AuthenticationPrincipal is empty when using EnableWebSecurity

As of Spring Security doc: 34.1 @EnableWebMvcSecurity states, the @EnableWebMvcSecurity was replaced by @EnableWebSecurity. But when I try to get the principal as below:

@RequestMapping(method = RequestMethod.POST)
@Secured("ROLE_USER")
public String postRoom(@Valid @ModelAttribute Room room, BindingResult result, Model model, @AuthenticationPrincipal Account principal) {
    if (result.hasErrors()) {
        return "room_form";
    }

    Account account = accountRepository.findByUsername(principal.getUsername());
    room.setAccountId(account.getId());
    room.setLastModified(new Date());
    roomRepository.save(room);
    return "room_list";
}

The principal.getUsername() is "", and all of the fields in principal are empty. below is my security configuration:

@Configuration
@EnableWebSecurity
@EnableGlobalMethodSecurity(securedEnabled = true)
public class WebSecurityConfig extends WebSecurityConfigurerAdapter {
    @Autowired
    private DataSource dataSource;

    @Autowired
    private SecurityProperties security;

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.authorizeRequests().anyRequest().permitAll()
            .and().formLogin().loginPage("/login").failureUrl("/login?error").permitAll()
            .and().logout().permitAll()
            .and().rememberMe()
            .and().csrf().disable();
    }

    @Autowired
    public void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
            auth.jdbcAuthentication().dataSource(this.dataSource).passwordEncoder(new BCryptPasswordEncoder(8));
    }
}

And the Account.java:

@Entity
@Table(name = "users")
public class Account implements Serializable {
    @Id
    @GeneratedValue
    private Long id;

    private String username;
    private String password;
    private boolean enabled;

    @Lob
    private byte[] avatar;

// getter / setter ...
}

But I can get the values if I use @EnableWebMvcSecurity.

So my question is, what the correct way to get the principal when I use @EnableWebSecurity?