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From Michal Kára <lemm...@ucw.cz>
Subject Re-rendering page on errors during onBeforeRender()
Date Tue, 19 May 2015 12:53:37 GMT
I got page with structure like:

- Page
  +- ListView -- items
  \ - Footer panel

Now the scenario (heavily simplified):

One of the items in ListView throws soft exception during onInitialize().
Required behavior in this case is to keep user on the page and replace
ListView with a panel stating "Sorry, backend outage".

So I catch the exception using aspect, replace view in the structure with
the panel and throw NonResettingRestartException to restart rendering the

Page is rendered and seems OK, BUT I realize onBeforeRender() of Footer
panel is never called.

I debugged it and found the cause:

ListView adds items in onPopulate() method which is called from
onBeforeRender(). What happens is that page starts rendering. Its
onBeforeRender() is called. Page does:


and calls onBeforeRenderChildren(). This calls onBeforeRender() of ListView,
causes exception, so that onBeforeRender() of the footer is not called this

When second rendering is attempted, call flow gets to internalBeforeRender()
of page. There is check:

if ((determineVisibility()) && !getFlag(FLAG_RENDERING) &&

But since FLAG_PREPARED_FOR_RENDER is set from previous attempt,
onBeforeRender() of the page is not called (again) which means
onBeforeRender() of the children is not called.

I found that if I call


on the page before throwing the NonResettingRestartException, all goes well.
But I am quite uneasy about this solution, so I have two questions:

1) Cannot this cause some unforeseen troubles? (Supposing my components are
correctly written so that their onBeforeRender() / onConfigure() /
onAfterRender() procedures can be safely called multiple times.)

2) Should not Wicket itself reset the FLAG_PREPARED_FOR_RENDER when response
restarting exception is caught and processed?

I created a Wicket quickstart project demonstrating the bug here:

Thank you,

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