How to get the POST request parameters of a jQuery $.post from a spring mvc controller?

Question

I need to load data from database dynamically on the page, without any page reload.

My code in JSP is like this:

<script>
$(document).ready(function() {
    $("#ClassNext").click(function(event){
        var data = {};
        var classArray = [];
        $.each($("input[name='classCheckbox']:checked"), function(){            
            classArray.push($(this).val());
        });
        console.log(classArray);
        data["list"] = classArray;
        data["var"] = "1";
        console.log(data);
        $.post('PopulateClassSample',data,function(responseJson){
            if(responseJson!=null){
                $.each(responseJson,function(key,value){
                    // do something
                })
            }
        })
    })
});
<script>
<c:forEach items="${classList}" var="Class">     
    <li>
        <input class="classCheckbox" type="checkbox" name="classCheckbox" value="${Class.classStart}"></input>
        <c:out value="${Class.showString}"/>
        <button style="display: none;" id="${Class.classStart}">Edit</button>
    </li>
</c:forEach>

<button id="ClassNext">Next</button>

Here I am posting a request to extract some information from the database through jQuery POST ($.post). I am expecting the response as a JSON object which contains the information from the database. The data variable is an object of two fields- one field contains an array of Strings, the other field contains a String. This data variable gets its required values correctly through javascript.

The method that catches this request in a controller:

@RequestMapping(value="/PopulateClassSample", method = RequestMethod.POST)
    @ResponseBody
    protected void SendResponse(@RequestParam("list") ArrayList<String> list, @RequestParam("start") String start, HttpServletResponse response) throws IOException
    {
        for(String s:list){
            System.out.println(s);

        }
        System.out.println(start);
        /*

        ArrayList<AcademicClass> classList = new ArrayList<AcademicClass>();
        DataAccess db = new DataAccess();
        classList = db.getClassList();
        Gson gson = new Gson();
        JsonElement element = gson.toJsonTree(classList,new TypeToken<List<AcademicClass>>() {}.getType());
        JsonArray jsonArray = element.getAsJsonArray();
        response.setContentType("application/json");
        response.getWriter().print(jsonArray);
        */
    }

Inside the DataAccess class, I am doing all the database query and returning the result through its methods. I need HttpServletResponse object here, because I will return something to the jQuery code as a JSON object. But I am getting errors like this in console:

jquery.min.js:4 POST http://localhost:8080/Mytuition/PopulateClassSample 400 ()

And the initial prints in the controller method doesn't gets executed.

I tried declaring a separate model class (which has two fields- an ArrayList of Strings and a String) and used the @ModelAttribute annotation. But then I get exceptions like this:

java.lang.NumberFormatException: For input string: ""

Is there any way to do it without @ModelAttribute annotation? And why isn't this working even if I am using the @ModelAttribute annotation?

Note: I will definitely return something to the JSP. That part is commented out only for testing purpose.

Answer 1

Your controller method its wrong. If you are doing an Ajax call to the server and need to retrieve the JSON response from the server you only need to do 2 things.

1. Define your method in the controller with @ResponseBody. Then Spring will translate your returning object to JSON format

2. Return some object to the view. You are returning "void"

Should be something like this

@RequestMapping(value="/PopulateClassSample", method = RequestMethod.POST)
    @ResponseBody
    protected List<AcademicClass> SendResponse(@RequestParam("list") ArrayList<String> list, @RequestParam("start") String start) throws IOException
    {
        /** DO your staff here and return the AcademicClass list */
        System.out.println(start);
        DataAccess db = new DataAccess();
        return db.getClassList();
    }