How to handle alert events in UIWebView

Hi

I have a url which shows a username,password fields and a submit button. On clicking the submit button it pops up a alert view in my device stating some message. I need to capture that message and use it in my application. Now that are java alerts that are being displayed.

So i need to find out a way so that as soon as the user clicks submit, I can disable the UIWebView and can take out the message from the pop up alert so that i can use it.