Need urgent help: how to avoid concurrent calls on statefull session beans

Question

Hi,
I need a little advice in designing a EJB session facade using JSPs, servlets, session and
entity beans.
My current design is:
- JSP pages: here are only getMethods for the session bean used. All set-methods are handled by a
- servlet: I have got one servlet handling several JSP pages. The servlet basically takes the
form fields and stores them in the session bean and than dispatches to the next JSP-page
- stateful session bean: here is, where all the business logic is conducted. There is one session
bean per servlet using several
- CMP entity beans: to talk to the database (Oracle 8i)
The application server is JBoss 3.0.3.
My problem is, if a user clicks on a submit button of a JSP page more than once before the next
page builds up, I may get a "javax.ejb.EJBException: Application Error: no concurrent calls on
stateful beans" error. I already synchronized (by the "session") the code in the servlet, but
it happens in the JSP pages as well.
I know, that Weblogic is able to handle concurrent calls, by JBoss isn't and it's clearly stated
in the spec, that a user should avoid to have concurrent calls to a stateful bean.
The big question is now: How can I avoid this? How can I prohibit the user to submit a form several
times or to ignore anything, which arrives after the first submit?
Thanks for any help,
Thorsten.

Answer 1

Synchronizing on the session is probably your best bet.
You'll need to do all the data access and manipulation in the servlet. Cache any data you need using request.setAttribute() and then not access the EJB on the JSP page.
If performance is an issue, you may also want to use create a user transaction to wrap all the EJB access in, otherwise each EJB call from the servlet is a new transaction. Just make sure you use a finally block to properly commit/rollback the transaction before you redirect to the JSP.
UserTransaction utx    = null;
synchronized (request.getSession())
  try {
    Context ctx = new InitialContext();
    utx = (UserTransaction) ctx.lookup("javax/transaction/UserTransaction");
    utx.begin();
    // ... Create session bean ...
    request.setAttribute("mydata", sessionBean.getMyData());
    try {
      utx.commit();
    catch (Exception ex) {
      log.warn("Transaction Rolled Back (" + ex.getClass().getName() + "): "
        + ex.getMessage(), ex);
    utx = null;
  } // try
  finally {
    if(utx != null)
      try {
        utx.rollback();
      catch (Exception e) {
        log.warn(e.getMessage(), e);
      } // catch
    } // if
  } // finally
} // syncrhonized(session)