The Romans specially designed the head of their "pilum" so that it bent on contact with the ground or the target. This prevented it being thrown back at the soldier. If it was lodged in a shield etc., then it was hard to extract.
Manuscript images of Saxon warriors show them holding a number of javelins in the same hand as their shield (putting a bit of a dampener on the idea of convex shields), allowing them to be readily grasped once the javelin in the primary hand has been thrown.
Depictions of javelin wielders onthe Bayeaux Tapestry hold the javelin behind the centre of gravity and experiments in holding a javelin in this manner have revealed it to be a surprisingly effective technique.