Extensibility

Extensibility

This page describes how to extend the capability of Saxon XSLT Stylesheets

Contents

Writing extension functions
Writing extension instructions
Writing input filters
Writing output filters
Implementing a collating sequence
Implementing a numbering sequence
Adding an output encoding

Writing extension functions

An extension function is invoked using a name such as prefix:localname(). The prefix must be the prefix associated with a namespace declaration that is in scope.

Extension functions must be implemented in Java.

Saxon supports the <saxon:script> element, based on the <xsl:script> element defined in the (now withdrawn) XSLT 1.1 working draft. This element defines a mapping between a namespace URI used in calls of extension functions, and a Java class that contains implementations of these functions. See saxon:script for details.

You can also use a short-cut technique of binding external Java classes, by making the class name part of the namespace URI. In this case, you don't need a <saxon:script> element.

With the short-cut technique, the URI for the namespace identifies the class where the external function will be found. The namespace URI must either be "java:" followed by the fully-qualified class name (for example xmlns:date="java:java.util.Date"), or a string containing a "/", in which the fully-qualified class name appears after the final "/". (for example xmlns:date="http://www.jclark.com/xt/java/java.util.Date"). The part of the URI before the final "/" is immaterial. The class must be on the classpath. For compatibility with previous releases, the format xmlns:date="java.util.Date" is also supported.

The Saxon namespace URI "http://saxon.sf.net/" is recognised as a special case, and causes the function to be loaded from the class net.sf.saxon.functions.Extensions. This class name can be specified explicitly if you prefer.

There are three cases to consider: static methods, constructors, and instance-level methods.

Static methods can be called directly. The localname of the function must match the name of a public static method in this class. The names match if they contain the same characters, excluding hyphens and forcing any character that follows a hyphen to upper-case. For example the XPath function call to-string() matches the Java method toString(); but the function call can also be written as toString() if you prefer. If there are several methods in the class that match the localname, the system attempts to find the one that is the best fit to the types of the supplied arguments: for example if the call is f(1,2) then a method with two int arguments will be preferred to one with two String arguments. The rules for deciding between methods are quite complex, and are not at present documented in detail, though they are based on the rules in the XSLT 1.1 Working Draft. If there are several methods with the same name and the correct number of arguments, but none is preferable to the others under these rules, an error is reported.

For example:


<xsl:value-of select="math:sqrt($arg)"
   xmlns:math="java:java.lang.Math"/>

This will invoke the static method java.lang.Math.sqrt(), applying it to the value of the variable $arg, and copying the value of the square root of $arg to the result tree.

Java constructors are called by using the function named new(). If there are several constructors, then again the system tries to find the one that is the best fit, according to the types of the supplied arguments. The result of calling new() is an XPath value of type Java Object; the only things that can be done with a Java Object are to assign it to a variable, to pass it to an extension function, and to convert it to a string, number, or boolean, using the rules given below.

Instance-level methods (that is, non-static methods) are called by supplying an extra first argument of type Java Object which is the object on which the method is to be invoked. A Java Object is usually created by calling an extension function (e.g. a constructor) that returns an object; it may also be passed to the style sheet as the value of a global parameter. Matching of method names is done as for static methods. If there are several methods in the class that match the localname, the system again tries to find the one that is the best fit, according to the types of the supplied arguments.

For example, the following stylesheet prints the date and time. This example is copied from the documentation of the xt product, and it works unchanged with Saxon, because Saxon does not care what the namespace URI for extension functions is, so long as it ends with the class name. (Extension functions are likely to be compatible between Saxon and xt provided they only use the data types string, number, and boolean).


<xsl:stylesheet
  version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:date="http://www.jclark.com/xt/java/java.util.Date">

<xsl:template match="/">
  <html>
    <xsl:if test="function-available('date:to-string') and function-available('date:new')">
      <p><xsl:value-of select="date:to-string(date:new())"/></p>
    </xsl:if>
  </html>
</xsl:template>

</xsl:stylesheet>

A wrapped Java object may be converted to another data type as follows.

It is converted to a string by using its toString() method; if the object is null, the result is the empty string "".
It is converted to a number by converting it first to a string, and then applying the XPath number() conversion. If it is null, the result is NaN.
It is converted to a boolean as follows: if it is null, the result is false, otherwise it is converted to a string and the result is true if and only if the string is non-empty.

A Java method called as an extension function may have an extra first argument of class net.sf.saxon.expr.XPathContext. This argument is not supplied by the calling XSL code, but by Saxon itself. The XPathContext object provides methods to access many internal Saxon resources, the most useful being current() which returns the current Item. The XPathContext object is not available with constructors.

If any exceptions are thrown by the method, or if a matching method cannot be found, processing of the stylesheet will be abandoned. If the tracing option has been set (-T) on the command line, a full stack trace will be output. The exception will be wrapped in a TransformerException and passed to any user-specified ErrorListener object, so the ErrorListener can also produce extra diagnostics.

The result type of the method is converted to an XPath value as follows.

If the method returns void, the XPath value is an empty sequence.
If the method returns null, the XPath value is a wrapped null Java Object.
If the method is a constructor, the XPath value is of type "wrapped Java object". The only way of using this is by passing it to another external function, or by converting it to one of the standard XPath data types as described above.
If the returned value is a Java boolean or Boolean, the XPath result is a boolean.
If the returned value is a Java double or Double, the XPath result is a double.
If the returned value is a Java float or Float, the XPath result is a float.
If the returned value is a Java int, short, or long, or one of their object wrapper equivalents, the XPath result is an integer.
If the returned value is a Java String, the XPath result is a string.
If the returned value is an instance of the Saxon class net.sf.saxon.value.value, the returned value is used unchanged.
If the returned value is an instance of the Saxon class net.sf.saxon.om.NodeInfo (a node in a Saxon tree), the XPath value will be a sequence containing a single node.
If the returned value is is an instance of the Saxon class net.sf.saxon.om.SequenceIterator (aan iterator over a sequence), the XPath value will be the sequence represented by this iterator. It is essential that this iterator properly implements the method getAnother() which returns a new iterator over the same sequence of nodes or values, positioned at the start of the sequence.
If the returned value is a DOM NodeList, the list of nodes is returned as a Saxon node-set. However, all the nodes must be instances of class net.sf.saxon.om.NodeInfo, that is, they must use Saxon's tree implementation, not some third-party DOM. But any implementation of NodeList can be used. The nodes can come from the original source tree, or from a newly-constructed tree, so long as it is constructed using Saxon.
If the returned value is a DOM Node that is not an instance of class net.sf.saxon.om.NodeInfo, it is rejected: the result must use Saxon's DOM implementation, not some third-party DOM.
If the result is any other Java object (including null), it is returned as a "wrapped Java object".

Note that Saxon's two principal tree structures both conform to the DOM Core Level 2 interface. However, it is read-only: any attempt to modify the tree causes an exception. Saxon's trees can only be built using the Saxon subclasses of the net.sf.saxon.tree.Builder class, and they cannot be modified in situ.

The system function function-available(String name) returns true if there appears to be a method available with the right name. It does not test whether this method has the appropriate number of arguments or whether the arguments are of appropriate types. If the function name is "new" it returns true so long as the class is not an abstract class or interface, and so long as it has at least one constructor.

There are a number of extension functions supplied with the Saxon product: for details, see extensions.html. The source code of these methods, which in most cases is extremely simple, can be used as an example for writing other user extension functions. It is found in class net.sf.saxon.functions.Extensions.

Writing extension instructions

Saxon implements the element extensibility feature defined in the XSLT standard. This feature allows you to define your own instruction types for use in the stylesheet. These instructions can be used anywhere within a content constructor, for example as a child of xsl:template, xsl:if, xsl:variable, or of a literal result element.

If a namespace prefix is to be used to denote extension elements, it must be declared in the extension-element-prefixes attribute on the xsl:stylesheet element, or the xsl:extension-element-prefixes attribute on any enclosing literal result element or extension element.

Note that Saxon itself provides a number of stylesheet elements beyond those defined in the XSLT specification, including saxon:assign, saxon:entity-ref, and saxon:while. To enable these, use the standard XSL extension mechanism: define extension-element-prefixes="saxon" on the xsl:stylesheet element, or xsl:extension-element-prefixes="saxon" on any enclosing literal result element.

To invoke a user-defined set of extension elements, include the prefix in this attribute as described, and associate it with a namespace URI that ends in "/" followed by the fully qualified class name of a Java class that implements the net.sf.saxon.style.ExtensionElementFactory interface. This interface defines a single method, getExtensionClass(), which takes the local name of the element (i.e., the name without its namespace prefix) as a parameter, and returns the Java class used to implement this extension element (for example, "return SQLConnect.class"). The class returned must be a subclass of net.sf.saxon.style.StyleElement.

The best way to see how to implement an extension element is by looking at the example, for SQL extension elements, provided in package net.sf.saxon.sql, and at the sample stylesheet books-sql.xsl which uses these extension elements. The main methods a StyleElement class must provide are:

prepareAttributes()	This is called while the stylesheet tree is still being built, so it should not attempt to navigate the tree. Its task is to validate the attributes of the stylesheet element and perform any preprocessing necessary. For example, if the attribute is an attribute value template, this includes creating an Expression that can subsequently be evaluated to get the AVT's value.
validate()	This is called once the tree has been built, and its task is to check that the stylesheet element appears in the right context within the tree, e.g. that it is within a template
process()	This is called to process a particular node in the source document, which can be accessed by reference to the XSLTContext object supplied as a parameter.
isInstruction()	This should return true, to ensure that the element is allowed to appear within a template body.
mayContainTemplateBody()	This should return true, to ensure that the element can contain instructions. Even if it can't contain anything else, extension elements should allow an xsl:fallback instruction to provide portability between processors

The StyleElement class has access to many services supplied either via its superclasses or via the XSLTContext object. For details, see the API documentation of the individual classes.

Any element whose prefix matches a namespace listed in the extension-element-prefixes attribute of an enclosing element is treated as an extension element. If no class can be instantiated for the element (for example, because no ExtensionElementFactory can be loaded, or because the ExtensionElementFactory doesn't recognise the local name), then fallback action is taken as follows. If the element has one or more xsl:fallback children, they are processed. Otherwise, an error is reported. When xsl:fallback is used in any other context, it and its children are ignored.

It is also possible to test whether an extension element is implemented by using the system function element-available(). This returns true if the namespace of the element identifies it as an extension element (or indeed as a standard XSL instruction) and if a class can be instantiated to represent it. If the namespace is not that of an extension element, or if no class can be instantiated, it returns false.

Writing input filters

Saxon takes its input from a SAX2 Parser reading from an InputSource. A very useful technique is to interpose a filter between the parser and Saxon. The filter will typically be an instance of the SAX2 XMLFilter class.

See the TrAX examples for hints on using a Saxon Transformer as part of a chain of SAX Filters.

Note that Saxon relies on the application to supply a well-balanced sequence of SAX events; it doesn't need to be well-formed (the root node can have any number of element or text children), but if it isn't well-balanced, the consequences are unpredictable.

Writing output filters

The output of a Saxon stylesheet can be directed to a user-defined output filter. This filter can be defined either as a standard SAX1 DocumentHandler, a SAX2 ContentHandler, or as a subclass of the Saxon class net.sf.saxon.output.Emitter. The advantage of using an Emitter is that more information is available from the stylesheet, for example the attributes of the xsl:output element.

When a ContentHandler is used, Saxon will by default always supply a stream of events corresponding to a well-formed document. (The XSLT specification also allows the output to be an external general parsed entity.) If the result tree is not well-formed, Saxon will notify the content handler of the fact by sending a processing instruction with the name "saxon:warning" and the text "Output suppressed because it is not well-formed". If the content handler is happy to accept output that is not well-formed, it can respond to this processing instruction by throwing a SAXException whose message text is "continue"; in this case subsequent events will be notified whether or not they are well-formed.

As specified in the JAXP 1.1 interface, requests to disable or re-enable output escaping are also notified to the content handler by means of special processing instructions. The names of these processing instructions are defined by the constants PI_DISABLE_OUTPUT_ESCAPING and PI_ENABLE_OUTPUT_ESCAPING defined in class javax.xml.transform.Result.

If an Emitter is used, however, it will be informed of all events.

The Emitter or ContentHandler to be used is specified in the method attribute of the xsl:output element, as a fully-qualified class name; for example method="prefix:com.acme.xml.SaxonOutputFilter". The namespace prefix is ignored, but must be present to meet XSLT conformance rules.

See the documentation of class net.sf.saxon.output.Emitter for details of the methods available, or implementations such as HTMLEmitter and XMLEmitter and TEXTEmitter for the standard output formats supported by Saxon.

It can sometimes be useful to set up a chain of emitters working as a pipeline. To write a filter that participates in such a pipeline, the class ProxyEmitter is supplied. Use the class XMLIndenter, which handles XML indentation, as an example of how to write a ProxyEmitter.

Rather than writing an output filter in Java, Saxon also allows you to process the output through another XSL stylesheet. To do this, simply name the next stylesheet in the saxon:next-in-chain attribute of xsl:output.

Any number of user-defined attributes may be defined on xsl:output. These attributes must have names in a non-null namespace, which must not be either the XSLT or the Saxon namespace. These attributes are interpreted as attribute value templates. The value of the attribute is inserted into the Properties object made available to the Emitter handling the output; they will be ignored by the standard output methods, but can supply arbitrary information to a user-defined output method. The name of the property will be the expanded name of the attribute in JAXP format, for example {http://my-namespace/uri}local-name, and the value will be the value as given, after evaluation as an attribute value template.

Implementing a collating sequence

It is possible to define a collating sequence for use by xsl:sort. This is controlled through the class attributes of the saxon:collation element. See saxon:collation for details.

Implementing a numbering sequence

It is possible to define a numbering sequence for use by xsl:number. This is controlled through the lang attribute of the xsl:number element. The feature is primarily intended to provide language-dependent numbering, but in fact it can be used to provide arbitrary numbering sequences: for example if you want to number items as "one", "two", "three" etc, you could implement a numbering class to do this and invoke it say with lang="alpha".

To implement a numberer for language X, you need to define a class net.sf.saxon.number.Numberer_X, for example net.sf.saxon.sort.Numberer_alpha. This must implement the interface Numberer. A (not very useful) Numberer is supplied for lang="de" as a specimen, and you can use this as a prototype to write your own. A numbering sequence is also supplied for lang="en", and this is used by default if no other can be loaded.

Note that any hyphens in the language name are ignored in forming the class name, but case is significant. For example if you specify lang="en-GB", the Numberer must be named net.sf.saxon.number.Numberer_enGB.

Adding an output encoding

If you want to use an output encoding that is not directly supported by Saxon (for a list of encodings that are supported, see conformance.html) you can do this by writing a Java class that implements the interface net.sf.saxon.charcode.PluggableCharacterSet. You need to supply two methods: inCharSet() which tests whether a particular Unicode character is present in the character set, and getEncodingName() which returns the name given to the encoding by your Java VM. The encoding must be supported by the Java VM. To use this encoding, specify the fully-qualified class name as the value of the encoding attribute in xsl:output.

Alternatively, it is possible to specify the CharacterSet class to be used for a named output encoding by setting the system property, e.g. -D"encoding.EUC-JP"="EUC_JP"; the value of the property should be the name of a class that implements the PluggableCharacterSet interface. This indicates the class to be used when the xsl:output element specifies encoding="EUC-JP".

Michael H. Kay
20 December 2001