An urgent issue of the XMLEncoder.

Question

Hi, all.
I have a very urgent problem of the XMLEncode.
Background:
My project is a web project, and the flows are as follow:
1. Get data entity from db by using toplink.
2. Set the data into a domain entity for a page.
3. Use XMLEncoder to convert the domain entity to an XML stream
4. Use Transformer to transform the XML stream to http response and displayed in the client browser with the XSL files.
Issue:
When two or more users visit the same page by the browser at the same time, the weblogic server will throw this exception:
java.lang.NullPointerException
Continuing ...
java.lang.Exception: discarding statement XMLEncoder0.writeObject(SearchWkOrdTyS
tDomainEntity0);
Continuing ...
I've found and read these two articles
http://forum.java.sun.com/thread.jsp?thread=262946&forum=62&message=1002333
and
http://forum.java.sun.com/thread.jsp?forum=63&thread=532252&tstart=30&trange=15
and know that it maybe cause by the Serialization of the classes, but I cannot find the method to solve this problem.
The code which I use to convernt the domain entity object to an XML document are as follow:
  public static Document object2Document(Object o)
  throws ReeferMnRException
    if (o==null) return null;
    Logger logger = LoggerFactory.getInstance().getLogger(XMLUtil.class.getName());
    logger.debug("Get into XMLUtil.object2Document===========================");
    try{
      ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
      XMLEncoder e = new XMLEncoder(
          new BufferedOutputStream(
          byteArrayOutputStream));
      e.writeObject(o);
      e.close();
      DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.
          newInstance();
      documentBuilderFactory.setNamespaceAware(true);
      StringBufferInputStream in = new StringBufferInputStream(new String(
          byteArrayOutputStream.toByteArray()));
      Document document = documentBuilderFactory.newDocumentBuilder().parse(in);
      return document;
    }catch(SAXException e){
      logger.error("XMLUtil-->object2Document-->SAXException:" + e.getMessage());
      throw new ReeferMnRException(
          ReeferMnRException.APPLICATION_SERVER_ERROR, ReeferMnRException.EXCEPTION);
    }catch(ParserConfigurationException e){
      logger.error("XMLUtil-->object2Document-->ParserConfigurationException:" + e.getMessage());
      throw new ReeferMnRException(
          ReeferMnRException.APPLICATION_SERVER_ERROR, ReeferMnRException.EXCEPTION);
    }catch(IOException e){
      logger.error("XMLUtil-->object2Document-->IOException:" + e.getMessage());
      throw new ReeferMnRException(
          ReeferMnRException.APPLICATION_SERVER_ERROR, ReeferMnRException.EXCEPTION);
    }catch(Exception e){
      logger.debug("XMLUtil-->object2Document-->Exception:" + e.getMessage());
      logger.error("XMLUtil-->object2Document-->Exception:" + e.getMessage());
      throw new ReeferMnRException(
          ReeferMnRException.APPLICATION_SERVER_ERROR,
          ReeferMnRException.EXCEPTION);
  }and an example of my domain entity to be convert into an XML document are as follow:
package reefer.mnr.domain.entity;
public class AbnInfoDomainEntity {
  private java.lang.String abnC;
  private java.lang.String abnId;
  private java.lang.String abnSubC;
  private java.lang.String abnSubDesc;
  private java.lang.String crtDt;
  public java.lang.String getAbnC() {
    return abnC;
  public java.lang.String getAbnId() {
    return abnId;
  public java.lang.String getAbnSubC() {
    return abnSubC;
  public java.lang.String getAbnSubDesc() {
    return abnSubDesc;
  public java.lang.String getCrtDt() {
    return crtDt;
  public void setAbnC(java.lang.String abnC) {
    this.abnC = abnC;
  public void setAbnId(java.lang.String abnId) {
    this.abnId = abnId;
  public void setAbnSubC(java.lang.String abnSubC) {
    this.abnSubC = abnSubC;
  public void setAbnSubDesc(java.lang.String abnSubDesc) {
    this.abnSubDesc = abnSubDesc;
  public void setCrtDt(java.lang.String crtDt) {
    this.crtDt = crtDt;
}Anyone know how to can solve this problem?
Thanks any more.

Answer 1

public static Document object2Document(Object o)Is XMLUtil implemented as a singleton? Is this method in a servlet class? Like the doctor says, remove the "static" keyword and try it.
public Document object2Document(Object o) { ... }
After that, I would try it with the "synchronized" keyword.
public synchronized Document object2Document (Object o) { ... }
And then try it with synchronized code block:
// This code writes the XML file of the object parameter
synchronized (this) {
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
XMLEncoder e = new XMLEncoder(
new BufferedOutputStream(byteArrayOutputStream));
e.writeObject(o);
e.close();
// This code returns XML document to calling object
DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance(); documentBuilderFactory.setNamespaceAware(true);
StringBufferInputStream in = new StringBufferInputStream(new String( byteArrayOutputStream.toByteArray()));
Document document = documentBuilderFactory.newDocumentBuilder().parse(in);
return document;