[FIXED] Sending information from a servlet back to a Filter

Issue

I have an old web application, running on Tomcat 7, that uses a very rudimentary open-session-in-view mechanism provided by a filter:

@Override public void doFilter (ServletRequest req, ServletResponse resp, FilterChain fc) 
    throws IOException, ServletException 
{
    try {
        HibernateUtil.beginTransaction();
        fc.doFilter(req, resp);
        HibernateUtil.commitTransaction();
    } catch (Throwable t) {
        Logger.exception(t, "processing servlet request");
        HibernateUtil.rollbackTransaction();
        throw new ServletException(t);
    }
}

I'm stuck with this now, and I think I'm running into one of the many flaws with OSIV (or at least this implementation of it) which is that I now want to be able to rollback transactions even without an exception being thrown. I want servlets to be able to control that, and I don't think I have much choice except to hack this functionality on somehow.

My question is: How can I communicate some sort of "rollback" flag from an arbitrary servlet back up to this filter? I want to be able to do something like this in the filter:

HibernateUtil.beginTransaction();
fc.doFilter(req, resp);
if (/* something that was set by a servlet/jsp */)
    HibernateUtil.rollbackTransaction();
else
    HibernateUtil.commitTransaction();

I'm not really sure what a reliable way to propagate information like that from a servlet back out to this filter is.

Solution

I don't advise using request attributes or thread-local variables, which has the following issues:

1. Your transaction is dependent on someone else having set a flag. If you work for a bank, I really don't wanna be a customer there.
2. Resource leaking if you don't clean up thread-local storage.
3. You can't write multithreaded code without manually copying stuff between thread-local storage.
4. If using request attribute, you'll have to extract the value in a Servlet and pass all the way to your DAO, assuming you're using a common multi layered architecture.

Instead, you can simply get the current transaction from the Hibernate session object and ask it to rollback. Session.getTransaction().rollback(). Best, scrap that code or find the person who wrote it and ask for a refund.

Answered By - Abhijit Sarkar
Answer Checked By - Marilyn (JavaFixing Volunteer)