Thymeleaf cannot resolve

Question

I have this Controller Class and an HTML Login page. Thymeleaf Cannot resolve 'loginRequest'.

@Controller                                                                                         
public class ProfessorController {

    private final ProfessorService professorService
  
    @Autowired
    public ProfessorController(ProfessorService professorService){
        this.professorService = professorService;
    }
  
    @GetMapping("/login")
    public String getLoginPage(Model model){
        model.addAttribute("loginRequest", new Professor());
        return "login_page";
    }

   @PostMapping("/login")
   public String login(@ModelAttribute Professor professor, Model model){
       System.out.println("login request: " + professor);
       Professor authenticated = 
       professorService.authenticate(professor.getUsername(),professor.getPassword());
       if(authenticated != null){
           model.addAttribute("userLogin",authenticated.getUsername());
           return "home";
       }else{
           return "error_page";
       }
   }
}

I have this in my html code: xmlns:th="http://www.thymeleaf.org"

<form class="login100-form validate-form" method="post" action="/login" th:object="${loginRequest}">
            

Answer 1

I had the same problem actually.. I had a project that was working fine, resolving Thymeleaf template variables.. so I compared the two projects..the only difference I could fine was the SpringBoot version in the POM. I was using 2.7 in my new project.. I reverted to 2.5.2 and it was able to resolve the variables. Hope this helps. Note: 2.6.8 appears to work fine as well.

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.5.2</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

Answer 2

I think I know the mistake, please try the below code in Thymeleaf template.

<form class="login100-form validate-form" method="post" th:action="@{/login}" th:object="${loginRequest} id="login_page"">

I think the issue is that you are missing the id property. Also for the action property the th: part is missing. it should be th:action and also modified the value @{/login}.