Japan that China intends to choke off the Japan's most important oil supply lines from Mideast to its homeland, once it commences the hostility against China.

Strait of Malacca (马六甲海峡) is the common route for ships less than 200K tonnes.  But it's narrow, shallow, and has become increasingly dangerous because it's too crowded and too prone of pirates.  Many of Japanese supertankers take the routes from Persian Gulf to Indian Ocean, then either (A) thru Sunda Strait (巽他海峡) , South China Sea, Bashi Channel, then turning northeast to Japan's homeland, or (B) thru Lombok Sela (龙目海峡), Makassar Strait (望加锡海峡), Celebes Sea (苏拉威西海), out to the western Pacific Ocean, then turning north to Japan's homeland. 

This time the round route Chinese South China Sea Fleet's warships are taking is exactly along these two most important strategic sea routes Japan depends on for its very energy survival.  That is why this training is said to be the near-real-war exercise.  This is the most serious warning signal China is sending to both Japan and the U.S., that China is really intending a BIG war if they continue down the current provocative route and behavior.  It' will not be a small skirmish as Japan and the U.S. are planning or dreaming at all!