How to assign map keys and values ​​- object's modelAttribute property in JSP forms using Spring?

I have a question web application in Spring. Here's the code to get the Q&A form and return it to exam.jsp:

@RequestMapping(value = "/")
public String getExam(ModelMap map) {

    List<Question> questions = new ArrayList<Question>();
    // Here I am getting the all data for questions list
    // and basically I am sending to the view the list of questions, where each question has list of variants
    map.addAttribute("questions", questions);
    return "exam";
}

      

        
        
        
      

    

My model classes:

public class Question {
    private int id;
    private String text;
    private List<Variant> variants;

    //getters and setters
}

public class Variant {
    private int id;
    private int questionId;
    private int correctness;
    private String text;

    //getters and setters
}

public class AnswerSheetWrapper {
    Map<Integer, Integer> answerSheet;

    //getter and setter
}

      

        
        
        
      

    

In my exam.jsp: I am getting the questions attribute from the getExam method from the controllers. I am creating groups of radio objects for each question and filling in the modelattribute "answerSheetWrapper" (I may be doing this wrong, so please tell me how to do it). I want the answerSheet map to contain the question id as keys and id variant as values :

<form:form action="/exam/calculate" modelAttribute="answerSheetWrapper">
    <c:forEach items="${questions}" var="question">
        ${question.text}<br />
        <c:forEach items="${question.variants}" var="variant">
            <form:radiobutton path="answerSheet['${question.id}']" value="${variant.id}"/>${variant.text} <!--Here code throws Exception when runned-->
        </c:forEach>
        <br />
    </c:forEach>
    <input type="submit" value="Göndər"/>
</form:form>

      

        
        
        
      

    

And this is my controller method where the form action takes place:

@RequestMapping(value = "/exam/calculate")
public String calculate(@ModelAttribute("answerSheetWrapper")AnswerSheetWrapper answerSheetWrapper) {

    // do processing with modelAttribute object
    return "someView";
}

      

        
        
        
      

    

I'm not sure if I give the path in the form: radiobutton correctly .

When I run the application, I get:

HTTP Status 500 - An exception occurred processing JSP page /resources/pages/exam.jsp at line 29

      

        
        
        
      

    

The main reason:

java.lang.IllegalStateException: Neither BindingResult nor plain target object for bean name 'answerSheetWrapper' available as request attribute

      

        
        
        
      

    

So where is the problem in my code? Any help is appreciated.