Japan has leaked military plans for dealing with possible future war with China. The war plans deal mainly with three situations. All assume that China will continue to increase its military power, the better to deal with Taiwan, and the United States. Thus the main scenario deals with how Japan would have to deal with a Chinese attack on Taiwan, and the likely involvement of the United States. America has bases in Japan, and there is well founded fear that China might attack these bases. This would draw Japan into the war. Not because Japan wants to be involved, but because its considered likely that Japanese public opinion would insist that Japan actively defend itself.
The other two scenarios cover possible Chinese military actions to assert claims over disputed islands, and potential underwater gas and oil fields, in the East China sea. The Senkakus island (Diaoyu Islands to China and the Tiaoyutais to Taiwan) are occupied by Japan. But China has a strong claim, partly for nationalistic and historical reasons, and possibly because of money (the possibility of oil and gas being found in the area.)
Japan is increasing its military spending, and is already, because of its large economy, the biggest military spender in the region. Japan sees the most immediate threat as a hostile North Korea, and the missiles and nuclear weapons being built there. There is also international terrorism for the Japanese military to contend with. But long term, and most worrisome, is China. There have been war plans dealing with China since the 1970s. These were last revised in the 1990s, but were generally kept quiet. No more, and that is a verbal shot across the bow the China. The warning is clear; Japan will be no pushover if it comes to war.