As it is an International connection, there is no need to go through Immigration or Customs. Just a security check.

Baggage can go astray in JNB often, and does not make the connecting flight. Just remember to pack some spare clothes in hand luggage. It usually catches up with you the next day.

Also don't pack valuables in your checked in luggage. If you can get it plastic wrapped, it will be a precautionary measure.