The Griffon SDK includes a class named GriffonExceptionHandler whose main responsibility is to catch any exceptions that your code did not catch, including those thrown inside the EventDispatchThread. It does so by implementing the Thread.UncaughtExceptionHandler interface and registering itself in the following manner:
This takes care of the first issue but it does not solve the second one. Another responsibility of this class is to propagate events through the Griffon event bus (Hamlet recently posted a screencast on this topic, go check it out!). Now any component that registers itself with the application as an event listener will get a chance to react to these type of exceptions. Let's see a quick example.
Assuming that you have a Griffon application with the following View and Controller classes
BombView.groovy
BombController.groovy
Clicking on the button triggers an exception but the Controller nor the View have code that can handle it. If GriffonExceptionHandler were not automatically registered at boot time then the application would end abruptly as soon after you click that button. Praise goodness that's not the case! But how do we add code that handles the exception? Of course you can add a try/catch block to the controller action but that would defeat the purpose of showing the event bus in action. Every Controller (and Service instance for that matter) in Griffon are registered as application event listeners automatically. We could add an even listener to the Controller, however that would be akin to adding a try/catch in the action, i.e, the component that throws the exception is also the one that handles it. We'll choose a different route and register a global event handler for this case.
Global event handlers are the same as any other event handlers, the only difference is that they are defined in a special script (griffon-app/conf/Events.groovy) or class (src/main/Events.java). Here's a quick way to deal with the event
Assuming that you have a Griffon application with the following View and Controller classes
griffon-app/conf/Events.groovy
This should result in a similar output on your console once you click on the button
Oops, got an unexpected exception bomb.BombException
If you look closer to the output on your console you should also see something akin to this
2011-03-22 12:45:41,157 [Thread-2] ERROR griffon.util.GriffonExceptionHandler - Uncaught Exception
bomb.BombException
at bomb.BombController$_closure1_closure2.doCall(BombController.groovy:5)
at bomb.BombController$_closure1_closure2.doCall(BombController.groovy)
bomb.BombException
at bomb.BombController$_closure1_closure2.doCall(BombController.groovy:5)
at bomb.BombController$_closure1_closure2.doCall(BombController.groovy)
That's right, GriffonExceptionHandler will also log the exception for you, it will even filter the stack trace so that you don't see any groovy internals nor java reflection bits. There's more information about this feature at the Griffon Guide, section 5.5.10. With a few more lines of code you can build an event handler that presents a dialog to the user, like this one
I'll show how to get it done on a future post but if you're feeling impatient then jump to Github and clone the sample bomb application.
Keep on Groovying!
