Exceptions and error pages

[Piotr Nowicki]

Howdy Ranchers!

I've got a problem. I have a servlet, which makes some computations. Let's say that this Servlet can (and will) throw an Illegal Argument Exception. Then I want to wrap this exception into the ServletException and throw it from the doGet(-) method, just like that:

...
public class BookServlet extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
		try {
			// some nasty stuff throwing an IAE
		}  catch (IllegalArgumentException e) {
			ServletException s = new ServletException("Customer ID is invalid", e); 
			s.printStackTrace();
			throw s; 
		}

		RequestDispatcher dispatcher = 	request.getRequestDispatcher("book.jsp");
		
		dispatcher.forward(request, response);
	}
}

I am also printing the stack trace to be sure what I am throwing, and it shows:

javax.servlet.ServletException: Customer ID is invalid
        at com.example.ocmjd.BookServlet.doGet(BookServlet.java:22)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:306)
(...) // buch of low-level stack trace elements

So, I've got my ServletException and message shown in the stack trace - fine. Now, my web.xml relevant part looks like:

<error-page> <exception-type>javax.servlet.ServletException</exception-type> <location>/servletException.jsp</location> </error-page>

So, the container is thinking - "I've bumped into the ServletException and I (the mighty container) will pass the exception to the /servletException.jsp file to cope with it":

<%@ page isErrorPage="true" %>
Exception: ${pageContext.exception}  <br />
Message: ${pageContext.exception.message}  <br />

And the result is:

Exception: java.lang.IllegalArgumentException
Message:

So, the exception is IllegalArgumentException - not a ServletException which I expected (as the IAE is just a cause for ServletException) and the message is empty.

Well, it looks like the container (?) is unwrapping the exception I am catching and serving just the cause of it. So the ${pageContext.exception} is the cause, not the real-one thrown exception.

Is it always like that? Can I access the original ServletException which was thrown from the Servlet? And, the most important, why is the container doing this to me? Why...?

Cheers!

[Frits Walraven]

Hi Pedro!

Well, it looks like the container (?) is unwrapping the exception I am catching and serving just the cause of it. So the ${pageContext.exception} is the cause, not the real-one thrown exception.

Is it always like that? Can I access the original ServletException which was thrown from the Servlet? And, the most important, why is the container doing this to me? Why...?

Yes, it is a bit hidden, but this is what the jsp2.0 spec says about it:

Table JSP.1-8 Page Directive Attributes
errorPage
"....If the URL names another JSP page then, when invoked that JSP page's exception implicit script variable shall contain a reference to the originating uncaught Throwable."

Why...?

Because

SRV.9.9.2 Error Pages
To allow developers to customize the appearance of content returned to a Web client
when a servlet generates an error, the deployment descriptor defines a list of error
page descriptions

When you don't define the errorPage in your web.xml you will see the ServletException. It is a possibility to customize the error-handling from exceptions thrown from the business layer.

Regards,
Frits

[Piotr Nowicki]

Thanks once again Frits :-)

I didn't know about that...

Oh, and BTW I've bumped into something like this:

Servlet 3.0 Final, 10.9.2 Error Pages

If no error-page declaration containing an exception-type fits using the class-hierarchy match, and the exception thrown is a ServletException or subclass thereof, the container extracts the wrapped exception, as defined by the ServletException.getRootCause method. A second pass is made over the error page declarations, again attempting the match against the error page declarations, but using the wrapped exception instead.

I really love this feeling while you learn something new :-)

[Frits Walraven]

Oh, and BTW I've bumped into something like this:
If no error-page declaration containing an exception-type fits using the class-hierarchy match, and the exception thrown is a ServletException or subclass thereof, the container extracts the wrapped exception, as defined by the ServletException.getRootCause method. A second pass is made over the error page declarations, again attempting the match against the error page declarations, but using the wrapped exception instead.

Yes I had also seen that part in Servlet2.4, and I was just wondering what will happen if you have defined both the page for the ServletException and the page for the IllegalArgumentException:
<error-page> <exception-type>javax.servlet.ServletException</exception-type> <location>/servletException.jsp</location> </error-page> <error-page> <exception-type>java.lang.IllegalArgumentException</exception-type> <location>/IllegalArgumentException.jsp</location> </error-page>

[Piotr Nowicki]

<error-page> <exception-type>java.lang.IllegalArgumentException</exception-type> <location>/roomNotFoundError.jsp</location> </error-page> <error-page> <exception-type>javax.servlet.ServletException</exception-type> <location>/servletException.jsp</location> </error-page>

It stopped at the ServletException, opened servletException.jsp. In this file I print what exception occured and it's IllegalStateException.

So, it's just like the spec says, but I still think it's a bit confusing at least for me :-) In this particular case it's even more confusing ;-)

Cheers!