There are different versions of SJSXP. One that comes from java.net and the other one from JDK. ZephyrWriterFactory.java(java.net) changes are not there in XMLOutputFactoryImpl.java(JDK6). JAX-WS relies on certain SJSXP features for correctness and performance. However this particular change

cvs diff -r 1.4 -r 1.5  ZephyrWriterFactory.java

is not there in JDK6 version. This causes some of the multi-thread tests to fail. Attaching a test case that fails on some machines. But it is easy to see why it fails(see the above diff). The following works with sjsxp from java.net but not with JDK6.

---------------------------------------
import javax.xml.stream.XMLEventFactory;
import javax.xml.stream.XMLStreamReader;
import javax.xml.stream.XMLStreamWriter;
import javax.xml.stream.XMLEventWriter;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.events.Attribute;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.StartDocument;
import javax.xml.stream.events.XMLEvent;
import java.io.*;

import com.sun.xml.internal.ws.api.streaming.XMLStreamReaderFactory;
import com.sun.xml.internal.ws.api.streaming.XMLStreamWriterFactory;

public class MultiThread {
    private static final XMLOutputFactory outputFactory =
        XMLOutputFactory.newInstance();
    private static final XMLInputFactory inputFactory =
        XMLInputFactory.newInstance();
    private static final int NO_THREADS = 3;

    public static void main(String[] args) throws Exception {
        Thread[] threads = new Thread[NO_THREADS];
        for(int i=0; i < NO_THREADS; i++) {
            threads[i] = new Thread(new MyRunnable(i));
        }
        for(int i=0; i < NO_THREADS; i++) {
            threads[i].start();
        }
        for(int i=0; i < NO_THREADS; i++) {
            threads[i].join();
        }
    }

    public static class MyRunnable implements Runnable {
        final int no;

        MyRunnable(int no) {
            this.no = no;
        }

        public void run() {
                try {
                     FileOutputStream fos = new FileOutputStream(""+no);
                     XMLStreamWriter w = getWriter(fos);
                     //System.out.println("Writer="+w+" Thread="+Thread.currentThread());
                     w.writeStartDocument();
                     w.writeStartElement("hello");
                     for(int j=0; j < 50; j++) {
                         w.writeStartElement("a"+j);
                         w.writeEndElement();
                     }
                     w.writeEndElement();
                     w.writeEndDocument();
                     w.close();
                     fos.close();

                     FileInputStream fis = new FileInputStream(""+no);
                     XMLStreamReader r = getReader(fis);
                     while(r.hasNext()) {
                         r.next();
                     }
                     r.close();
                     fis.close();
                } catch(Exception e) {
                     e.printStackTrace();
                }
        }
    }

    public static /* synchronized */ XMLStreamReader getReader(InputStream is)
        throws Exception {
        //return inputFactory.createXMLStreamReader(is);
        return XMLStreamReaderFactory.create(null, is, true);
    }

    public static /* synchronized */ XMLStreamWriter getWriter(OutputStream os)
        throws Exception {
        //return outputFactory.createXMLStreamWriter(os);
        return XMLStreamWriterFactory.createXMLStreamWriter(os);
    }

}
------------------------------
$ javac -verbose -Xbootclasspath:$JAVA_HOME/jre/lib/rt.jar MultiThread.java
$ java MultiThread
XMLOutputFactory of = XMLOutputFactory.newInstance();
        XMLStreamWriter w = of.createXMLStreamWriter(new ByteArrayOutputStream());
        System.out.println(w);

The above code throws the following exception when run with JDK6.

Exception in thread "main" java.lang.UnsupportedOperationException
        at com.sun.xml.internal.stream.writers.XMLStreamWriterImpl.entrySet(XMLStreamWriterImpl.java:2126)
        at java.util.AbstractMap.toString(AbstractMap.java:478)
        at java.lang.String.valueOf(String.java:2827)
        at java.io.PrintStream.println(PrintStream.java:771)
        at WhichSjsxp.main(WhichSjsxp.java:21)