China isn't a threat to Japan. China needs Japan and a stable Eastern hemisphere to compete economically with the rest of the world, so they can make enough $$$ to bring the masses out of poverty and avoid being lined up against a courtyard wall when the regime falls.
Because of this they are maybe the best hope when it comes to keeping North Korea 'in-line'. It is highly unlikely that North Korea would try anything radical on a large scale without the tacit approval or clearance of China. This is mostly due to the fact that the North Korean leadership *knows* that if they get too annoying and screw up the economics of Asia by starting another war 'all of a sudden' China is not afraid to 'spend' the men and equipment to walk in to North Korea and set up a North Korea that China thinks is 'acceptable'. And if they did do this, North Korea knows that Japan, the U.S., and the rest of the world might 'formally' protest such actions but behind closed doors the thought of the day would be 'It is impossible for China to run North Korea worse than the present North Korean leadership from a world-stability standpoint'.
Japan isn't going to build offensive strategic weapons to counter North Korea either. They know for a fact that the U.S. would intervene in a very direct manner if North Korea attacked Japan with strategic weapons of any sort. And 'M.A.D.', which was idiotic against the Russians - who had leadership that actually did care if their people were vaporized in a war - is totally worthless when applied to North Korea: a Nation run by a madman who has proven that he doesn't give a damn about the citizens he rules over.
'My possibly educated $.02'.
Mike/wulfie