SpringMVC 提交的问题，很奇怪

// 大致是这样的
public class User {
 
    private String name;
 
   // ... get and set
}
 
public class Book {
 
    // ... other fild
 
   private User user;
 
   // ... get and set
 
}

然后在表单提交的时候用

报错，
org.springframework.beans.NullValueInNestedPathException: Invalid property 'user' of bean class [com.lqz.Book]: Cannot access indexed value of property referenced in indexed property path 'user': returned null

at org.springframework.beans.BeanWrapperImpl.getPropertyValue(BeanWrapperImpl.java:547)
at org.springframework.beans.BeanWrapperImpl.getNestedBeanWrapper(BeanWrappe