. . . repaired and ultimately end up having to be replaced.
The problem is that (especially on the threads but also elsewhere in the barrel) a pen is subject to some forces during closing and removal of the cap and during writing as well, and sooner or later the crack tends to separate if you've tried to glue it; and then of course it will usually grow over time until it becomes large enough to permanently mechanically (and cosmetically) compromise your Fine Writing Instrument
I'd just replace the barrel if at all possible.