IMO somehow it's an extention of domestic politics. What is the most effective way to arouse 'nationalism' and to a certain degree divert public attention to external affairs? Japan the old rival, presented a perfect case - ramming a trawler, detaining the captain over the alleged Japan's Senkaku (Diaoyu) waters under their 'domestic' jurisdiction. The public perception is CCP has been an 'ostrich' avoiding confrontations with foreign 'aggressors' after 'opening and reform' with economic development as priority, and also lots of 'national interests' have been 'ceded' in exchange for a 'peaceful envionment'. Therefore getting assertive with Japan has won a lot of 'bravo' among Chinese for the guts to 'defend Chinese interest'.
In addition to Diaoyu (Senkaku) China and Japan also quarrel over East China Sea demarcation (oil/gas fields). China for a while was willing to nego. a 'joint' exploration while 'shelfing' disputes. But once China realizes Japan is anyway a 'hardliner' in the US's camp of 'containment conspiracy', China feels it's not worthwhile to 'compromise' with Japan. Of course a nuke war is very impossible when everybody incl JP has the deterrent . IMO, by so doing China also indirectly asserts herself to the US and tests the US's 'bottomline' i.e. how far the US would like to go in such conflicts on South or East China Sea for 'allies'.
Besides JP as a 'democracy' lacks 'consistency' in foreign policies (how many PMs Japan has got in just a few years?) in the eyes of Beijing. Therefore in simple words, Beijing doesn't regard Kan administration as a 'serious' counterpart.
By the way CCP can't be seen in the light of 'outdated' ideology. It has to affirm its 'legitimacy' in ruling by catering for "the people's will" although it doesn't have to be 'proofread' by election from time to time. When Indians refer to CCP it often sounds like CCP is 'evil' as a blockade for 'progress'. It's not yet true "for the time being".