Pass through runtime exceptions
-------------------------------

         Key: VELOCITY-429
         URL: http://issues.apache.org/jira/browse/VELOCITY-429
     Project: Velocity
        Type: Improvement
  Components: Source  
    Reporter: Will Glass-Husain
Assigned to: Will Glass-Husain
    Priority: Minor
     Fix For: 1.5


Thought it might be useful to archive the brief discussion on this topic since it's a cross-cutting type of issue.  I'm going to link to this issue in the change log.

----- Original Message -----
From: "Will Glass-Husain" <wglass@fori...>
To: <velocity-dev@jaka...>
Sent: Monday, January 02, 2006 7:03 AM
Subject: RuntimeExceptions

First, happy new year to the Velocity developer community...

I've been pondering how Velocity handles exceptions. Right now, almost every call to a plugin has a catch-all Exception handler which does little more than log the Exception.  There's been a couple of requests in the past few months to pass RuntimeExceptions or similar through.  

* In Velocity-424, Malcolm Edgar wanted NPEs from a call to toString to pass through a #parse.  (we did this).
* Llewelyn Falco created a "TestableUberspect" which interrupted processing upon an invalid method call.
* There was someone else (can't find the reference) who wanted to interrupt processing when confronted with an invalid method call.

I was wondering, why does Velocity intercept every exception?  Isn't the point of a RuntimeException that it signals an application-level problem that should be returned to the calling code?  In addition, we should allow plugins (event handlers, uberspectors, resource loaders) to interrupt processing for critical problems.  To me, there seems three choices.

(1) Catch every unexpected condition and do something with it (e.g. catch NPE from toString() and wrap it in a MethodInvocationException).  Logging does not count as doing something.

(2) Create a special VelocityRuntimeException that plugins can use to interrupt processing.  Every generic catch (Exception E) wrapper would pass this through.  (I've coded this though not applied the patch).

(3) Remove all generic Exception catch's.  (or if not feasible, always pass RuntimeException's through).

To me, option #1 seems infeasible.  (if we start wrapping toString, we'd need to wrap a lot of other little stuff).  Option #2 is useful for plugins to interrupt processing.  But I'm wondering, why not do option #3 and just pass through all RuntimeExceptions.  Any reason why we *should* be catching unexpected RuntimeException's?

Appreciate other thoughts...

Best,
WILL

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