The Chinese are not allowed to forget about Nanjing. Every time anything in the news that involves a problem with Japan the old films are rolled out on TV. Japan is not China's problem. Their problem is about 850,000,000 Chinese who are just above peasant level who are looking at TV screens showing China's economic expansion and asking, "where is my share?"
Revolutions in China have always been started by peasants and the Chinese leadership is all too aware of its own recent history. There job is to avoid this at all costs and the defence against that is continuous growth through overseas trade. They are not going to rock the boat by starting a war. They still haven't got over the drubbing they received from Viet Nam.
Some islands, like the Paracel Islands, have been accepted internationally as being part if China for over a hundred years. Other have the temptation of mineral rights, especially hydro-carbons. If they didn't have that potential China would not be interested in them; nor would Japan, or the Philippines, or Taiwan, or Vietnam, or Malaysia.