This article wonders of Japan is a toothless tiger. It is not battle tested, to be sure, but as the article discusses, it is well equipped and does train to high standards. So Japan's military has teeth.
The problem is the lack of tail--the logistics backing to maintain a fight or fight away from Japan--as the article notes:
However, the truth is that Japan's military would not be able to defend Japan alone in the long-run nor go on the offensive, primarily because of its lack of offensive weapons, limited manpower and equipment pool.
Behind the JSDF stands the US, and therein lies any strength it might wield.
Too much can be made of lack of "offensive" weapons. F-16s are fighter planes but with precision weapons do well for ground support in practice. And while Japan does not have aircraft carriers, they do have "helicopter destroyers" that look suspiciously like small aircraft carriers. F-35s as well as helicopters can fly from them.
For a fight over the Senkaku Islands, as long as we help Japan behind the front, I think Japan could win.
But it is true that Japan would rely on us for a lot of logistics to keep fighting for long. That is a common issue with our allies (in part deliberately encouraged to keep allies on a short leash so they don't drag us into war). See our resupply efforts for NATO allies in the war against Libya's tiny military in 2011.
I'd be happier if Japan had a more powerful air force. But short of nuclear weapons, Japan would be a formidable foe for China.