JSP form automatically submits

Question

The problem is that the JSP page automatically submits and transfers me to the action servlet. How do I make it wait until the user makes a selection in the dropdown option menu?

        <form name="mainForm" action="psm.srv" method="POST" onSubmit="return setParam(event)">
            <select id="main" onchange="document.mainForm.submit()">
                <option value="null">Choose chart</option>
                <option value="pays">PAYS</option>
                <option value="expenses">EXPENSES</option>
        </select></form>

<script type="text/javascript">
   function setParam(event){
        event.preventDefault();
        var select = document.getElementById("main");
        var value = select.options[select.selectedIndex].value;
        document.mainForm.action = "psm.srv?optionSelected=" + value ;
        document.mainForm.submit();
   }
</script>

Answer 1

The problem is this line:

onchange="document.mainForm.submit()"

What that means is that whenever the select input changes, the form will submit. You don't have to manually set this value. Add a 'name' attribute to the select field and just get the parameter in your servlet like you normally do. The value gets set automatically. You need a submit button though.

  <form name="mainForm" action="psm.srv" method="POST">

            <select name="selectField">
                <option value="null">Choose chart</option>
                <option value="pays">PAYS</option>
                <option value="expenses">EXPENSES</option>
             </select>

             <input type="submit" value="Submit"/>

   </form>

In your servlet you simply do:

String fieldValue = request.getParameter("selectField");

Answer 2

Remove on submit and onchange method and You can do simply by Jquery :

$(document).on('change', '#main', function () {

    var selectedValue=$(this).val();
    var url = "psm.srv?optionSelected=" + selectedValue;
    window.location = url;

    });

This will get your current value and take to the action url.