Last month, China made an official protest to Japan, complaining that Japanese aircraft and ships harassed Chinese warships training in international waters (the East China Sea). Japan rejected the protest and reminded the Chinese that anyone can watch anyone on the high seas, and it's been that way for everyone, for a long time. The Chinese don't agree with this custom, and have become increasingly aggressive at keeping foreign intelligence collecting ships and aircraft away from its coastal waters, and from Chinese warships training far out in the ocean. This unique Chinese interpretation of intelligence collection at sea puts it at odds with the rest of the world.
China is becoming more active at sea. While there are plenty of places the Chinese Navy can send its ships for training, dispatching them to international waters near Okinawa has the added advantage of reminding Japan that China could use force to assert its rights over small islands Japan also claims. Some of these islands have oil and natural gas deposits nearby. Chinese leaders are very concerned about energy supplies, because China has to import nearly all its petroleum, and consumption is growing rapidly.
China is also still very concerned with Japanese military and economic power. Add to that the smoldering resentment for Japanese occupation and atrocities from the 1880s to the 1940s, and you have potential for war that is little appreciated outside the region. In fact, China has tense relationships with all its neighbors, in addition to a long history of Chinese aggression. The growing economic power of China is seen, by Chinese neighbors, as another weapon the Chinese will use against them.
Over the last three years, Japan has increased anti-submarine patrols in international waters, just outside Japanese territorial waters. Chinese submarines are apparently exercising there more frequently, looking for Japanese, South Korean and American warships to play tag with. The U.S. has also redirected more of its space based naval search capabilities to assist the Japanese.
Over the last two years, it's been more common for Chinese Song class diesel electric and Han class nuclear powered boats to be detected and tracked by American and Japanese warships and aircraft. Some of these Chinese subs have been found to be stalking American and Japanese warships.