When is it ok to throw a ServletException from a Servlet?

Question

I have been throwing ServletExceptions in the past, when something/anything goes wrong in a Servlet, mostly just wrapping the exception in ServletException.

Now I am thinking it is actually better to not throw a ServletException but to respond with response.sendError(sc) and use the correct HTTP status codes.

If I can't send an error using reponse.sendError, (IOException), I wrap the IOException in a ServletException.

Is the above a better way to respond? When is it OK to just throw a ServletException?

Answer 1

I have just come to the opposite conclusion to @alamar. My situation is writing a REST-style servlet to be called by code in an Oracle database, not by humans.

I want to return the HTTP code 400 Bad Request to the caller when the request information is invalid or missing. Throwing a ServletException causes the container to return 500 Internal Server Error, which states there is something wrong with the server, not with the request.

Here is the solution I adopted.

1. Create a simple exception class RequestException that extends Exception.
2. Check the validity of the request with method(s) that throw new RequestException(message).

3. Catch RequestException in the servlet’s doPost method and call HttpServletResponse.sendError() like this:

   try {
       validateRequest(request);
       // Do stuff with a valid request.
   } catch (RequestException e) {
       response.sendError(HttpServletResponse.SC_BAD_REQUEST, e.getMessage());
   }
   

The message is returned to the caller as it would be with a ServletException.

Answer 2

It is better to throw a ServletException.

You can later actually catch this exception with error-page directive in your web.xml, and make appropriate actions: display a fancy error page to user, send the exception along with the stack trace to developers, write it to logs and so on.

If you just use sendError(), you can't do that, to various extents.