Need Help for query

Question form JP Morgan i was selected for second round : but he gave assignment , question is : Currently JP Morgan Banking Application is deployed / running successfully on client or customer Machine , The JP Morgan App may Throw Run time Exceptions , Now We have to Write 3rd party Java Program to capture Run Time Exception and write those Runtime Exceptions to New file

constraints : 1, We should not modify to any of class of Production which is currently Running . Could you please Help on this .

Note : Capture the Runtime Exceptions & we have to Write to Different file

Thanks in Advance