It's a bit long winded but in short I have confidence that the JSDF could stop an air and sea invasion of Japan in the start of a conflict, but long term could be tricky. The JMSDF receives a lot of funding specifically because of concerns regarding the PLAN. the Japanese MOD has put a lot of time and effort into weapons programs and ship programs to maximize the use of a hull and minimize the amount of crew required to run it. What is most important in the start of a conflict is blunting the PLAN spear before it can establish safe transit for amphibious landings. To that end, Japan has invested in a multi layered defense plan starting with ships and all the way to mobile land based anti-ship missile systems that can be deployed anywhere. The JSDF is also constantly training and has a high standard for entrance and training alike. The current doctrine is defensive and plays into that well.
The issues stem from a potential long lasting conflict against China, a nation that not only has immense manpower but immense people to draft. Military service in Japan is thankless hard work and you have to love your nation or love the military to make it a career. Unlike the U.S there's no discounts, benefits or healthcare subsidies after leaving service. You have your pension but everyone in Japan does. You have your healthcare but so does everyone else. All this is to say military service is unpopular and could be a source of weakness when replacing manpower. China's ships are impressive on paper but if their best are countered early on, Japan's life gets easier. There is an expectation that the U.S would join in a conflict but it's not relied upon as a crutch. That being said Japanese realists within government and military expect the opening of a conflict between JP and CN would rely on Japan holding out until the gears in the U.S start turning.
As far as military preparedness goes. Japan is well off. Not in the amount of stuff they have now but the industry they can tap in to during a conflict. Major contractors like MHI reserve the right to cancel any non defense related project and produce what is needed for a hot conflict. This is on top of non in use capabilities (like building nuclear reactors for ships and long range delivery systems) are still practiced. It's glacially slow but the skilled workers are there working on rockets or new ships. If they didn't they would lose the skilled labor in question. That's why filling out the MODs Type 90 MBT order took so long and why the U.S still makes Abrams the Amry doesn't need. If you slow it down you preserve skilled workers incase you need to ramp up industry.
In most Japanese war games China holds and endge in endurance but Japan's current doctrine is deterrance through cost. Much like the swiss, the idea is to make every advance cost more than it's worth. How many ships and aircraft is China willing to lose, how many carriers and subs etc. So long as advancement to the mainland can be stifled it buys time for things at home to change. Recruiting reforms, enlistment incentives etc. Japan doesn't plan on winning a prolonged conflict with China through taking land but bleeding as much as possible. To this end I feel the JSDF is ready for a conflict just not a long one without outside support past the 6 month mark. I belive after that time if no large reforms have been passed at home, some islands might be taken like Tsushima and possibly some in the Ryukyu chain or those closest to Taiwan.
Japan produces a lot of munitions and stores them away. For example old tanks as far back as the Type 61, decomissioned decades ago, go to cold storage for rainy days and Japanese naval procurment emphesiszes building good ships, going as far as decomissioning ships with a decade left on their life just to ensure no capability is lost. None of this is to say Japan could "demolish china" on it's own but I am of the opinion that Japan could shut down Chinese advances for a while. Not forever, and without outside help things would gets worse over time. But if Ukraine has taught me anything it's that wars are very random and things that are inoccuous could turn out to be very important.
I didn't want to throw up numbers and percentages and so on because this is long enough already, but what it comes down to for a seaborne conflict is how many ships with decent systems can be produced in a set amount of time. Which is another headache entirely. Though good training and good ships go a long way.
