As far as "repairing the cracks", you really can't. If the glass is cracked on your screen the only thing you can do is replace the screen. On your phone it's not a particularly difficult repair; in fact, it's one of the easiest I've seen as far as cell phones go. Here's a video showing you what you need to do.
how to samsung galaxy j2 broken lcd replacement - YouTube
The one thing I take exception to on this video is that they skipped the step of cleaning the old adhesive off the frame. If you don't do that you take the risk that the old glue will keep the new from sticking well and could eventually lead to the screen coming off. Go around the edge with Q-Tips and some 90% or higher concentration isopropyl (rubbing) alcohol and you should be good.
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