problem with Chinese

I write Chinese characters with Xwiki. I have change the encoding to UTF-8. Futhermore, I set mysql default encode to UTF-8. The Wiki can work well to display Chinese in the edit situation and view situation. After I change the wiki by writing Chinese in it. And change it again. If I roll back the version, the Chinese characters will become something like "?????". How to solve this problem? Thanks, Wang Ning