Accessing a java HashMap from JSP page using JavaScript

Question

I have a controller class in Spring MVC, where i am returning a HaspMap as Model attribute -

@ModelAttribute("regPrefix")
public Map<String, String> getRegPrefixesOfDepartmentforGroup(final Model model, final HttpServletRequest request) {
    final Map<String, String> regPrefixOfDept = new HashMap<String, String>();
    regPrefixOfDept.put(regKey, regPrefix);
    return regPrefixOfDept;
}

Now in the corresponding JSP page, i am trying to access the Hashmap and store the key/value pairs of the Hasmap in a variable using JavaScript. I am trying like below but its not right. Can anyone suggest how to do that

<script>
        $("#deptIdSelection").change(function()
        {
            var obj = document.getElementById("deptIdSelection");
            var textItem = obj.options[obj.selectedIndex].text;
            alert(textItem);
            var mapObj = "${regPrefix}";
            for(var key in mapObj)
                alert(key +" ->"+ mapObj[key]);

         }
        );
</script>

Kirill Simonov · Accepted Answer · 2018-01-28 18:16:36Z

Try to access map values like this:

${regPrefix.key}

If you want to iterate through the map keys, it is not so easy: JSTL is executed on the server side and is rendered to a plain text - no JavaScript objects are created. The following line var mapObj = "${regPrefix}"; will be rendered to a string representation of HashMap, not to a JavaScript object.

To convert your map to a JavaScript object I suggest you to use one of the following methods:

1) Convert your map to JSON and pass it as a String. So when it is rendered to a JavaScript code, it will look like a regular JS object: var mapObj = {"key": "value", ...};. You can use Gson or any other JSON library to achieve this:

final Map<String, String> regPrefixOfDept = new HashMap<String, String>();
regPrefixOfDept.put(regKey, new Gson().toJson(regPrefixOfDept));

And then var mapObj = ${regPrefix}; (note that you need no quotes around because you want mapObj to be a JS object, not a string)

2) Iterate through your map using <c:forEach> to generate a JS object:

var mapObj = {
    <c:forEach items="${regPrefix}" var="item" varStatus="loop">
      ${item.key}: '${item.value}' ${not loop.last ? ',' : ''}
    </c:forEach>
};

In both cases you should be able to then call

for(var key in mapObj)
   alert(key +" ->"+ mapObj[key]);

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