The Japanese government recently announced a major military exercise, which will include amphibious lands on a Japanese island located 250 miles to the south east of Okinawa. The drill is located within what is called the Ryukyu island passage way that forms the first island change that the US, and her allies, could use to try contain China in the event of hostilities.
Military maneuvers between the United States and Japan are nothing new, however, this test is different in that it is a very large joint exercise, someone correct me if I am wrong but I do not think Japan has initiated a military exercise of this size that involves all branches of the military in quiet some time if ever, and the introduction of Japanese anti-ship missiles that, up to know, have not been deployed in Okinawa or the surrounding islands. This is a direct challenge, though Japanese officials would never admit it as such, to China's claims to have finally developed the military capability to surmount the obstacle that the island chains pose to China's navy.
In the event of hostilities, determining whether Japan has military strength alone to thwart the Chinese or vice versa is virtually impossible. The fact is there are few navies that have engaged in any sort of real combat operations, much less large scale military operations, since the end of world war II. China has made some big leaps in terms of their naval capability, however, their navy is still very inexperienced in the type of coordinated maneuvers that would be required for that kind of operation. On the other side is Japan, with a large and capable navy, but has followed military doctrine that puts them in a support position to the United States. This brings us to the other side of the coin, nothing develops in a vacuum.
In terms of trying to maintain military hegemony over the region, the United States has a vested interest in continued cooperation and facilitation with Japan. It would be a stretch the think of the Japanese navy as an extension of the United States navy, however, it is highly likely, almost assured, that if Japan were drawn into conflict with China that the nation would seek, and probably receive, assistance from the United States. Whether the converse is true is another story. If conflict broke out between the US and China there is no guarantee that Japan would get involved, though I imagine that it would be likely, and the US ability contain the Chinese is greatly increased with Japanese aid.
Revisiting the image from Stratfor above, the two closet passage ways that China could realistically breech would be the Ryukyu island chain and the Luzon straight; I highly doubt the Chinese would even attempt to pass through the sea of Japan. The US already has a high degree of report with Japan, Taiwan and the Philippines, and in regards to the Philippines it is only growing stronger. If the US could force a situation were the only realistic naval avenue for Chinese military, and commercial shipping, could transit is towards the Indonesian Sea, it would be a major boon for the US as it would force military operations farther away from the Chinese mainland, and the land based anti-naval support that could be brought to bear, and into an area where the US military strengths would come into play; open ocean self supported warfare.
China is most definitely concerned about these operations because the PRC realizes the danger that a strengthened Ryukyu presents them. Economically China, like the United States, depends on unfettered access to the oceans for both resources and markets. If China doesn't have reasonable ability to at least check military power in the East or South China seas then she isn't economically or geopoliticallu secure, much less a super power of the same magnitude as the United States that can dictate policy and economies half a world away.