View System Documentation - Handling System Errors

• HTTP Errors

  HTTP errors occur when a user requests a resource (via a URL) that is not mapped to the system (via the servlet mapping contained within the web.xml) and that file/resource is not found or has access issues etc (If not mapped then the app server treats the request as a normal web server would). HTTP errors can also occur in the MVC portion of the application for mapped resources but these are typically set by the Exception handler or if an uncaught Java runtime error occurs. The Java application server deals with these types of errors via the handlers defined within the app's "WEB-INF/web.xml" file (all standard HTML errors 404...500 - see table below). These typically are mapped to the "jsp/ErrorPage.jsp" page to be displayed to the user.

• Caught MVC Java Exceptions

  Java exceptions can be thrown within the any portion of the MVC pattern. This includes catchable view errors like XML file not found errors, most null pointers, and all throws that are controller and model related. Most view rendering related errors are able to be caught (see next item).
  
  The Exception class is used to catch and display exceptions thrown by the application. Logging also shows all exceptions thrown. See the WEB-INF/config/spring/common.xml" which contains a bean id="Default.ExceptionHandler" that specifies the exception error handler class to be used; typically class="org.ibisph.mvc.ExceptionHandler". When errors like this occur, they are typically logged and displayed to the via the "jsp/ErrorPage.jsp" page.

• Caught "View Rendering" Exceptions

  This type of error is typically when an XSLT has a problem. The error is caught by the view object but since the user's response stream has already been opened the Spring MVC RequestDispatcher servlet can not do anything about displaying an error page. The user is typically shown a blank page but the exception is logged in the various log files.

• Uncaught Java Runtime Errors

  Hopefully these rarely happen. Java runtime errors are unexpected and not caught by the Spring MVC RequestDispatcher servlet or the MVC code. This type of problem is seen when a system object is null or if the the system runs out of memory etc). Errors like this typically do not show the user anything other than a blank screen. Most of the time, unless the JVM crashes, the error will show up in the output logs.

HTTP Errors Defined in WEB-INF/web.xml