UserDetails
UserDetailsService返回 {security-api-url}org/springframework/security/core/userdetails/UserDetails.html[UserDetails]。DaoAuthenticationProvider验证 UserDetails，然后返回 Authentication，其主体是已配置的 UserDetailsService`返回的 `UserDetails。
{security-api-url}org/springframework/security/core/userdetails/UserDetails.html[UserDetails] is returned by the UserDetailsService.
The DaoAuthenticationProvider validates the UserDetails and then returns an Authentication that has a principal that is the UserDetails returned by the configured UserDetailsService.