Re: Problem with sharing sessions/ multithreading

[Ralph Goers]

Torsten Curdt wrote:
Don't want to be picky ...but better don't use DCL
http://www.javaworld.com/javaworld/jw-05-2001/jw-0525-double.html
cheers
 

Thanks, that article wasn't too helpful in figuring out why the 
technique doesn't work. However, it refreences an article from Alan 
Holub that does. From his description, the following version of the DCL 
should work fine.  I'm not sure the function actually has to be static. 
From what I could tell synchronized should be sufficient.

Document getDocument()
{
       Request request = ObjectModelHelper.getRequest(objectModel);
       Session session = request.getSession(true);
       Document doc = (Document)session.getAttribute("myAttribute");
       if   (doc == null)
       {
            // It is possible for more than one thread to get here at 
the same time with doc == null
            syncronize(LOCK);
            {     // So check again. Only the first caller should 
actually create the document.
                  doc = (Document)session.getAttribute("myAttribute");
                  if (doc == null)
                  {     
                       DocumentBuilderFactory dbf  = 
DocumentBuilderFactory.newInstance();                             
                    
                       doc = buildDocument(dbf);
                       session.setAttribute("myAttribute", doc);
                   }                        
            }
       }
       return doc;
}

private static synchronized Document 
buildDocument(DocumentBuilderFactory dbf)
{
   return dbf.newDocumentBuilder().newDocument();
}

--
Torsten