Japan will get involved in a near-future possible conflict but S.Korea would most likely stay neutral. Note that Japan is in the quad and is actively working to get inside the FIVE Eyes intelligence network as a full member which as you know entails ahem, certain duties like this one for exampleExactly which allies are you talking I am not sure if Japan or Korea will get involved They can count of Australia, Britain and Canada but they are minnow as far as naval asset concern In the next decade even counting Japan and Korea Chinese navy will have the preponderance number of asset even the next 5 years you will have 50 destroyers, 50 frigates and 80 to 100 corvette plus 40 conventional sub, 3 or 4 carrier, 7 or 8 LPD , 4 or 5 LHD
In case the Japanese dont actively participate then they will certainly aid the US by allowing the use of its bases, radars, targeting data, intelligence etc or they might just try to divert a portion of China's strength by making major military exercises in a different theater to draw China's attention.
So as you can see there are many ways Japan can and will use to aid the US. On the other hand, if by 2030 China keeps growing at the current trend along with its military then i fully expect for Japan to go at the S.Korea route by staying neutral