What's wrong in this transaction?

Hi guys,
i've developed a jsf application.
I do lots of prepared statement in a session,what i want is do all in a
transaction. So i've created an object that collects all the prepared
statement of the session.
When the user click a final submit botton transaction has to b
executed.
My problem is that if a sql exception comes where object is executed
rollback isn't called and transaction is partial executed!!!!!!!
Why this?
Can you help me finding error in my simple code?
Thanks very much

This is my method
public String executeTransaction() throws SQLException {
        Connection conn=null;
        DataSource dataSource = Singleton.getInstance().getDataSource();
        conn = dataSource.getConnection();

            Experiment exp = (Experiment) ViewUtils
            .eval("#{experiment}");

            List results=new ArrayList();
            ResultSet rs=null;
            Statement stmt=null;
            boolean allgood=false;
            if (conn != null) {

                conn.setAutoCommit(false);

                for (int i=0;i<exp.getExperiments().size();i++){
                    try{

                    ((java.sql.PreparedStatement)
exp.getExperiments().get(i)).executeUpdate();
      if(i==(exp.getExperiments().size()-1))
      conn.commit();
                    }
                    catch (Exception e) {
                        conn.rollback();
                        e.printStackTrace();
                    }

                }

            }
            else System.out.println("connessione non disponibile");

        return "submit";
    }

please help me