[FIXED] Thymeleaf preprocessing returns null when calling an entity get method

Issue

I'm using preprocessing in thymeleaf to append a class in <i> based on whether the user solved a challenge or not. I'm using a method isSolvedBy that takes a username string to check if the user solved the challenge. In the template, the username string is passed via calling another method getUsername that returns the username of the logged-in user. When calling getUsername() method it passes a null value to the caller function.

Controller snippet:

model.addAttribute("user", user.get());
model.addAttribute("score", user.get().getScore());
model.addAttribute("rank", user.get().getUserRank());

model.addAttribute("challenges", challengeList);

String challengesNumber = Integer.toString(challengeList.size());
model.addAttribute("challengesNumber", challengesNumber);

return "challenges";

challenges view snippet:

<a th:each="challenge: ${challenges}" th:href="@{/challenge(id=${challenge.challengeId})}" class="challenge-card white-text" style="text-decoration: none">
    <h3 style="margin-top: 5px;" th:text="${challenge.challengeTitle}" class="golden-text"></h3>
    <i th:classappend="${challenge.isSolvedBy(__${user.getUsername()}__)}"></i>  <!-- here is the preprocessing  -->
    <div>
        <h4 class="card-info">Points: <span class="golden-text" th:text="${challenge.challengeScore}"></span></h4>
        <h4 class="card-info">Level: <span class="" th:text="${challenge.level}"></span></h4>
    </div>
</a>

This is the challenge.isSolvedBy method:

    public String isSolvedBy(String username)
    {
        System.out.println(username);       // this prints null

        return "solved-challenge";      // temporary return value
    }

This user.getUsername() method:

    public String getUsername() {
        System.out.println(username);   // this prints the username value when called by thymeleaf
        
        return username;
    }

When the controller is called everything works fine and the preprocessing statement "${challenge.isSolvedBy(__${user.getUsername()}__)}" is executed without any errors but it passes a null value to challenge.isSolvedBy() method even though user.getUsername() will actually prints the actual username when it is called. Here is the actual output:

2022-07-11 01:09:07.365  INFO 19416 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring DispatcherServlet 'dispatcherServlet'
2022-07-11 01:09:07.365  INFO 19416 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'
2022-07-11 01:09:07.366  INFO 19416 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 1 ms
yousef [printed from getUsername method]
null [printed from isSolvedBy method]
yousef [printed from getUsername method]
null [printed from isSolvedBy method]

I'm still learning spring MVC and Thymeleaf, so clearly I missed something but I can't figure what it is.

Anyone know what is the problem?

Thanks in advance.

Solution

It seems that thymeleaf did not passed the returned value from __${user.getUsername()}__ as a string. Single quote marks are needed because after the first pass in thymeleaf the result of ${challenge.isSolvedBy(__${user.getUsername()}__)}" will be ${challenge.isSolvedBy(<result>)}. I don't know why it passed it as null and did not throw an error. The correct expression should include ' around the expression that needs to be passed as a string, because when writing the preprocessing statement like this ${challenge.isSolvedBy('__${user.getUsername()}__')} the result after the first thymeleaf pass will be ${challenge.isSolvedBy('<result')} and thus thymeleaf will pass the result as a string to the method.

Answered By - Yousef Ali
Answer Checked By - Katrina (JavaFixing Volunteer)