For springSecurityService.principal.id, since there is no field for dependency injection for springSecurityService, it cannot work, so you need to call a call springSecurityService.principal.id-org.springframework.security.core.context.SecurityContextHolder.context.authentication.principal.id
To fix the problem with the user, you will need the full name of the class with the package. So it should be
{com.yourcompany.yourapp.User.get(org.springframework.security.core.context.SecurityContextHolder.context.authentication.principal.id)}