SAXON: the Java API

This document describes how to use SAXON as a Java class library, without making any use of XSLT stylesheets. If you want to know how to control stylesheet processing from a Java application, see using-xsl.html.

Note: The Java API was provided in SAXON long before the XSL interface. Most of the things that the Java API was designed to do can now be done more conveniently in XSL. Reflecting this, some of the features of the API have been withdrawn as redundant, and the focus in SAXON will increasingly be on doing everything possible through XSL.

The Java processing model in SAXON is an extension of the XSL processing model:

• You write handlers for elements and other nodes in the document.
• You define the rules that associate a particular handler with particular elements or other nodes. These rules are expressed as XSL-compatible patterns. The node handlers are analogous to XSL templates.
• Within a node handler, you can select other nodes (typically but not necessarily the immediate children) for processing. The system will automatically invoke the appropriate handler for each selected node. Alternatively, you can use SAXON API calls to navigate directly to other nodes in the document.

You can process some elements in Java and others in XSLT if you wish.

The library of standard node handlers issued with previous versions of Saxon is no longer available. These handlers all performed tasks that are done much more easily using XSLT directly.

When a Java node handler is invoked, it is provided with information about the node via a NodeInfo object (usually you will be processing element nodes, in which case the NodeInfo will be an ElementInfo object). The node handler is also given information about the processing context, and access to a wide range of processing services, via a Context object.

The NodeInfo implements the DOM Node interface, allowing navigation around the tree. It also provides facilities to:

• determine the node's name, string-value, and attributes as defined in the XPath tree model
• associate user data with a node for the duration of the current session
• copy the node to the result tree

The Context object allows the node handler to:

• access any parameters associated with the applyTemplates() call that invoked this node handler
• get information about the current node list (the list of nodes being processed by this handler: for example, to determine if this is the last node in the list)
• get rapid access to nodes based on registered keys and identifiers
• declare and reference variables
• set an output destination for output from this node or its children (useful when splitting an XML document heirarchically)
• write output to the current destination

SAXON: comparison with SAX and DOM

There are two standard APIs for processing XML documents: the SAX interface, and the DOM. SAX (see http://www.megginson.com/SAX/index.html) is an event-driven interface in which the parser reports things such as start and end tags to the application as they are encountered, while the Document Object Model (DOM) (see http://www.w3.org/dom is a navigational interface in which the application can roam over the document in memory by following relationships between its nodes.

SAXON offers a higher-level processing model than either SAX or DOM. It allows applications to be written using a rule-based design pattern, in which your application consists of a set of rules of the form "when this condition is encountered, do this processing". It is an event-condition-action model in which the events are the syntactic constructs of XML, the conditions are XSLT-compatible patterns, and the actions are Java methods.

If you are familiar with SAX, some of the differences in SAXON are:

• You can provide a separate handler for each element type (or other node), making your application more modular
• SAXON supplies context information to your application, so you can find out, for example, the parent element of the one you are currently processing
• SAXON provides facilities for organizing the output of your application, allowing you to direct different parts of the output to different files. SAXON is a particularly convenient tool for splitting a large document into page-sized chunks for viewing, or into individual records for storing in a relational or object database.
• SAXON allows you to register your preferred SAX-compliant XML parser; you do not need to hard-code the name of the parser into your application or supply it each time on the command line. SAXON also works with several DOM implementations.
• SAXON extends the SAX InputSource class allowing you to specify a file name as the source of input.

Serial and Direct processing: preview mode

An earlier release of SAXON allowed a purely serial mode of processing: each node was processed as it was encountered. With experience, this proved too restrictive, and caused the internal architecture to become too complex, so it was withdrawn. It has been replaced with a new facility, preview mode. This is available both with XSL and with the Java API.

Preview mode is useful where the document is too large to fit comfortably in main memory. It allows you to define node handlers that are called as the document tree is being built in memory, rather than waiting until the tree is fully built which is the normal case.

When you define an element as a preview element (using the setPreviewElement() method of the PreviewManager class), its node handler is called as soon as the element end tag is encountered. When the node handler returns control to SAXON, the children of the preview element are discarded from memory.

This means, for example, that if your large XML document consists of a large number of chapters, you can process each chapter as it is read, and the memory available needs to be enough only for (a) the largest individual chapter, and (b) the top-level structure identifying the list of chapters.

Processing is controlled by a class called the Controller. Some of the functions of this class are relevant only to XSLT transformation, but most can also be used when Saxon is used purely from Java. Each application run must instantiate a new Controller.

There are several classes used to define the kind of processing you want to perform. These are the RuleManager for registering template rules, the KeyManager for registering key definitions, the PreviewManager for registering preview elements, the Stripper for registering which elements are to have whitespace nodes stripped, and the DecimalFormatManager for registering named decimal formats. These classes can all be reused freely, and they are thread safe once the definitions have been set up. All of these objects are registered with the Controller using methods such as setRuleManager() and setKeyManager().

The Builder class is used to build a document tree from a SAX InputSource. Its main method is build(). The builder can be serially reused to build further documents, but it should only be used for one document at a time. The builder needs to know about the Stripper if whitespace nodes are to be stripped from the tree, and it needs to know about the PreviewManager if any elements are to be processed in preview mode. The relevant classes can be registered with the builder using the setStripper() and setPreviewManager() methods.

The Controller class is used to process a document tree by applying registered node handlers. Its main method is run(). The controller is responsible for navigating through the document and calling user-defined handlers which you associate with each element or other node type to define how it is to be processed. The controller can also be serially reused, but should not be used to process more than one document at a time. The Controller needs to know about the RuleManager to find the relevant node handlers to invoke. If keys are used it will need to know about the KeyManager, and if decimal formats are used it will need to know about the DecimalFormatManager. These classes can be registered with the Controller using setRuleManager(), setKeyManager(), and setDecimalFormatManager() respectively. If preview mode is used, the PreviewManager will need to know about the Controller, so it has a setController() method for this purpose.

Node handlers are called to process each node, in the same way as template rules are used in XSLT. They node handler can choose whether or not subsidiary elements should be processed (by calling applyTemplates()), and can dive off into a completely different part of the document tree before resuming. A user-written node handler must implement the NodeHandler interface; for convenience and compatibility with previous releases, the classes ElementHandler and ElementHandlerBase are also provided.

A node handler can write to the current output destination. The controller maintains a stack of outputters. Your node handler can switch output to a new destination by calling setOutputDetails(), and can revert to the prevoius destination by calling resetOutputDetails(). This is useful both for splitting an input XML document into multiple XML documents, and for creating output fragments that can be reassembled in a different order for display. Details of the output format required must be set up in an OutputDetails object, which is supplied as a parameter to setOutputDetails(). The actual control of output destinations rests with a class called the OutputManager, but you will normally interact with this via wrapper methods in the Controller.

SAXON provides a layer of services on top of a SAX-compliant XML parser. It will work with any Java-based XML parser that implements the SAX1 or SAX2 interface.

SAXON uses the configuration file ParserManager.properties to decide which SAX parser to use. This file identifies a default parser and a list of alternatives. As issued, it lists some popular (and free) SAX-compliant parsers which have been tested with SAXON. The default is a version of the AElfred parser which is bundled with SAXON. If you want to specify a different parser, or change the default, simply edit the ParserManager.properties file.

If you want to use different parsers depending on the URI of the document being read, you can achieve this by writing a URIResolver that nominates the parser to be used for each input file.

The Controller and Builder

A simple application proceeds as follows:

1. Create an instance of the RuleManager.
2. Define a number of node handlers using the setHandler() method of the RuleManager class.
3. Create an instance of the Controller.
4. Register the node handlers with the Controller using the setRuleManager() method.
5. Create an instance of the Builder.
6. Supply an input XML document to build a document tree using the build() method of the Builder class. This returns a DocumentInfo object
7. Supply this DocumentInfo object to the run() method of the Controller class. This will start processing at the root node, calling your node handlers as appropriate.

An element handler is one kind of NodeHandler. We focus here on handlers for elements rather than other kinds of node, because they are the most common.

User-written element-handlers are written to implement the interface ElementHandler. Optionally, you can define them as subclasses of the system-supplied class ElementHandlerBase, an element handler that does nothing.

Always remember that if you want child elements to be processed recursively, your element handler must call the applyTemplates() method.

The element handler is supplied with an ElementInfo object which provides information about the current element, and with a Context object that gives access to a range of standard services such an Outputter object which includes a write() method to produce output.

Normally you will write one element handler for each type of element, but it is quite possible to use the same handler for several different elements. You can also write completely general-purpose handlers. You define which elements will be handled by each element handler using a pattern, exactly as in XSLT.

You only need to provide one method for the selected element type. This is:

Patterns are used in the setHandler() interface to define which nodes a particular handler applies to. Expressions are used in the applyTemplates() interface to control which nodes are selected for processing. Patterns and expressions used in the SAXON Java API have exactly the same form as in XSLT.

The detailed rules for patterns can be found in patterns.html, and for expressions in expressions.html

Expressions and Patterns are represented in the API by classes Expression and Pattern respectively. These include static methods to create an Expression or Pattern from a String. A few convenience methods also allow Expressions and Patterns to be supplied directly as Strings.

When you create an Expression or Pattern using the methods Expression.make() and Pattern.make() you may supply a StaticContext object. This object provides the information needed to interpret certain expressions and patterns: for example, it provides the ability to convert a namespace prefix within the expressions into a URI. In an XSLT stylesheet, the StaticContext provides information the expression can get from the rest of the stylesheet; in a Java application, this is not available, so you must provide the context yourself. If you don't supply a StaticContext object, a default context is used: this will prevent you using context-dependent constructs such as variables and namespace prefixes.

The ElementInfo object represents an element node of the XML document (that is, a construct with a start and end tag). It is a subclass of NodeInfo, which represents a general Node. The Saxon classes such as NodeInfo and ElementInfo correspond to the DOM classes Node and Element, but with many additional methods provided.

The main purpose of the ElementInfo object is to provide element handlers with information about the element. The most commonly-used methods include:

Michael H. Kay
11 September 2000