How can I effectively debug JSPs which directly or indirectly (thru helper classes) use databases (JDBC)?

For eg. I want to debug a JSP file which shows a NullPointerException when a PreparedStatement in a DBHandler class tries an executeUpdate(). The same JSP file (with the same data) works fine on restarting Tomcat. The java-bean and the DBHandler (executeUpdate()) work fine in standalone mode (ie as an application). Now where & how do I start debugging?