variables in Thymeleaf

I am using thymeleaf to send variables from the controller to the view (HTML file) but it says that is null.

This is my HelloController (src/main/java/controller/HelloController):

@Controller
    public class HelloController {
        public String mensaje = "Springboot";
        @GetMapping("/")
        public String index(Model model){
            model.addAttribute("mensaje", mensaje);
            return "index";
        }
    }

This is the HTML file (src/main/resouces/templates/index.html):

<!DOCTYPE html>
<html lang="en" xmlns:th="http://www.w3.org/1999/xhtml">
<head>
    <meta charset="UTF-8">
    <title>Index</title>
</head>
<body>
<h1 th:text="'Hello' + ${mensaje} + '!'"></h1>
</body>
</html>

The message in the browser should be "Hello Springboot!" but it says "Hellonull!"

Any help?

Solution

try the below and let me know.

  @Controller
public class HelloController {

    private String mensaje;

    public HelloController() {
        this.mensaje = "Springboot";
    }


    @GetMapping("/")
    public String index(Model model){
        model.addAttribute("mensaje", mensaje);
        return "index";
    }
}

in the above i tried by adding constructor to the HelloController class , that will make sure that variable mensaje is initialized correctly.