Javelins were used, sometimes in small numbers and sometimes in large numbers. For example, during the 611 (Sui Dynasty) invasion of Korea (more precisely, of Koguryo), the Chinese army included 30,000 javelineers as well as 30,000 crossbowmen [1].
In Japan, there was the uchi-ne, a short javelin. This was mostly used as a self-defence weapon, notably when travelling by palanquin. The short length meant that it could be thrown from inside the palanquin. It could be used as a stabbing weapon in the confines of a palanquin, but fletched examples were clearly made to be thrown. Some examples of uchi-ne:
Also, here is an example of a Qing Dynasty javelin head, identified as such on the basis of its small size and the barbed head:
Peter Dekker also includes information about Chinese javelins, including the specifications for javelins for the Green Standard Army (the Qing non-Banner Chinese regular army troops): about 4 feet/120 cm long, with a 14cm barbed head. The total weight would have been about 900-1000g.
Javelins have some advantages over bows and crossbows. Heavy javelins can deliver tremendous energy, and therefore can be effective against armour (an Olympic javelin, a little lighter than the Qing javelin above, can be thrown with well over 300J of energy, 2-3 times what can be expected from even powerful bows). A javelin can also be used with a shield.
EDIT: A Han Dynasty javelin was recently posted in https://old.reddit.com/r/ArmsandArmor/comments/fgte15/a_han_dynasty_javelin_the_shaft_is_iron_too_tokyo/
References:
[1] David A. Graff, Medieval Chinese Warfare, 300-900, Routledge, 2002, pg 146.