Japan fears nukes. Majourity of Japanese fear nukes. China has the infrastructure to and has already expanded its nuke arsenal. But its 600+ bombs are mostly ICBM and SLBM warheads aimed at the US and Russia (India to a lesser extend). Not sure if China could quickly develop a new warhead for DF-17 type medium range HGVs that could easily penetrate Japan multi-layered air defense system.Knowing Japan, they will scrape the bottom of the barrel to come up with any excuse to get USA to save them.
After all when you look at location, Japan has no friends. Even South Korea which is supposed to be its vassal colleagues doesn't like it. Its surrounded by people who want to get even with it. That puts it in a very hard spot considering all except South Korea have nukes.
Deterrence is ideal. But China really needs to be prepared to go all out for this. There are no half measures which means a lot of people will probably die.
Maybe a viable strategy from Beijing’s position could be should Tokyo drop or amend the Article 9, Beijing would respond by dropping its no first use policy, as well as resuming nuclear testing (both underground AND airburst, including mating warheads to missiles in full range test like the one conducted in 1966) in Lop Nur. Keep in mind that due to China’s limitations places under the 1996 CTBT after ONLY 46 tests (but Moscow and Washington already had thousands), Beijing stands to gain the most should all great powers resume nuclear testing.

