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5.2. Introduction to JAXP 1.1

TrAX was a great idea, and the original work and concepts behind it were absorbed into JAXP Version 1.1. If you search for TrAX on the Web and get the feeling that the effort is waning, this is only because focus has shifted from TrAX to JAXP. Although the name has changed, the concept has not: JAXP provides a standard Java interface to many XSLT processors, allowing you to choose your favorite underlying implementation while retaining portability.

First released in March 2000, Sun's JAXP 1.0 utilized XML 1.0, XML Namespaces 1.0, SAX 1.0, and DOM Level 1. JAXP is a standard extension to Java, meaning that Sun provides a specification through its Java Community Process (JCP) as well as a reference implementation. JAXP 1.1 follows the same basic design philosophies of JAXP 1.0, adding support for DOM Level 2, SAX 2, and XSLT 1.0. A tool like JAXP is necessary because the XSLT specification defines only a transformation language; it says nothing about how to write a Java XSLT processor. Although they all perform the same basic tasks, every processor uses a different API and has its own set of programming conventions.

JAXP is not an XML parser, nor is it an XSLT processor. Instead, it provides a common Java interface that masks differences between various implementations of the supported standards. When using JAXP, your code can avoid dependencies on specific vendor tools, allowing flexibility to upgrade to newer tools when they become available.

The key to JAXP's design is the concept of plugability layers. These layers provide consistent Java interfaces to the underlying SAX, DOM, and XSLT implementations. In order to utilize one of these APIs, you must obtain a factory class without hardcoding Xalan or SAXON code into your application. This is accomplished via a lookup mechanism that relies on Java system properties. Since three separate plugability layers are used, you can use a DOM parser from one vendor, a SAX parser from another vendor, and yet another XSLT processor from someone else. In reality, you will probably need to use a DOM parser compatible with your XSLT processor if you try to transform the DOM tree directly. Figure 5-1 illustrates the high-level architecture of JAXP 1.1.

Figure 5-1

Figure 5-1. JAXP 1.1 architecture

As shown, application code does not deal directly with specific parser or processor implementations, such as SAXON or Xalan. Instead, you write code against abstract classes that JAXP provides. This level of indirection allows you to pick and choose among different implementations without even recompiling your application.

The main drawback to an API such as JAXP is the "least common denominator" effect, which is all too familiar to AWT programmers. In order to maximize portability, JAXP mostly provides functionality that all XSLT processors support. This means, for instance, that Xalan's custom XPath APIs are not included in JAXP. In order to use value-added features of a particular processor, you must revert to nonportable code, negating the benefits of a plugability layer. Fortunately, most common tasks are supported by JAXP, so reverting to implementation-specific code is the exception, not the rule.

Although the JAXP specification does not define an XML parser or XSLT processor, reference implementations do include these tools. These reference implementations are open source Apache XML tools,[18] so complete source code is available.

[18] Crimson and Xalan.

5.2.1. JAXP 1.1 Implementation

You guessed it -- we will now reimplement the simple example using Sun's JAXP 1.1. Behind the scenes, this could use any JAXP 1.1-compliant XSLT processor; this code was developed and tested using Apache's Xalan 2 processor. Example 5-3 contains the complete source code.

Example 5-3. SimpleJaxp.java

package chap5;

import java.io.*;

/**
 * A simple demo of JAXP 1.1
 */
public class SimpleJaxp {

    /**
     * Accept two command line arguments: the name of an XML file, and
     * the name of an XSLT stylesheet. The result of the transformation
     * is written to stdout.
     */
    public static void main(String[] args)
            throws javax.xml.transform.TransformerException {
        if (args.length != 2) {
            System.err.println("Usage:");
            System.err.println("  java " + SimpleJaxp.class.getName( )
                    + " xmlFileName xsltFileName");
            System.exit(1);
        }

        File xmlFile = new File(args[0]);
        File xsltFile = new File(args[1]);

        javax.xml.transform.Source xmlSource =
                new javax.xml.transform.stream.StreamSource(xmlFile);
        javax.xml.transform.Source xsltSource =
                new javax.xml.transform.stream.StreamSource(xsltFile);
        javax.xml.transform.Result result =
                new javax.xml.transform.stream.StreamResult(System.out);

        // create an instance of TransformerFactory
        javax.xml.transform.TransformerFactory transFact =
                javax.xml.transform.TransformerFactory.newInstance( );

        javax.xml.transform.Transformer trans =
                transFact.newTransformer(xsltSource);

        trans.transform(xmlSource, result);
    }
}

As in the earlier examples, explicit package names are used in the code to point out which classes are parts of JAXP. In future examples, import statements will be favored because they result in less typing and more readable code. Our new program begins by declaring that it may throw TransformerException:

public static void main(String[] args)
        throws javax.xml.transform.TransformerException {

This is a general-purpose exception representing anything that might go wrong during the transformation process. In other processors, SAX-specific exceptions are typically propagated to the caller. In JAXP, TransformerException can be wrapped around any type of Exception object that various XSLT processors may throw.

Next, the command-line arguments are converted into File objects. In the SAXON and Xalan examples, we created a system ID for each of these files. Since JAXP can read directly from a File object, the extra conversion to a URI is not needed:

File xmlFile = new File(args[0]);
File xsltFile = new File(args[1]);

javax.xml.transform.Source xmlSource =
        new javax.xml.transform.stream.StreamSource(xmlFile);
javax.xml.transform.Source xsltSource =
        new javax.xml.transform.stream.StreamSource(xsltFile);

The Source interface is used to read both the XML file and the XSLT file. Unlike the SAX InputSource class or Xalan's XSLTInputSource class, Source is an interface that can have many implementations. In this simple example we are using StreamSource, which has the ability to read from a File object, an InputStream, a Reader, or a system ID. Later we will examine additional Source implementations that use SAX and DOM as input. Just like Source, Result is an interface that can have several implementations. In this example, a StreamResult sends the output of the transformations to System.out:

javax.xml.transform.Result result =
        new javax.xml.transform.stream.StreamResult(System.out);

Next, an instance of TransformerFactory is created:

javax.xml.transform.TransformerFactory transFact =
        javax.xml.transform.TransformerFactory.newInstance( );

The TransformerFactory is responsible for creating Transformer and Template objects. In our simple example, we create a Transformer object:

javax.xml.transform.Transformer trans =
        transFact.newTransformer(xsltSource);

Transformer objects are not thread-safe, although they can be used multiple times. For a simple example like this, we will not encounter any problems. In a threaded servlet environment, however, multiple users cannot concurrently access the same Transformer instance. JAXP also provides a Templates interface, which represents a stylesheet that can be accessed by many concurrent threads.

The transformer instance is then used to perform the actual transformation:

trans.transform(xmlSource, result);

This applies the XSLT stylesheet to the XML data, sending the result to System.out.

5.2.2. XSLT Plugability Layer

JAXP 1.1 defines a specific lookup procedure to locate an appropriate XSLT processor. This must be accomplished without hardcoding vendor-specific code into applications, so Java system properties and JAR file service providers are used. Within your code, first locate an instance of the TransformerFactory class as follows:

javax.xml.transform.TransformerFactory transFact =
        javax.xml.transform.TransformerFactory.newInstance( );

Since TransformerFactory is abstract, its newInstance( ) factory method is used to instantiate an instance of a specific subclass. The algorithm for locating this subclass begins by looking at the javax.xml.transform.TransformerFactory system property. Let us suppose that com.foobar.AcmeTransformer is a new XSLT processor compliant with JAXP 1.1. To utilize this processor instead of JAXP's default processor, you can specify the system property on the command line[19] when you start your Java application:

[19] System properties can also be specified in Ant build files.

java -Djavax.xml.transform.TransformerFactory=com.foobar.AcmeTransformer MyApp

Provided that JAXP is able to instantiate an instance of AcmeTransformer, this is the XSLT processor that will be used. Of course, AcmeTransformer must be a subclass of TransformerFactory for this to work, so it is up to vendors to offer support for JAXP.

If the system property is not specified, JAXP next looks for a property file named lib/jaxp.properties in the JRE directory. A property file consists of name=value pairs, and JAXP looks for a line like this:

javax.xml.transform.TransformerFactory=com.foobar.AcmeTransformer

You can obtain the location of the JRE with the following code:

String javaHomeDir = System.getProperty("java.home");
NOTE: Some popular development tools change the value of java.home when they are installed, which could prevent JAXP from locating jaxp.properties. JBuilder, for instance, installs its own version of Java 2 that it uses by default.

The advantage of creating jaxp.properties in this directory is that you can use your preferred processor for all of your applications that use JAXP without having to specify the system property on the command line. You can still override this file with the -D command-line syntax, however.

If jaxp.properties is not found, JAXP uses the JAR file service provider mechanism to locate an appropriate subclass of TransformerFactory. The service provider mechanism is outlined in the JAR file specification from Sun and simply means that you must create a file in the META-INF/services directory of a JAR file. In JAXP, this file is called javax.xml.transform.TransformerFactory. It contains a single line that specifies the implementation of TransformerFactory: com.foobar.AcmeTransformer in our fictitious example. If you look inside of xalan.jar in JAXP 1.1, you will find this file. In order to utilize a different parser that follows the JAXP 1.1 convention, simply make sure its JAR file is located first on your CLASSPATH.

Finally, if JAXP cannot find an implementation class from any of the three locations, it uses its default implementation of TransformerFactory. To summarize, here are the steps that JAXP performs when attempting to locate a factory:

  1. Use the value of the javax.xml.transform.TransformerFactory system property if it exists.

  2. If JRE/lib/jaxp.properties exists, then look for a javax.xml.transform.TransformerFactory=ImplementationClass entry in that file.

  3. Use a JAR file service provider to look for a file called META-INF/services/javax.xml.transform.TransformerFactory in any JAR file on the CLASSPATH.

  4. Use the default TransformerFactory instance.

The JAXP 1.1 plugability layers for SAX and DOM follow the exact same process as the XSLT layer, only they use the javax.xml.parsers.SAXParserFactory and javax.xml.parsers.DocumentBuilderFactory system properties respectively. It should be noted that JAXP 1.0 uses a much simpler algorithm where it checks only for the existence of the system property. If that property is not set, the default implementation is used.

5.2.3. The Transformer Class

As shown in Example 5-3, a Transformer object can be obtained from the TransformerFactory as follows:

javax.xml.transform.TransformerFactory transFact =
        javax.xml.transform.TransformerFactory.newInstance( );
javax.xml.transform.Transformer trans =
        transFact.newTransformer(xsltSource);

The Transformer instance is wrapped around an XSLT stylesheet and allows you to perform as many transformations as you wish. The main caveat is thread safety, because many threads cannot use a single Transformer instance concurrently. For each transformation, invoke the transform method:

abstract void transform(Source xmlSource, Result outputTarget)
    throws TransformerException

This method is abstract because the TransformerFactory returns a subclass of Transformer that does the actual work. The Source interface defines where the XML data comes from and the Result interface specifies where the transformation result is sent. The TransformerException will be thrown if anything goes wrong during the transformation process and may contain the location of the error and a reference to the original exception. The ability to properly report the location of the error is entirely dependent upon the quality of the underlying XSLT transformer implementation's error reporting. We will talk about specific classes that implement the Source and Result interfaces later in this chapter.

Aside from actually performing the transformation, the Transformer implementation allows you to set output properties and stylesheet parameters. In XSLT, a stylesheet parameter is declared and used as follows:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="html"/>
  <xsl:param name="image_dir" select="'images'"/>
  
  <xsl:template match="/">
    <html>
      <body>
        <h1>Stylesheet Parameter Example</h1>
        <img src="{$image_dir}/sample.gif"/>
      </body>
    </html>
  </xsl:template>
</xsl:stylesheet>

The <xsl:param> element declares the parameter name and an optional select attribute. This attribute specifies the default value if the stylesheet parameter is not provided. In this case, the string 'images' is the default value and is enclosed in apostrophes so it is treated as a string instead of an XPath expression. Later, the image_dir variable is referred to with the attribute value template syntax: {$image_dir}.

Passing a variable for the location of your images is a common technique because your development environment might use a different directory name than your production web server. Another common use for a stylesheet parameter is to pass in data that a servlet generates dynamically, such as a unique ID for session tracking.

From JAXP, pass this parameter via the Transformer instance. The code is simple enough:

javax.xml.transform.Transformer trans =
        transFact.newTransformer(xsltSource);
trans.setParameter("image_dir", "graphics");

You can set as many parameters as you like, and these parameters will be saved and reused for every transformation you make with this Transformer instance. If you wish to remove a parameter, you must call clearParameters( ), which clears all parameters for this Transformer instance. Parameters work similarly to a java.util.Map; if you set the same parameter twice, the second value overwrites the first value.

Another use for the Transformer class is to get and set output properties through one of the following methods:

void setOutputProperties(java.util.Properties props)
void setOutputProperty(String name, String value)
java.util.Properties getOutputProperties( )
String getOutputProperty(String name)

As you can see, properties are specified as name/value pairs of Strings and can be set and retrieved individually or as a group. Unlike stylesheet parameters, you can un-set an individual property by simply passing in null for the value. The permitted property names are defined in the javax.xml.transform.OutputKeys class and are explained in Table 5-1.

Table 5-1. Constants defined in javax.xml.transform.OutputKeys

Constant

Meaning

CDATA_SECTION_ELEMENTS

Specifies a whitespace-separated list of element names whose content should be output as CDATA sections. See the XSLT specification from the W3C for examples.

DOCTYPE_PUBLIC

Only used if DOCTYPE_SYSTEM is also used, this instructs the processor to output a PUBLIC document type declaration. For example: <!DOCTYPE rootElem PUBLIC "public id" "system id">.

DOCTYPE_SYSTEM

Instructs the processor to output a document-type declaration. For example: <!DOCTYPE rootElem SYSTEM "system id">.

ENCODING

Specifies the character encoding of the result tree, such as UTF-8 or UTF-16.

INDENT

Specifies whether or not whitespace may be added to the result tree, making the output more readable. Acceptable values are yes and no. Although indentation makes the output more readable, it does make the file size larger, thus harming performance.

MEDIA_TYPE

The MIME type of the result tree.

METHOD

The output method, either xml, html, or text. Although other values are possible, such as xhtml, these are implementation-defined and may be rejected by your processor.

OMIT_XML_DECLARATION

Acceptable values are yes and no, specifying whether or not to include the XML declaration on the first line of the result tree.

STANDALONE

Acceptable values are yes and no, specifying whether or not the XML declaration indicates that the document is standalone. For example: <?xml version="1.0" encoding="UTF-8" standalone="yes"?>.

VERSION

Specifies the version of the output method, typically 1.0 for XML output. This shows up in the XML declaration as follows: <?xml version="1.0" encoding="UTF-8"?>.

It is no coincidence that these output properties are the same as the properties you can set on the <xsl:output> element in your stylesheets. For example:

<xsl:output method="xml" indent="yes" encoding="UTF-8"/>

Using JAXP, you can either specify additional output properties or override those set in the stylesheet. To change the encoding, write this code:

// this will take precedence over any encoding specified in the stylesheet
trans.setOutputProperty(OutputKeys.ENCODING, "UTF-16");

Keep in mind that this will, in addition to adding encoding="UTF-16" to the XML declaration, actually cause the processor to use that encoding in the result tree. For a value of UTF-16, this means that 16-bit Unicode characters will be generated, so you may have trouble viewing the result tree in many ASCII-only text editors.

5.2.4. JAXP XSLT Design

Now that we have seen some example code and have begun our exploration of the Transformer class, let's step back and look at the overall design of the XSLT plugability layer. JAXP support for XSLT is broken down into the packages listed in Table 5-2.

Table 5-2. JAXP transformation packages

Package

Description

javax.xml.transform

Defines a general-purpose API for XML transformations without any dependencies on SAX or DOM. The Transformer class is obtained from the TransformerFactory class. The Transformer transforms from a Source to a Result.

javax.xml.transform.dom

Defines how transformations can be performed using DOM. Provides implementations of Source and Result: DOMSource and DOMResult.

javax.xml.transform.sax

Supports SAX2 transformations. Defines SAX versions of Source and Result: SAXSource and SAXResult. Also defines a subclass of TransformerFactory that allows SAX2 events to be fed into an XSLT processor.

javax.xml.transform.stream

Defines I/O stream implementations of Source and Result: StreamSource and StreamResult.

The heart of JAXP XSLT support lies in the javax.xml.transform package, which lays out the mechanics and overall process for any transformation that is performed. This package mostly consists of interfaces and abstract classes, except for OutputKeys and a few exception and error classes. Figure 5-2 presents a UML class diagram that shows all of the pieces in this important package.

Figure 5-2

Figure 5-2. javax.xml.transform class diagram

As you can see, this is a small package, indicative of the fact that JAXP is merely a wrapper around the tools that actually perform transformations. The entry point is TransformerFactory, which creates instances of Transformer, as we have already seen, as well as instances of the Templates abstract class. A Templates object represents a compiled stylesheet and will be covered in detail later in this chapter.[20] The advantage of compilation is performance: the same Templates object can be used over and over by many threads without reparsing the XSLT file.

[20] The exact definition of a "compiled" stylesheet is vague. XSLT processors are free to optimize cached stylesheets however they see fit.

The URIResolver is responsible for resolving URIs found within stylesheets and is generally something you will not need to deal with directly. It is used when a stylesheet imports or includes another document, and the processor needs to figure out where to look for that document. For example:

<xsl:import href="commonFooter.xslt"/>

ErrorListener, as you may guess, is an interface that allows your code to register as a listener for error conditions. This interface defines the following three methods:

void error(TransformerException ex)
void fatalError(TransformerException ex)
void warning(TransformerException ex)

The TransformerException has the ability to wrap around another Exception or Throwable object and may return an instance of the SourceLocator class. If the underlying XSLT implementation does not provide a SourceLocator, null is returned. The SourceLocator interface defines methods to locate where a TransformerException originated. In the case of error() and warning(), the XSLT processor is required to continue processing the document until the end. For fatalError(), on the other hand, the XSLT processor is not required to continue. If you do not register an ErrorListener object, then all errors, fatal errors, and warnings are normally written to System.err. TransformerFactoryConfigurationError and TransformerConfigurationException round out the error-handling APIs for JAXP, indicating problems configuring the underlying XSLT processor implementation. The TransformerFactoryConfigurationError class is generally used when the implementation class cannot be found on the CLASSPATH or cannot be instantiated at all. TransformerConfigurationException simply indicates a "serious configuration error" according to its documentation.



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