They have got large stocks of reprocessed plutonium; I have read an article saying that Japan can manufacture dozens of crude nukes in less than a week. China won't likely be able to gather the necessary Intel quickly.
This is pretty accurate and has been true since the late 20th century.
The difference is, China has been working on various BMD systems since 1960s and establishing BMD systems since 2000s. Japan would require many hundreds of high yield weapons to do serious damage to China but such damage is close to civilisation ending. China only require several dozen high yield weapons to end Japan's civilisation and make the entire set of islands completely uninhabitable.
Japan would be opening Pandora's box by actually assembling those crude nuclear weapons and launching them at China. BTW those weapons cannot be high yield thermonuclear weapons. So Japan would actually need thousands of XX kt yield weapons to cover China meanwhile they will be opening the door to China ending them with only a few MRBM missiles. Not to mention Chinese nuclear tipped cruise missiles would be more likely to carry out the blow rather than just MRBM.
Japan has non-existent BMD at the moment. Some long range Patriot SAMs (of which they number in the handful of units) are mostly anti-aircraft optimised types and some for intercepting subsonic cruise missiles, not even terrain hugging cruise missiles like the already antiquated CJ-10.
Japan taking a nuclear posture against China is a 1 in 1000 scenario and would only open the door to China ending them. They need 1000 to take on China while China needs 1 to take out Japan. Pretty foolish if you ask me when the US has a nuclear posture against China on Japan's behalf. Unless the US abandons Japan, there is zero good sense for Japan to paint itself as a nuclear target which it currently isn't owing to not just China's NFU policy but relatively cooperative relations. Escalating needlessly to a nuclear posture even in a Taiwan armed reunification scenario doesn't make sense for any Japanese politician that doesn't want Chinese MT yield nuclear warheads detonating all around major Japanese cities.
Japan is sliding into second tier power nation status with China and South Korea eating up industries formerly dominated by Japan. They talk a big game about Taiwan but when the bullets fly, pillow fucking weebs are not going to find the balls to become corpses on the beaches of Taiwan to be picked off by the tens of thousands of Chinese drones and armed robots. Talk is for politicians to do their "democracy performative art"... This is a genuine weakness of democracy, there is so much more performance and bullshitting around. An autocracy when it comes to war truly has a major advantage. They don't need to do any internal, domestic posturing to gain favour. If it's war, the Chinese people are genuinely quite prepared. Whereas the exact opposite is true for the anime degenerates. These people are not samurais or banzai fanatics, they are pillow fuckers with no jobs and severe vitamin d deficiency.
Visit Japan and look at their men, then visit China and look at the men. Japanese men can not fight their way out of wet tissue these days. They'd be trying to take upskirt photos of Taiwanese women before they learn how to load a rifle. You'd love to see them join a Taiwan war, absolutely love it. The most ball-less pathetic male species right now is quite objectively the Japanese "man". Riddled with various forms of degeneracy and lack of masculinity. Even by east asian standards these men are pathetic. I've visited several times and anyone who thinks japan is still a fighting capable nation has definitely never visited. The men there kill themselves when they get bad grades or get offended at work.
