xdocs/sources/xalan/faq.xml

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<faqs title='Frequently asked questions'>
  <group title='General XSLT'>
    <faq title='Where do I go to learn about XSLT'>
      <q>Where do I go to learn about XSLT?</q>
      <a>
        <p>The definitive sources are the W3C XSLT and XPath recommendations:
          <resource-ref idref='xslt' /> and
          <resource-ref idref='xpath' />.
        </p>
        <p>For a brief listing of tutorials, discussion forums, and other materials, see
          <link anchor='uptospeed' idref='overview'>Getting up to speed with XSLT</link>.
        </p>
      </a>
    </faq>
    <faq title='Where can I ask questions about XSLT'>
      <q>Where can I ask questions about XSLT?</q>
      <a>
        <p>xalan-dev and xalan-j-users are for Xalan-Java questions only. This is not the best forum to ask general XSLT questions. The
          <resource-ref idref='mulberryxsl-list' /> is an excellent place to ask XSLT questions; please search
          <resource-ref idref='dpawsonxslfaq' /> to ensure your question has not been already asked.
        </p>
      </a>
    </faq>
    <faq title='TrAX and JAXP'>
      <q>What are TrAX and JAXP, and are they related?</q>
      <a>
        <p>TrAX is the Transformation API for XML. In November 2000, TrAX was revised and incorporated into JAXP, the JAVA API for XML
    Processing. JAXP (including TrAX) provides users a standard, vendor-neutral API for working with (and transforming) XML documents. 
    You can use this API to build applications that are not bound to the particular implementation details of a given XML parser or XSL
    transformer.</p>
        <p>Xalan-Java includes the JAXP packages, implements the TrAX portion of that API (javax.xml.transform....), and includes xercesImpl.jar from
    Xerces-Java, which implements the parser portion of the API (javax.xml.parser....).</p>
        <p>For more information, see
          <link idref='trax'>TRaX (Transformation API for XML)</link> and
          <resource-ref idref='jaxp11' />.
        </p>
      </a>
    </faq>
  </group>
  <group title='Versions'>
    <faq title='Determining Xalan-Java Version'>
      <q>How do I see what version of Xalan-Java I'm running? How do I determine which parser I'm using?</q>
      <a>
        <p>
          <anchor name='environmentcheck' />
        </p>
        <p>
          <em>Using the EnvironmentCheck utility:</em> To help diagnose classpath problems and also determine which version of Xalan-Java is being used, try running Xalan-Java's environment checking utility, checked in at 
    xml-xalan/java/src/org/apache/xalan/xslt/EnvironmentCheck.
        </p>
        <p>You can run this utility from the command line as follows:</p>
        <p>
          <code>java org.apache.xalan.xslt.EnvironmentCheck [-out outFile]</code>
        </p>
        <p>You can also call this utility from within your application. For example,</p>
        <p>
          <code>boolean environmentOK = (new EnvironmentCheck()).checkEnvironment (yourPrintWriter);</code>
        </p>
        <p>Be sure to run EnvironmentCheck in the environment where you are experiencing the problem. For example, if you get a 
    NoClassDefFound error from a command-line application, run EnvironmentCheck on the command line with exactly the same 
    classpath. If the error occurs inside your Java application (or in a servlet, etc.), be sure to call the 
    EnvironmentCheck checkEnvironment(...) method from within your running application.</p>
        <p>Best of all, you can call checkEnvironment from a stylesheet using extensions:</p>
        <source>
&lt;?xml version="1.0"?&gt;
&lt;xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"
    xmlns:xalan="http://xml.apache.org/xalan"
    exclude-result-prefixes="xalan"&gt;
&lt;xsl:output indent="yes"/&gt;

&lt;xsl:template match="/"&gt;
  &lt;out&gt;
    &lt;xsl:copy-of select="xalan:checkEnvironment()"/&gt;
  &lt;/out&gt;
&lt;/xsl:template&gt;
&lt;/xsl:stylesheet&gt;</source>
      </a>
    </faq>
    <faq title='Which version of Xerces should I be using?'>
      <q>Which version of Xerces should I be using?</q>
      <a>
        <p><resource-ref idref='xslt4j-current' /> has been tested with <resource-ref idref='xml4j-used' />. See
          <link anchor='status' idref='readme'>Status</link>.
        </p>
      </a>
    </faq>
    <faq title='Compatibility with Xalan-Java version 1'>
      <q>How do I run applications that use the Xalan-Java version 1 API with Xalan-Java 2</q>
      <a>
        <p>The Xalan-Java 1 compatibility API has been deprecated and now purged, so you must use the Xalan-Java 2 API. We strongly encourage you to
   use the JAVAX 1.1/TrAX API. For more information ,see the next FAQ.</p>
      </a>
    </faq>
    <faq title='Issues running Xalan-Java on JDK 1.3'>
      <q>I'm having a problem building or running Xalan-Java on the JDK 1.3.</q>
      <a>
        <p>The JDK 1.3 automatically places everything in the lib/ext directory in front of everything you place on the classpath. If this directory contains a version of DOM, JAXP, or Xerces that predates the Xalan-Java distribution you are using, you may have problems!</p>
        <p>The IBM JDK 1.3 includes an earlier version of xerces.jar in the lib/ext directory, a version that does not implement the JAXP 1.1 interfaces and therefore does not work with the current Xalan-Java release. Accordingly, you must either purge the xerces.jar that is in that directory or replace it with the xercesImpl.jar that is included with the Xalan-Java distribution.</p>
        <p>The SUN JDK 1.3 includes a pre-1.1 version of the JAXP in crimson.jar. Either purge the crimson.jar in that directory or overwrite it with a newer crimson.jar that includes and implements the JAXP 1.1 interfaces.</p>
      </a>
    </faq>
    <faq title='Issues running Xalan-Java on JDK 1.4'>
      <q>I'm having a problem running Xalan-Java on the JDK 1.4.</q>
      <a>
        <p>The Sun JDK 1.4 is packaged with an old version (2.2D11) of Xalan-Java. The JDK 1.4 will attempt to use this version instead of any on the classpath. Unfortunately, this causes problems when attempting to use a newer version of Xalan-Java with the Sun JDK 1.4.</p>
        <p>You can always determine which version of Xalan-Java you are running by using the
          <link anchor='environmentcheck'>EnvironmentCheck</link> class or by using the xalan:checkEnvironment extension function. It is highly recommended that you use this method to verify the version of Xalan-Java you are running, especially before opening a bug report.
        </p>
        <p>There are several ways to use a later version of Xalan-Java and override the one packaged with the JDK:</p>
        <ul>
          <li>For the SUN JDK 1.4, use the
            <resource-ref idref='endorsed' />. Place the xalan.jar, xercesImpl.jar, and xml-apis.jar in the
            &lt;java-home&gt;\lib\endorsed directory, where
            &lt;java-home&gt; is where the runtime software is installed.
          </li>
          <li>Use the -Xbootclasspath java commandline option to prepend the new xalan.jar, xercesImpl.jar, and xml-apis.jar to the boot class path. When running Xalan:
            <br />
            <br />
	    <code>java -Xbootclasspath/p:&lt;path&gt;
		    org.apache.xalan.xslt.Process</code>
            <br />
	    <br />where &lt;path&gt; is a colon seperated lists of the paths to the files xalan.jar, xercesImpl.jar, and xml-apis.jar (e.g. bin/xalan.jar:bin/xercesImpl.jar:bin/xml-apis.jar) containing the new-version of Xalan-Java.
          </li>
        </ul>
        <p>The following methods
          <em>do not work</em>:
        </p>
        <ul>
          <li>Using the CLASSPATH environment variable or using -classpath to place the new classes in the classpath.</li>
          <li>Using the -jar option to explicitly execute the classes inside the new jar files.</li>
        </ul>
      </a>
    </faq>
    <faq title="Using the 'signature' file to verify a download">
      <q>How do I use the "signature" file to verify my download?</q>
      <a>
        <p>For each Xalan-Java download file in
          <resource-ref idref='xslt4j-distdir' />, there is a corresponding signature file. 
        The signature file for xalan-j_2_0_1.tar.gz, for example, is xalan-j_2_0_1.tar.gz.sig.
        </p>
        <p>The .sig files are PGP signatures of the actual .zip or .tar.gz
        download files.  You can use these files to verify the authenticiy of the download. You do not need the .sig file to 
        use the corresponding donwload file.</p>
        <p>To check the authenticity of a Xalan-Java distribution, you need a copy of
        PGP which is available in a number of licenses, including some free
        non-commercial licenses, either from an mit.edu site or on
        the pgp.com site. Once you have a version of PGP installed, you
        should be able to 'verify the signature' of the .sig file, which basically verifies that the corresponding 
        .zip or tar.gz file has not been changed since we signed it.</p>
      </a>
    </faq>
  </group>
  <group title='Performance Issues'>
    <faq title='Speeding up transformations'>
      <q>What can I do to speed up transformations?</q>
      <a>
        <p>In the ongoing development of Xalan-Java, enhancing performance is the primary goal of the Xalan-Java team.
    Here are some preliminary suggestions for you to keep in mind as you set up your applications:</p>
        <ul>
          <li>Use a Templates object (with a different Transformers for each transformation) to perform multiple transformations with the same 
    set of stylesheet instructions (see
            <link anchor='multithreading' idref='usagepatterns'>Multithreading</link>).
            <br />
            <br />
          </li>
          <li>Set up your stylesheets to function efficiently.
            <br />
            <br />
          </li>
          <ul>
            <li>Don't use "//" (descendant axes) patterns near the root of a large document.
              <br />
              <br />
            </li>
            <li>Use xsl:key elements and the key() function as an efficient way to retrieve node sets.
              <br />
              <br />
            </li>
            <li>Where possible, use pattern matching rather than xsl:if or xsl:when statements.
              <br />
              <br />
            </li>
            <li>xsl:for-each is fast because it does not require pattern matching.
              <br />
              <br />
            </li>
            <li>Keep in mind that xsl:sort prevents incremental processing.
              <br />
              <br />
            </li>
            <li>When you create variables, <br/><code>&lt;xsl:variable name="fooElem" select="foo"/&gt;</code><br/> is usually faster than
              <br/><code>&lt;xsl:variable name="fooElem"&gt;&lt;xsl:value-of-select="foo"/&gt;&lt;/xsl:variable&gt;</code>
              <br />
              <br />
            </li>
            <li>Be careful using the last() function.
              <br />
              <br />
            </li>
            <li>The use of index predicates within match patterns can be expensive.
              <br />
              <br />
            </li>
            <li>Decoding and encoding is expensive.
              <br />
              <br />
            </li>
          </ul>
          <li>For the ultimate in server-side scalability, perform transform operations on the client. For examples, see
            <link anchor='appletxmltohtml' idref='samples'>appletXMLtoHTML</link> and
            <link anchor='get-todo-list' idref='samples'>get-todo-list</link>.
          </li>
        </ul>
      </a>
    </faq>
  </group>
  <group title='Namespace Related'>
    <faq title='Retrieving nodes in the default namespace'>
      <q>XPath isn't retrieving nodes that are in the default namespace I defined. How do I get them?</q>
      <a>
        <p>If you are looking for nodes in a namespace, the XPath expression must include a namespace prefix that you have mapped to the
       namespace with an xmlns declaration. If you have declared a default namespace, it does not have a prefix (see
          <jump href='http://www.w3.org/TR/xpath.html#node-tests'>XPath Node Tests</jump>). In order to construct XPath expressions 
       to retrieve nodes from this namespace, you must add a namespace declaration that provides a prefix you can include in the XPath
        expressions.
        </p>
        <p>Suppose, for example, you you want to locate nodes in a default namespace declared as follows:
          <br />
          <code>xmlns="http://my-namespace"</code>
        </p>
        <p>Add a nampespace declaration with a prefix:
          <br />
          <code>xmlns:foo="http://my-namespace"</code>
        </p>
        <p>Then you can use foo: in your XPath expression.</p>
        <p>Hint: Don't use default namespaces, and the problem doesn't arise.</p>
      </a>
    </faq>
    <faq title='Setting the parser to be namespace aware'>
      <q>
        How do I set my parser to be namespace aware?
      </q>
      <a>
        <p><anchor name='namespace-aware' /></p>
        <p>If you use a TransformerFactory to process a stylesheet Source and generate a Transformer, the
  TransformerFactory instructs the  SAXParserFactory to set the parser's namespaceAware property to true.
  But if you call the parser directly, you may need to set the namespaceAware property yourself. For
  example:</p>
        <source>javax.xml.parsers.SAXParserFactory spFactory =
  javax.xml.parsers.SAXParserFactory.newInstance();
spFactory.setNamespaceAware(true);</source>
        <note>For more information about setting the namespaceAware property, and SAX2 vs. JAXP default settings, see
          <jump href='http://xml.apache.org/~edwingo/jaxp-faq.html#nsDefaults'>JAXP FAQ: Warning about namespace processing defaults</jump>.
        </note>
      </a>
    </faq>
  </group>
  <group title='Common Errors'>
    <faq title='NoClassDefFound errors'>
      <q>I'm getting a NoClassDefFound error. What has to be on the classpath?</q>
      <a>
        <ol>
          <li>xalan.jar, xml-apis.jar, and xercesImpl.jar -- or the XML parser you are using -- must always be on the classpath.
            <br />
            <br />
          </li>
          <li>To run the samples in the samples subdirectories, xalansamples.jar must be on the classpath. To run the servlet (in
    samples/servlet), xalanservlet.jar must be on the classpath along with the javax.servlet and javax.servlet.http packages. Sun distributes
    the javax.servlet packages in the JSWDK servlet.jar file.
            <br />
            <br />
          </li>
          <li>To run extensions which use the component and script extension elements (including the samples in samples/extensions), bsf.jar must be on the 
    classpath. To run extensions implemented in JavaScript, js.jar must also be on the classpath. For information on what 
    you need to run extensions implemented in other scripting languages, see
            <link anchor='supported-lang' idref='extensions'>Supported languages</link>.
          </li>
        </ol>
    You can check the correctness of your environment with the
        <link anchor='environmentcheck'>EnvironmentCheck</link> feature.
        <p>For more information, see
          <link anchor='classpath' idref='getstarted'>Setting up the system classpath</link>.
        </p>
      </a>
    </faq>
    <faq title='a &quot;DOM006 Hierarchy request error&quot;'>
      <q>Why do I get a "DOM006 Hierarchy request error" when I try to transform into a DOM Document node?</q>
      <a>
        <p>This error occurs when Xalan-Java tries to add a Node to a Document node where it isn't allowed. For example, attempting to add non-whitespace
  text to the DOM Document node produces this error.</p>
        <p>The error can also occur when a Document node is created with the DOMImplementation createDocument() method, which takes a qualified name
    as an argument and creates an element node.  If you then pass the returned Document node to Xalan-Java, you get a "DOM006 Hierarchy request
    error" when Xalan-Java tries to add a second element to the Document node. The solution is to either use the DocumentBuilder newDocument() method
    to create a Document that does not contain an element node, or use a DocumentFragment. It should be noted that the 
    DocumentBuilder newDocument() method is "Non-preferred" according to the JAXP 1.1 documentation.</p>
      </a>
    </faq>
    <faq title='Namespace not supported by SAXParser'>
      <q>Why am I getting a "Namespace not supported by SAXParser exception?</q>
      <a>
        <p>We have seen this probem arise for two quite different reasons:</p>
        <ul>
          <li>SAX1 interfaces are on your classpath in front of the SAX2 interfaces provided with your XML
  parser.
            <br />
            <br />
  or
            <br />
            <br />
          </li>
          <li>The parser you are using to process a stylesheet Source and generate a Transformer does not have the 
  namespaceAware property set to true.</li>
        </ul>
        <p>
          <em>SAX1 on the classpath</em>
        </p>
        <p>SAX1 should not be on your classpath. The SAX1 interfaces and implementations of the SAX1 SAXPparser
  are not namespace aware.</p>
        <p>To help diagnose your classpath, you can use the
          <link anchor='environmentcheck'>EnvironmentCheck
  utility</link>. If you are running under JDK 1.3, see
          <link anchor='jdk13'>Issues running Xalan-Java on JDK
  1.3</link>. If you are running a servlet, make sure the servlet engine is not placing SAX1 on the
  classpath.
        </p>
        <p>When you create a Transformer, you must use a
          <link anchor='namespace-aware'>namespace-aware</link> parser to parse the stylesheet.
        </p>
      </a>
    </faq>
    <faq title='Missing xsl:version attribute error'>
      <q>I'm getting an error about my stylesheet missing the xsl:version attribute - what's wrong?</q>
      <a>
        <p>There are two common causes for this error.</p>
        <ol>
          <li>Using the wrong URI for the xsl: prefix will cause this message. URIs, and namespace URIs in particular, are case sensitive. Ensure the URI for the xsl namespace is "http://www.w3.org/1999/XSL/Transform".</li>
          <li>The parser you are using to process a stylesheet Source and generate a Transformer does not have the namespaceAware property set to true.<br/>
          When you create a Transformer, you must use a <link anchor='namespace-aware'>namespace-aware</link> parser to parse the stylesheet.</li>
        </ol>
      </a>
    </faq>
    <faq title='StackOverflowError with recursive stylesheet'>
      <q>Xalan-Java dies with a java.lang.StackOverflowError when I run a deeply recursive stylesheet. The same stylesheet worked fine in the past (or on other machines). What's happening?</q>
      <a>
        <p>That may not be our fault.  As of JDK 1.3.x, many Java Virtual Machine publishers reduced the default size of a thread's call stack from 1MB to 256KB. This allows more threads to run simultaneously, but it means that each thread is more limited in how deeply its function calls can be nested.</p>
	<p>Some JVMs may offer an option that allows you to raise this limit. For example, in Sun JDK 1.3.1 you can start JVM with the -Xss1m option to allow each thread to use a full megabyte.  In IBM's JDK you can start with the -Xss1m and -Xoss1m options.  Other JVMs may set this in other ways, or may not allow you to control it at all; check the documentation on your system for details.</p>
        <p>Note too that on some platforms 1MB is an architectural upper limit on the stack size, so setting -Xss2m (or equivalent) may not allow deeper recusion than -Xss1m.</p>
      </a>
    </faq>
    <faq title='OutOfMemoryError processing multiple documents'>
      <q>I get a java.lang.OutOfMemoryError when I try to process multiple documents with the document() function. What can I do?</q>
      <a>
        <p>As a general rule, Xalan-Java currently caches all of the documents that you read in with the document() function during a transformation.</p>
        <p>If your objective is to transform a series of documents, you can break the process into a series of transformations. 
The
          <link anchor='pipedocument' idref='extensionslib'>PipeDocument</link> extension element provides one strategy for batching a 
series of parallel transformations.
        </p>
        <p>Another alternative is to place your document() call in the select attribute of an xsl:for-each instruction element 
and use a custom PI (Processing Instruction) to turn off document caching. Include an XPath expression in your document() call if you do not 
need to process the entire document.</p>
        <p>Sample stylesheet fragment:</p>
        <source>
&lt;xsl:template match="doc"&gt;
  &lt;xsl:for-each select="document(@href)/bar/zulu"&gt;
    &lt;?xalan:doc-cache-off?&gt;
    &lt;!-- process each document --&gt;
  &lt;xsl:for-each&gt;
&lt;/xsl:template&gt;</source>
        <note>PIs do not ordinarily uses namespaces, so "xalan:" is a 'fake' namespace we have included to indicate that this is not a standard PI.</note>
        <p>If you include an XPath expression in your document() call, you can also turn on
          <link anchor='incremental' idref='dtm'>incremental transform</link>
to eliminate the need to read in the entire document. In fact, you can take advantage of the incremental transform feature even if you are not turning
off document caching.
        </p>
        <p>You can also increase your jvm heap size with the -Xmx or -mx flag, depending on which JVM you are using (you can include both flags, and the JVM will ignore the one it doesn't understand). For example, to give your JVM 64 meg, try
          <br />
          <code> java -Xmx64m -mx64m
            <ref>Class</ref>
          </code>
        </p>
      </a>
    </faq>
    <faq title="File Not Found error">
      <q>Why do I get 'file not found' when I pass c:\path\filename.txt?</q>
      <a>
        <p>Xalan often requires legal URLs as system identifiers, not local pathnames (this is partly due to underlying parsers requiring this). A simple (but not always correct!) way to change a local pathname into a URL in Java 1.1x is:</p>
        <source>
  public static String filenameToURL(String filename)
  {
    File f = new File(filename);
    String tmp = f.getAbsolutePath();
    if (File.separatorChar == '\\')
    {
      tmp = tmp.replace('\\', '/');
    }
    // Note: gives incorrect results when filename already begins with
file:///
    return "file:///" + tmp;
  }
        </source>
        <p>For a slightly more detailed example, see org.apache.xml.utils.SystemIDResolver.</p>
      </a>
    </faq>
  </group>
  <group title='Miscellaneous'>
    <faq title='Chaining transformations'>
      <q>How do you chain together a series of transformations?"</q>
      <a>
        <p>Xalan-Java supports two strategies for chaining together a series of transformations such that the output of each 
    transformation provides input for the next transformation.</p>
        <ul>
          <li>For each transformation in the series, you can set one SAX ContentHandler to process the input, and another ContenHandler to process
       the output.
            <br />
            <br />
          </li>
          <li>You can also set up a series of parent-child relationships between an XMLReader and one or more XMLFilters.</li>
        </ul>
        <p>For the details and links to examples, see
          <link anchor='outasin' idref='usagepatterns'>Using transformation output as input for 
    another transformation</link>.
          <anchor name='jdk13' />
        </p>
      </a>
    </faq>
    <faq title='Stylesheet validation'>
      <q>How do I validate an XSL stylesheet?</q>
      <a>
        <p>An XSL stylesheet is an XML document, so it can have a DOCTYPE and be subject to validation, right?</p>
        <p>The XSLT Recommendation includes a
          <jump href='http://www.w3.org/TR/xslt#dtd'>DTD Fragment 
      for XSL Stylesheets</jump> with some indications of what you need to do to create a complete DTD for a given
      stylesheet. Keep in mind that stylesheets can include literal result elements and produce output that is not valid
      XML.
        </p>
        <p>You can use the xsl:stylesheet doctype defined in xsl-html40s.dtd for stylesheets that generate HTML.</p>
      </a>
    </faq>
    <faq title='Setting output encoding'>
      <q>Why is the output character encoding I set in the stylesheet not being used?</q>
      <a>
        <p>If you use a character output stream to instantiate the
          <jump href='apidocs/javax/xml/transform/stream/StreamResult.html'>StreamResult</jump> object which holds the 
    transformation output, the Writer uses its own encoding, not the encoding you specify
    in the stylesheet.
        </p>
        <p>If you want to use the stylesheet output encoding, do not use StreamResult(java.io.Writer) to
    instantiate the holder for the output. Alternatively, you can specify the encoding when you create a Writer 
    (java.io.OutputStreamWriter). Once the Writer exists, you cannot change its encoding.</p>
      </a>
    </faq>
    <faq title='Getting line and column numbers for errors in XML input documents and XSL stylesheets'>
      <q>How do I get line numbers for errors in the XML or XSL input when I am performing a transformation?</q>
      <a>
        <p>Use or mimic the command-line processor (
          <jump href='apidocs/org/apache/xalan/xslt/Process.html'>org.apache.xalan.xslt.Process</jump>).
        </p>
        <p>A
          <jump href='apidocs/javax/xml/transform/TransformerException.html'>TransformerException</jump> generally wraps another exception, often a SAXParseException. The command-line processor uses the static
          <jump href='apidocs/org/apache/xml/utils/DefaultErrorHandler.html'>org.apache.xml.utils.DefaultErrorHandler</jump> printLocation() method to chase down the exception cause and get a
          <jump href='apidocs/javax/xml/transform/SourceLocator.html'>SourceLocator</jump>that can usually report line and column number.
        </p>
        <p>Suppose you wanted to modify the ValidateXMLInput sample in the samples/Validate subdirectory to include line and column numbers . All you
need to do is call DefaultErrorHandler.printLocation() in the the Handler internal class error() and warning() methods. For example, replace</p>
        <source>public void error (SAXParseException spe)
  throws SAXException
{
  System.out.println("SAXParseException error: " + spe.getMessage());
}</source>
        <p>with</p>
        <source>public void error (SAXParseException spe)
  throws SAXException
{
  PrintWriter pw = new PrintWriter(System.out, true);
  org.apache.xml.utils.DefaultErrorHandler.printLocation(pw, spe);
  pw.println("SAXParseException error: " + spe.getMessage());
}</source>
        <p>You can also replicate code from the printLocation() method to obtain a SourceLocator, and then use the SourceLocator getLineNumber() and getColumnNumber() methods. The getRootSourceLocator() method below returns a SourceLocator.</p>
        <source>
import javax.xml.transform.SourceLocator;
import javax.xml.transform.TransformerException;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.apache.xml.utils.SAXSourceLocator;
import org.apache.xml.utils.WrappedRuntimeException;
....
public static SourceLocator getRootSourceLocator(Throwable exception)
{
  SourceLocator locator = null;
  Throwable cause = exception;
    
  // Try to find the locator closest to the cause.
  do
  {
    if(cause instanceof SAXParseException)
    {
      locator = new SAXSourceLocator((SAXParseException)cause);
    }
    else if (cause instanceof TransformerException)
    {
      SourceLocator causeLocator = 
                    ((TransformerException)cause).getLocator();
      if(null != causeLocator)
        locator = causeLocator;
    }
    if(cause instanceof TransformerException)
      cause = ((TransformerException)cause).getCause();
    else if(cause instanceof WrappedRuntimeException)
      cause = ((WrappedRuntimeException)cause).getException();
    else if(cause instanceof SAXException)
      cause = ((SAXException)cause).getException();
    else
      cause = null;
  }
  while(null != cause);
        
  return locator;
}</source>
        <note>
          <em>Xalan-Java exception handling:</em>  The exception architecture in Xalan-Java and with transforms in general is tricky because of multiple layers of exception handling, involving movement back and forth between SAX and Transformer exceptions and across pipes. Xalan-Java often uses a WrappedRuntimeException to throw over many layers of checked exceptions, in order not to have every possible checked exception be declared for every function in the stack, which means it has to catch this exception at the upper levels and unwrap the exception to pass it on as a TransformerException.
          <br />
          <br />A JAXP 1.1 TransformerException often wraps another exception. Two of the TransformerException structures that are frequently used to construct contained exceptions in JAXP 1.1 do not set the locator.  The locator is not set because we don't know the type of exception that the Throwable argument represents.  The solution is to chase up the contained exceptions to find the root cause, which will usually have a location set for you.  This can be somewhat tricky, as not all the exceptions may be TransformerExceptions.  A good sample is in the DefaultHandler static printLocation() method, which the Xalan-Java command-line processor uses to report errors. You can also roll your own functions along the lines of the getRootSourceLocator() example above.
        </note>
      </a>
    </faq>
    <faq title='Servlet unable to find classes for extension functions/elements'>
      <q>My servlet cannot find classes that implement extension functions or elements. What can I do?</q>
      <a>
        <p>If you install xalan.jar in the servlet engine's lib directory (e.g., tomcat/lib), as opposed to the servlet's 
    lib directory, then the Xalan-Java classes are loaded by a classloader that does not see the classes in the servlet's
    classloader (i.e., the extension classes, if you placed them there). The Xalan-Java classes try to load the extension 
    classes using their own classloader, and that attempt fails.</p>
        <p>Workaround: place xalan.jar in the servlet's lib directory and NOT in the servlet engine's lib directory. 
    Another workaround is to place the extension classes also in the servlet engine's lib directory, but you
    generally want to avoid cluttering that directory.</p>
        <p>Thanks to Gunnlauger Thor Briem (gthb@dimon.is) for providing this information.</p>
      </a>
    </faq>
  </group>
</faqs>