@NoArg
data class User(
    var username: String,
    var password: String,
    // ...
)

// bean
public class User {
    private int id;
    private String firstname;
    private String lastname;
    private String username;
    private String password;
    private String email;
    // ...
    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", firstname='" + firstname + ''' +
                ", lastname='" + lastname + ''' +
                ", username='" + username + ''' +
                ", password='" + password + ''' +
                ", email='" + email + ''' +
                '}';
    }
}

// controller
@Controller
public class UserController {
    @PostMapping("/user")
    public String getUser(User user){
        System.out.println(user);
        return "success";
    }
}

<form th:action="@{/user}" method="post">
    username:
    <label>
        <input type="text" name="userName">
    </label>
    password:
    <label>
        <input type="password" name="passWord">
    </label>
    <!-- .... -->
    <input type="submit" value="submit">
</form>

form The name field of the form is consistent with the Java properties will work fine, what if the form has a lot of content and the fields are inconsistent with the Java bean properties. As shown above.
😊

The above code will work fine.
output:

User{username=a, password=a}