Strange Behaviour java variable

Question

I have encountered a strange behaviour which i cannot explain. Here is my JSP pages code

<%@ page contentType="text/html; charset=iso-8859-1" language="java" import="java.sql.*,java.util.*" errorPage="" %>   
<%
String qid = request.getParameter("qid");
int qno;
int choice = 0;
if(qid==null)
qno = 1;
else
qno = Integer.parseInt(qid);
%>
<script>
    function f1()
    {
      var check;
    if(document.getElementById('r1').checked||document.getElementById('r2').checked||document.getElementById('r3').checked||document.getElementById('r4').checked)
    {   
        check = true;
        if(document.getElementById('r1').checked){
        <% choice = 1; %>
        }
        else if(document.getElementById('r2').checked){
        <% choice = 2; %>
        }
        else if(document.getElementById('r3').checked){
            <% choice = 3; %>
        }
        else {
          <% choice = 4; %>
        }
    }
    else{
        alert("Select an answer");
        check = false;
    }
     if(check){

    <%
    out.println("document.form1.action=\"starttest.jsp?qid="+qno+"&choice="+choice+"\"");
    %>
    document.form1.method="post";
    document.form1.submit();
    }
    }
</script>
<form id ="form1" name="form1" method="post" action="">
<%
try{
Connection conn=DriverManager.getConnection("jdbc:mysql://localhost/examination?" +"user=root&password=nawed");
PreparedStatement ps=conn.prepareStatement("select question_text from question_master where question_id=?");
ps.setInt(1,qno);
ResultSet rs=ps.executeQuery();
while(rs.next()){
out.println("1><B>"+rs.getString(1)+"</B><br/>");
}
ps=conn.prepareStatement("select choice_1,choice_2,choice_3,choice_4,answer from multichoice_question where ques_id=?");
ps.setInt(1,qno);
rs= ps.executeQuery();
while(rs.next()){
  session.setAttribute("pans",rs.getString(5));
%>
<input type="Radio" name="radio" id="r1"/><%=rs.getString(1)%><br/>
<input type="Radio" name="radio" id="r2"/><%=rs.getString(2)%><br/>
<input type="Radio" name="radio" id="r3"/><%=rs.getString(3)%><br/>
<input type="Radio" name="radio" id="r4"/><%=rs.getString(4)%><br/>
<input type="button" name="button" value="Submit" onclick="f1()"/>
<%
}
ps.close();
rs.close();
}catch(Exception e){}
%>
</form>
</body>
</html>

The problem is that i am getting choice value 4 everytime in query string but intrestingly problems get solved when i use javaScript variable instead like:

<script>
    function f1()
    {
      var check;
    if(document.getElementById('r1').checked||document.getElementById('r2').checked||document.getElementById('r3').checked||document.getElementById('r4').checked)
    {   var choice;
        check = true;
        if(document.getElementById('r1').checked){
         choice = 1;
        }
        else if(document.getElementById('r2').checked){
         choice = 2;
        }
        else if(document.getElementById('r3').checked){
             choice = 3;
        }
        else {
           choice = 4; 
        }
    }
    else{
        alert("Select an answer");
        check = false;
    }
     if(check){

    <%
    out.println("document.form1.action=\"starttest.jsp?qid="+qno+"&choice=\"+choice");
    %>
    document.form1.method="post";
    document.form1.submit();
    }
    }
</script>

I cant explain this Strange behaviour.It will be very helpful if anyone can explain this in simple way :)

Answer 1

JavaScript runs on the client side. Java runs on the application / web server.

Think of a JSP as a big Java method which outputs the client's document (HTML, JavaScript, CSS, etc.).

Therefore, in the below snippet:

    if(document.getElementById('r1').checked){
    <% choice = 1; %>
    }
    else if(document.getElementById('r2').checked){
    <% choice = 2; %>
    }
    else if(document.getElementById('r3').checked){
        <% choice = 3; %>
    }
    else {
      <% choice = 4; %>
    }

These 4 lines execute sequentially. You can look at the compiled JSP code to verify this.

choice = 1;
choice = 2;
choice = 3;
choice = 4;

Therefore, choice will end up being 4. The if statements are JavaScript, which the browser will execute and have nothing to do with the Java logic.