As for weapons, well, what's a war without ships?
Junk ~ A junk is an ancient Chinese sailing vessel/ship design still in use today. Junks were developed during the Song Dynasty (960-1129) and were used as seagoing vessels as early as the 2nd century CE. They evolved in the later dynasties, and were used throughout Asia for extensive ocean voyages. They were found, and in lesser numbers are still found, throughout South-East Asia and India, but primarily in China, perhaps most famously in Hong Kong. Found more broadly today is a growing number of modern recreational junk-rigged sailboats.
And due to the limited number of 'J' items, I bring you a 'J' related, medieval sport: