I think this is more of a courtesy warning rather than a threat.  Japanese government's should take it serously because there might not be a second chance to regret.

http://www.ibtimes.co.uk/beijing-hinted-military-action-against-japan-over-south-china-sea-dispute-1577231?utm_source=yahoo&utm_medium=referral&utm_campaign=rss&utm_content=/rss/yahoous/news&yptr=yahoo