Minor Description
Either there is a bug in an implementation or misunderstanding in docs.
RandomStringUtils.random (count, letters, numbers) is documented so that:
letters if true, generated string will include alphabetic characters
numbers if true, generated string will include numeric characters
But apparently the current implementation supports only that generated string may include either only letters, only numbers or both.
This is current implementation:
if (letters && Character.isLetter(ch) || numbers && Character.isDigit(ch) || !letters && !numbers)
So there may be situation when generated string is not containing numbers at all which is in contrary with what the docs say.