Why does getRealPath() return null when deployed with a .war file?

getRealPath() is returning the actual path in the local system, but returns null when deployed with a .war file.

<%@ page import="java.io.*" %>
<%@ page contentType="text/html;charset=ISO-8859-1" %> 
<%
int iLf = 10;
char cLf = (char)iLf;
String a= application.getResource("/");
//String myfile = application.getRealPath("/")+ "generate.xml";
//String myfile = request.getContextPath()+"generate.xml";
//String myfile = request.getRealPath("/")+"generate.xml";

out.println(myfile);    
File outputFile = new File(myfile);
outputFile.createNewFile();
FileWriter outfile = new FileWriter(outputFile);
outfile.write(" <?xml version='1.0' encoding='UTF-8'?> "+cLf);
outfile.write(" <playlist version='1' xmlns = 'http://xspf.org/ns/0/' > " +cLf);
outfile.write(" <title>My Band Rocks Your Socks</title> "+cLf); 
outfile.write("<trackList>"+cLf); 
%>
 <%! String[] sports; %>
 <%
    sports = request.getParameterValues("sports");

    out.println("<html><body><h1>hello</h1></body></html>");

    if (sports != null)
    { 
         for (int i = 0; i < sports.length; i++)
         { 
              // outfile.writeln (sports[i]); 
              String total=sports[i];
              String[] sa=total.split("[,]");
              // String[] sub=new String();
              outfile.write("<track>"+cLf);
              for (int j=0;j<sa.length;j++)
              {
                // outfile.writeln(sa[j]);
                // outfile.writeln("sa["+j+"]="+sa[j]);
                if( j == 0)
                {
                     outfile.write("<location>" + sa[0] +"</location>"+cLf); 
                }
                else if (j == 1)
                     {
                        outfile.write("<image>" + sa[1] +"</image>"+cLf); 
                     }
                     else if( j==2)
                          {