The Chinese ships were near the Senkaku Islands in the East China Sea. The disputed island chain, which is nothing more than uninhabited rocks, is administered by Japan but claimed by China.
Flare-ups of this sort have occurred between Beijing and Tokyo in the past. And, despite the Hague's decision to reject China's claim on the South China Sea, the rapid expansion of China's navy paired with Japan's reliance on sea-lanes is a recipe for awakening Japan's pacifist slumber.