China is preparing for the worst scenario.

China military is trying to familiarize with the aviation routes in preparation for the possible worst scenario in the future.

http://www.asahi.com/ajw/articles/AJ201701100021.html

If Trump rejects the one-China policy upon inauguration, China will impose economic sanctions against Taiwan and also take military actions to surround Taiwan.  As China is anticipating USA and Japan to come to help Taiwan, it has to familiarize with the aviation routes around Japan for future military confrontations.  Tension is indeed rising in South China Sea.