How to know if the current application jar file has been overwritten?

When I overwrite a jar file that is being used, the buttons that call new windows stop responding silently but is there some check I can make from within the application to know if the jar file has been overwritten, so that I can warn the user to restart it?
I know this sounds bad so let me explain why I would want this, if you don't care then skip everything below this point.
I have close to a hundred of users executing Java applications from a central repository in the network. This is important because the applications are always changing and evolving so it's important that I only have to update one binary. I do most updates after work hours so I don't have a problem there but sometimes there are critical errors that need to be corrected immediately. Depending on the amount of people using an application it may be impractical to warn and wait for everyone to close it and make everyone else wait for that so I just overwrite the files that need to be updated and then warn everyone to restart the application. The result is that buttons that call for frames that haven't been called yet stop working silently. Some users already know what to do when that happens but I want a more graceful approach. I wanted to check for this situation from inside the application so that I could inform the user of the update and provide information on what to do next. Can I do this? I tried some try and catch blocks of code but I can't seem to catch anything when that happens.

You can write your own class loader that loads classes from that jar file. Make sure it isn't on the class path and when you want to create an object from that jar file, call up your class loader to do that. (You only have to do that for your "top-level" objects, within those objects creating another object will automatically be done by your class loader.)
Periodically check whether the jar has been modified. If it has, refresh the application by creating a new instance of your class loader and using that to create new instances of all your objects.
But that sounds like a lot of work just to avoid setting up an emergency change procedure where you get everybody to close their applications.

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