Tag Archives: exception

Lesson learned from the Exception Filter

Long time ago, while I’m still a student, I have a chance to read Foundation of Programming“. “All exception should be captured, to show a understandable, friendly error message to customer”.

But now I realize that it’s not enough. In a recent web project, I created an exception filter to handle all exceptions popping out:

@Override
public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		String errorMessage = "";

		try {
			chain.doFilter(request, response);
		} catch (SessionExpiredException e){
		    log.info(e.getMessage(), e);
                    errorMessage = "Your session has been expired";
                    request.sendRedirect(...);
		} catch (InvalidFacebookIdException e) {
                   // ...
		} catch(FacebookAccountAlreadyRegisteredException e) {
                   // ...
		} catch (Exception e) {
                   // ...
		}
	}

Do you notice the duplicate code? At any exception caught, the system must log the error, compose the mesage, then redirect to the error page.There are more than dozen of exceptions like that. How can we “clean” the code?

Thanks bro Tinh, here’s a (customized) solution:

@Override
public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
//...
		try {
			chain.doFilter(request, response);
		} catch (Exception e) {
                   errorMessage = e.getMessage();
                   log.error("Error occur: " + e.getMessage(), e);
                   res.sendRedirect(...);
		}
	}

All errors now are caught in one place: general Exception instead of concrete type.Two problems remain:

  • How to catch the right exception, because they are wrapped by Servlet Exception.
  • Guarantee all Customized Exception have a message

The answer for the first question:

private String getUsefulMessage(Exception e) {
Exception default = e.getMessage();
do {
if (e.getClass().getName().equals(InvalidFacebookId.class.getName())) return e.getMessage();
// ... same for other exceptions
e = e.getCause();
} while(e.getCause() != null);

return default;
} 

It’s the same, yes, for logic. But the later is more clearer.

The second question is more about coding policy than a technical problem, so I leave it out. In the case of our team, we decide that each exception should have an understandable getMessage().