Struts Calling Action on page load

Question 1

You need to call your action like this if you want to navigate to another page,

function callAction() {
    window.location = "navigation.do?parameter=showNavigation";
}

else if you want to call the same page (without loading another page), you need to use ajax like this. Here i used jQuery ajax

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js">
                                                                    </script>
<script type="text/javascript">
$(document).ready(function() {
    $.ajax({
        url : "navigation.do?parameter=showNavigation",
        type : "POST",
        success : function(data) {
            alert(data);
        }
    });
});
</script>

Hope this helps.

Question 2

@user3333294: You can also do in the following way:

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<html>
    <head>
        <script type="text/javascript">
            function submitPage(){
                document.forms[0].action = "navigation.do?parameter=showNavigation";
                document.forms[0].submit();
            }
        </script>
    </head>
    <body id="mainBody">
        <form method="post" onload="submitPage()">
            ...
        </form>
    </body>
</html>