Spring security returns String as principal instead of UserDetails on controller

The question is: I have an Auth server(spring security) and a Resource server，The Resource server can't get principal user correctly.

Spring Boot Version: 2.0.7 Spring Cloud Version: Finchley.SR2

Firstly, I go to get access token from Auth-server, and It is the success. Then, I use the token to post the Resource-server, and the controller of Resource-server can get a principal. However, the principal is a string, not a UserDetails Object.

Auth Server websecurity config like this:

@Configuration
@EnableWebSecurity
@EnableGlobalMethodSecurity(prePostEnabled = true, securedEnabled = true)
public class WebSecurityConfig extends WebSecurityConfigurerAdapter {
    @Autowired
    private MaliUserDetailsService maliUserDetailsService;

    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder();
    }

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
            .anyRequest().authenticated()
        .and()
            .csrf().disable();
    }

    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
  auth.userDetailsService(maliUserDetailsService).passwordEncoder(passwordEncoder());
}

    @Override
    @Bean
    public AuthenticationManager authenticationManagerBean() throws Exception {
        return super.authenticationManagerBean();
    }
}

Auth Server UserDetailsService：

@Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
    User user = userRepository.findByUsername(username);
    if (user == null) {
        throw new UsernameNotFoundException(username);
    }
    return user;
}

Resouce Server Controller：

@GetMapping("/test")
public String test(Authentication authentication) {
    UserDetails userDetails = (UserDetails) authentication.getPrincipal();

    return "hello " + userDetails.getUsername();
}

When I post to resource server, it cast a exception:

java.lang.ClassCastException: class java.lang.String cannot be cast to class org.springframework.security.core.userdetails.UserDetails (java.lang.String is in module java.base of loader 'bootstrap'; org.springframework.security.core.userdetails.UserDetails is in unnamed module of loader 'app')

=============== Then I tried to change to controller code like this:

@GetMapping("/test")
public String test(Authentication authentication) {
    return "hello " + authentication.getPrincipal().toString();
}

The result is "hello admin". (admin is my username)

Why the principal in the controller is a string instead of UserDetails Object? How to solve it.