The Chinese don't hate the Japanese per say - no Chinese person I know does.
Sorry, but then you only know Chinese overseas, who are not representative of the whole. Even then, people don't usually come out and say "I hate Japanese." I've spent years between both countries.
Of course, not all Chinese hate Japanese, but enough do that it could start a war. Of course they tend to be less educated and people who don't know any Japanese. Whenever there's a flare-up, Chinese tourism to Japan mostly continues until the government orders agents to shut it down.
Still...When Japanese are attacked in the streets, it gives them little comfort to say "not all Chinese hate you." Obviously, enough do that it's not safe.
You don't think that perhaps some of stems from War Crimes and the approximately 10,000,000 people that the Japanese killed during WW II?
Lookup The Nanjing Massacre.
The events have been denied/played down etc. by Japanese diplomats/politicians as recently as 2010.
Actually, I was surprise this wasn't brought up several posts back. People need a reason to hate, no matter how long ago. Japanese politicians are mostly scum, so what? You don't see 1000s of Japanese protesting the Mongol invasions or attacking Chinese in the streets in Japan.
And whatever the number of those killed by Japanese in the war, it was 80 years ago. Most people on either side were not even born when those events occurred. Many wars and hate campaigns are spurred by distant memories of injustice. That doesn't make it right.
And much more recently, many, many times the death and destruction was brought to China by their own. And the leader of that genocide (both cultural and human) is honored with his portrait looking over Tiananmen Square.
I am making a simple assertion here, really. There's a lot of hate/bigotry against the Japanese in China, and the government uses it successfully to hold onto power. I know this from personal experience, I've lived in both countries and I have close friends in both countries. It's a serious issue because it could lead to war. If China becomes unstable, some aspiring dictator could do very well by promising to crush Japan. Sure, many Chinese would be against that - but they might not be able to stop it. Racism/bigotry is powerful, and it often works.