Re: [Springframework-developer] Fw: [Springframework-user] MVC Exception Handling

[Rod J. <rod...@in...>]

Good work.

I agree that being able to throw Exception is valuable. After all, the
implementation isn't _forced_ to declare such a broad throws clause if
people want more help from the compiler.

I agree that Struts-style "declarative" exception handling makes sense. That
was one of the worthwhile features of Struts 1.1. Of course our
implementation, in true Spring style, will be more open :-)

Regards,
Rod

----- Original Message ----- 
From: "jürgen höller [werk3AT]" <jue...@we...>
To: <spr...@li...>
Sent: Saturday, November 22, 2003 5:14 PM
Subject: [Springframework-developer] Fw: [Springframework-user] MVC
Exception Handling


Everybody,

Continuing the tradition of last minute changes: I've repeatedly thought
about exception handling in the web tier, especially when analyzing Struts
1.1 and WebWork2. Both of the latter allow an action to throw any Exception
(Struts 1.0 did not, BTW); Struts additionally supports declarative mapping
of exception to exception handlers.

Steve, who's mail I've quoted below, has the use cast that all of his
business methods throw checked exceptions, but our web controllers just
allow ServletException. Instead of wrapping his exceptions all the time, it
would be much nicer if our web controllers would allow any Exception to be
thrown, just like Struts 1.1. and WebWork.

A poster on our forums recently noted that he would like to be able to
forward to a specific error page if he encounters certain conditions in his
form controller methods (like referenceData or formBackingObject). This is
valid concern: Such exceptions effectively exit from the normal workflow to
a well-known error page within the web MVC framework.

Therefore:

1. I've just changed all our web controller signatures from
ServletException+IOException to Exception. DispatcherServlet wraps
Exceptions other than Servlet/IO in ServletExceptions.

2. I've introduced an ExceptionResolver mechanism: DispatcherServlet
registers all implementors of the interface ExceptionResolver and allows
them to resolve exceptions to ModelAndView objects, for forwarding to
specific error pages within the web MVC framework. Of course,
ExceptionResolvers can be ordered: They get invoked one by one until a
ModelAndView was returned; if none could resolve the exception, default
handling applies.

3. I've added a simple implementation of ExceptionResolver that allows to
map exception names to view names: SimpleMappingExceptionResolver (note that
by default, no exception resolver is used). Exception names can be any part
of the exception class or the exception message. An "exceptionAttribute" is
supported: The exception gets exposed in the model under that name, by
default "exception".

4. For exiting the workflow to a specific error page, I've introduced
ModelAndViewDefiningException: If a handler throws that exception, passing a
specific ModelAndView into its constructor, DispatcherServlet will
automatically forward to the specified view - without the need for an
ExceptionResolver! This is handy for known error conditions that lead to
well-known error pages, to be checked for example at form setup in a form
controller.

I'm quite keen on introducing this already for M3, as it removes a
restriction that neither Struts nor WebWork have, and allows for cool
exception handling strategies :-) Client code should just be affected at
very specific points if at all: for example, if calling showForm in custom
handleInvalidSubmit methods, the method needs to throw Exception now instead
of ServletException - should be straightforward to adapt. It is a good thing
that we mainly work with template methods: Allowing for more exception types
to get thrown does not affect client code there.

I'll wait till tomorrow with committing these changes: If there any
objections or further suggestions, please voice them promptly!

Juergen


________________________________

Von: spr...@li... im Auftrag von Steve
Gulics
Gesendet: Di 18.11.2003 22:25
An: springframework-user
Betreff: [Springframework-user] MVC Exception Handling



I've been looking at the sample apps and have some
questions on how Spring handles exceptions.

For example, in the Pet Clinic example, if a checked
exception is thrown when
getClinic().storeOwner(owner); is called in
AddOwnerForm I want to go to a specific error page
based on that exception. I guess I could put the call
in a try-catch block and based on the exception I
return the appropriate ModelAndView. The problem with
this approach is its very tedious. Especially since
all our current BusinessDelegates throw checked
exceptions.

Stuts has a clean approach which has the Struts
Action.excute method throw Exception which allows you
to define in the struts-config.xml a handler for the
exception that will control what page to forward to.
For example
<global-exceptions>
    <exception
      key="some.key"
      type="java.io.IOException"
      handler="com.yourcorp.ExceptionHandler"/>
</global-exceptions>

See the following url for more info:
http://jakarta.apache.org/struts/userGuide/building_controller.html#exception_handler

My question is what is the perferred Spring way to
handle these situations? Is it just try-catch blocks?
I understand that the Spring approach is not to use
Checked Exceptions but I cannot control that since all
the business delegates we currently have throw checked
exception and I cannot rewrite them at this time.

thanks,
Steve


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