of XMLReader. It is most useful in conjunction with its XMLFilterImpl helper class; see the EventFilter javadoc for information contrasting that XMLFilterImpl approach with the relevant parts of this pipeline framework. Briefly, such XMLFilterImpl children can be either producers or consumers, and are more limited in configuration flexibility. In this framework, the focus of filters is on the EventConsumer side; see the section on pipe fitting below.

Consume to Standard or Custom Data Representations

Many consumers will be used to create standard representations of XML data. The TextConsumer takes its events and writes them as text for a single XML document, using an internal XMLWriter. The DomConsumer takes its events and uses them to create and populate a DOM Document.

In other cases, you will write consumers yourself. For example, you might use a particular unmarshaling filter to produce objects that fit your application's requirements, instead of using DOM. Such consumers work at the level of XML data models, rather than with specific representations such as XML text or a DOM tree. You could convert your output directly to WAP format data (WBXML).

Pipe Fitting

Pipelines are composite event consumers, with each stage having the opportunity to transform the data before delivering it to any subsequent stages.

The PipelineFactory class provides access to much of this functionality through a simple syntax. See the table in that class's javadoc describing a number of standard components. Direct API calls are still needed for many of the most interesting pipeline configurations, including ones leveraging actual or logical concurrency.

Four basic types of pipe fitting are directly supported. These may be used to construct complex pipeline networks.

Note that filters can be as complex as XSLT transforms (not yet available) on input data, or as simple as removing simple syntax data such as ignorable whitespace, comments, and CDATA delimiters. Some simple "built-in" filters are part of this package.

Coding Conventions: Filter and Terminus Stages

If you follow these coding conventions, your classes may be used directly (give the full class name) in pipeline descriptions as understood by the PipelineFactory. There are four constructors the factory may try to use; in order of decreasing numbers of parameters, these are:

Of course, classes may support more than one such usage convention; if they do, they can automatically be used in multiple modes. If you try to use a terminus class as a filter, and that terminus has a constructor with the appropriate number of arguments, it is automatically wrapped in a "tee" filter.

Debugging Tip: "Tee" Joints can Snapshot Data

It can sometimes be hard to see what's happening, when something goes wrong. Easily fixed: just snapshot the data. Then you can find out where things start to go wrong.

If you're using pipeline descriptors so that they're easily administered, just stick a write ( filename ) filter into the pipeline at an appropriate point.

Inside your programs, you can do the same thing directly: perhaps by saving a Writer (perhaps a StringWriter) in a variable, using that to create a TextConsumer, and making that the first part of a tee -- splicing that into your pipeline at a convenient location.

You can also use a DomConsumer to buffer the data, but remember that DOM doesn't save all the information that XML provides, so that DOM snapshots are relatively low fidelity. They also are substantially more expensive in terms of memory than a StringWriter holding similar data.

Debugging Tip: Non-XML Producers

Producers in pipelines don't need to start from XML data structures, such as text in XML syntax (likely coming from some XMLReader that parses XML) or a DOM representation (perhaps with a DomParser).

One common type of event producer will instead make direct calls to SAX event handlers returned from an EventConsumer. For example, making ContentHandler.startElement calls and matching ContentHandler.endElement calls.

Applications making such calls can catch certain common "syntax errors" by using a WellFormednessFilter. That filter will detect (and report) erroneous input data such as mismatched document, element, or CDATA start/end calls. Use such a filter near the head of the pipeline that your producer feeds, at least while debugging, to help ensure that you're providing legal XML Infoset data.

You can also arrange to validate data on the fly. For DTD validation, you can configure a ValidationConsumer to work as a filter, using any DTD you choose. Other validation schemes can be handled with other validation filters.

Source code is GPL'd in the JAXP subproject at
This documentation was derived from that source code on 2001-07-12.