Awkward NullPointerException in Xalan transformations

Question

L.S,
I'm having a serious problem with XSL transformations using Apache Xalan 2.6.0: a transformation works once, but multiple calls to the method will cause a null-pointer exception to occur within the TrAXFilter object taking care of the transformation.
The class responsible for transforming any XML messages in our system looks like this (class MessageWriter appearing at line 14 is a JAXB marshalling class we've developed; class FilterManager appearing at line 21 returns an array of 1 hard-coded XMLFilter object):
public class MessageBuilder {
  private final static SAXParserFactory XML_PARSER = SAXParserFactory.newInstance();
  public static String getXmlMessage(MessageSDO message) {
    String xml = MessageWriter.doMarshal(message);
    try {
      SAXParser parser = XML_PARSER.newSAXParser();
      XMLReader reader = XMLReaderFactory.createXMLReader();
      InputSource srcInput = new InputSource(new StringReader(xml));
      FilterManager ftrManager = (FilterManager) ConfigurationManager.
        getInstance(ConfigurationManager.BOUNDARY).getBean(FilterManager.class);
      XMLFilter[] filters = ftrManager.getOutputFilters(message);
      if (filters.length > 0) {
        for (int i=0;i<filters.length;i++) {
          filters.setParent((i==0)? reader : filters[i-1]);
Serializer serializer = SerializerFactory.getSerializer(OutputPropertiesFactory.getDefaultMethodProperties("xml"));
ByteArrayOutputStream output = new ByteArrayOutputStream();
serializer.setOutputStream(output);
filters[filters.length-1].setContentHandler(serializer.asContentHandler());
filters[filters.length-1].parse(srcInput);
xml = output.toString();
} catch (Exception e) {
throw new RuntimeException(e);
return xml;
The test I've run (if 2nd block commented out test succeeds, otherwise it fails on that 2nd attempt):
public void testTransformedOutput() {
  System.out.println("Transformed output test:\n\n");
  MessageSDO msg = MockSDOFactory.getInstance().getMessageSDO();
  String xmlMsg = MessageBuilder.getXmlMessage(msg);
  /*System.out.println("Transformed output test, 2nd pass:\n\n");
    MessageSDO msg2 = MockSDOFactory.getInstance().getMessageSDO();
    String xmlMsg2 = MessageBuilder.getXmlMessage(msg2);*/
}The exception message:
java.lang.RuntimeException: java.lang.NullPointerException
at org.apache.crimson.parser.Parser2.parseInternal(Parser2.java:658)
at org.apache.crimson.parser.Parser2.parse(Parser2.java:334)
at org.apache.crimson.parser.XMLReaderImpl.parse(XMLReaderImpl.java:448)
at org.apache.xalan.transformer.TrAXFilter.parse(TrAXFilter.java:134)
at org.boundary.helper.MessageBuilder.getXmlMessage(MessageBuilder.java:36)
// Line 36 in MessageBuilder is:
// filters[filters.length-1].parse(srcInput);What I don't yet understand is how TrAXFilter manages to somehow not have a parent in the 2nd pass of the test, while it does in the 1st pass. I've tried a few things already, but so far haven't been able to find a solution.
Does anyone know of a bug like this? Or is it clear to someone what I'm doing completely wrong here? I'd appreciate any help you can offer.

Answer 1

I've been able to solve this problem and since others might be interested, I thought I'd post the solution here as well.
It turns out that XMLFilter objects cannot be reused. This isn't documented very well, and undoubtedly doesn't come as a surprise to the more experienced XML specialists out there, but it's something you need to know. As a result, instead of using XMLFilter objects for repetetive transformations, you need to use an alternative, namely [url http://java.sun.com/j2se/1.4.2/docs/api/javax/xml/transform/Templates.html]Templates objects:
final SAXTransformerFactory FACTORY = (SAXTransformerFactory) TransformerFactory.newInstance();
File xslFile = new File("someStylesheet.xsl");
StreamSource src = new StreamSource(xslFile);
Templates template = FACTORY.newTemplates(src);
// Later on, you can repeatedly do this:
String xml = getSomeXMLDocument();
InputSource srcInput = new InputSource(new StringReader(xml));
XMLFilter filter = FACTORY.newXMLFilter(template); // Use cached template instead of parsing XSL file every time...
XMLReader reader = XMLReaderFactory.createXMLReader();
filter.setParent(reader);
Serializer serializer = SerializerFactory.getSerializer(OutputPropertiesFactory.getDefaultMethodProperties("xml"));
ByteArrayOutputStream output = new ByteArrayOutputStream();
serializer.setOutputStream(output);
filter.setContentHandler(serializer.asContentHandler());
filter.parse(srcInput);
String xml = output.toString();