How is catch{} code handled 2004-01-02 - By Merrill Cornish
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My sudden IllegalStateException problem turned out to be caused by my
error handling technique, but I'm not sure why.
When I started this project (as a way or learning JSP), I have
EVERYTHING in JSP pages, meaning that there was a lot of Java code in <%
... %> sections of the JSP page. I added an errorPage declaration to
each of my JSP pages to divert all errors to my standard error handling
page--and all was well.
Later, I decided to limit the JSP pages to HTML as much as possible and
move all of the Java program logic into servlets. I left the errorPage
declarations in the JSP pages, although there wasn't much left there to
throw an exception.
In the servlets, I used the standard try/catch constructs to intercept
exceptions. However, I decided I wanted the exceptions caught in
servlets to be handled by the same error page as the JSP pages used.
Rather than have EVERY catch{} clause do the redirect, I defined a
utility subroutine named errorPage() that collected various information
in the catch{} clause, then called sendRedirect() to the error page.
Separate from these catch{} clauses, whenever the processing in a
servlet was complete, it ended with a sendRedirect() to the next JSP
page followed immediately by a return. I had assumed that the
sendRedirect()s in the main servlet code were "safe" from the
sendRedirect() in the catch{} cause since--as I understood it--once the
exception was thrown and the catch{} entered, nothing else in the
servlet was processed.
However, my "IllegalStateException" experience suggests there is
something going on with catch{} that I don't understand. Or, as a
friend of mine used to say, "I don't understand all I know about that." :-)
To recap, I got an IllegalStateExceptioni pointing to a sendRedirect()
in a servlet until I effectively removed the sendRedirect() by returning
before the sendRedirect() could be reached. Only then did I see an
SQLException intercepted by a catch{} and redirected with
sendRedirect() to the error page.
Why did main servlet processing appear to continue (allowing the second
sendRedirect() to cause a problem) after the exception was triggered?
Merrill Cornish
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