Session Values on the JSP Pages Comes Only After Refreshing the Page and Seen null Until Refreshing it

Question

I am developing a web project with spring boot. I am using JSP on the frontend. I save some calculations I made in my service to session and then I show them in my JSP file with getAttribute() method. My problem is this: When I open my JSP page for the first time, these values ​​freeze to null. When I refresh the page, the values ​​appear as they should be, but it always takes time for the values ​​to arrive, and I have to manually refresh the page in order for these values ​​to appear.

I don't think there is a way to zero this wait time and a way to show session values ​​instantly.

Here is my example service java code:

@Override
    public void testService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        String testValue = "testValue";
        request.getSession().setAttribute("testValue", testValue);
}

Here is an example of JSP file:

<div id="testValue">
                    <%=request.getSession().getAttribute("testValue")%>
 </div>

The screenshots of problem statement:

• the value before refresing the page
• the value after refreshing the page

As seen in the images, I can see the value I want when I manually refresh the page. But we cannot expect the user to manually refresh the page. I don't know if there is a way to directly wait for these session values ​​without waiting. I tried to write an ajax call that will update the relevant party when the session values ​​are set:

<script>
                    function updateSessionVariables() {
                        var url = "/diger/indexpage/searchnew.oim";
                        var xmlHttpRequest = new XMLHttpRequest();
                        xmlHttpRequest.onreadystatechange = function () {
                            if (xmlHttpRequest.readyState == 4) {
                                if (xmlHttpRequest.status == 200) {
                                    document.getElementById("testValue").innerHTML = xmlHttpRequest.responseText;
                                }
                            }
                        };
                    }
                    updateSessionVariables();
                </script>

But there was no change, I had to manually refresh the page again to see the values. How can I solve this problem? I want to show these values ​​to the user directly without waiting or at least without having to manually refresh the page.