Symfony2 - Doctrine -类xxx不是一个有效的实体或映射的超类

I have seen many questions in the past about this subject, but none with working solutions. I'm following the Symfony book and cookbook. I used doctrine to fill in getter/setter methods in one of the examples, but it didn't work when I tried to repeat it with another example. When I went back to the previous exercise, it stopped working there too.I have seen many questions in the past about th