How to handle exceptions thrown in Wicket custom model?

Question

I have a component with a custom model (extending the wicket standard Model class). My model loads the data from a database/web service when Wicket calls getObject().

This lookup can fail for several reasons. I'd like to handle this error by displaying a nice message on the web page with the component. What is the best way to do that?

public class MyCustomModel extends Model {

    @Override
    public String getObject() {
        try {
            return Order.lookupOrderDataFromRemoteService();
        } catch (Exception e) {
            logger.error("Failed silently...");
            // How do I propagate this to the component/page?
        }           
        return null;
}

Note that the error happens inside the Model which is decoupled from the components.

Answer 1

Handling an exception that happens in the model's getObject() is tricky, since by this time we are usually deep in the response phase of the whole request cycle, and it is too late to change the component hierarchy. So the only place to handle the exception is very much non-local, not anywhere near your component or model, but in the RequestCycle.

There is a way around that though. We use a combination of a Behavior and an IRequestCycleListener to deal with this:

• IRequestCycleListener#onException allows you to examine any exception that was thrown during the request. If you return an IRequestHandler from this method, that handler will be run and rendered instead of whatever else was going on beforehand.

  We use this on its own to catch generic stuff like Hibernate's StaleObjectException to redirect the user to a generic "someone else modified your object" page. If you

• For more specific cases we add a RuntimeExceptionHandler behavior:

  public abstract class RuntimeExceptionHandler extends Behavior {
      public abstract IRequestHandler handleRuntimeException(Component component, Exception ex);
  }
  

  In IRequestCycleListener we walk through the current page's component tree to see whether any component has an instance of RuntimeExceptionHandler. If we find one, we call its handleRuntimeException method, and if it returns an IRequestHandler that's the one we will use. This way you can have the actual handling of the error local to your page.

  Example:

  public MyPage() {
    ...
    this.add(new RuntimeExceptionHandler() {
      @Override public IRequestHandler handleRuntimeException(Component component, Exception ex) {
        if (ex instanceof MySpecialException) {
          // just an example, you really can do anything you want here.
          // show a feedback message...
          MyPage.this.error("something went wrong"); 
          // then hide the affected component(s) so the error doesn't happen again...
          myComponentWithErrorInModel.setVisible(false); // ...
          // ...then finally just re-render this page:
          return new RenderPageRequestHandler(new PageProvider(MyPage.this));
        } else {
          return null;
        }
      }
    });
  }
  

  Note: This is not something shipped with Wicket, we rolled our own. We simply combined the IRequestCycleListener and Behavior features of Wicket to come up with this.