Question

It’s not documented. Why does it return an array of strings? (If it had return a string, I wouldn’t have asked)

Answer 1

Actually it says on the page you link to:

ThrowableInformation is log4j’s internal representation of throwables.
It essentially consists of a string array, called ‘rep’, where the
first element, that is rep[0], represents the string representation of
the throwable (i.e. the value you get when you do
throwable.toString()) and subsequent elements correspond the stack
trace with the top most entry of the stack corresponding to the second
entry of the ‘rep’ array that is rep[1].