This tip shows how to check if a file was modified on the server!

 URL u =null;
long timestamp = 0;
getImage(getDocumentBase(), "test.gif");

try {

     u = new URL(getDocumentBase(), "test.gif");
     URLConnection uc = u.openConnection();

     ** use timestamp has a reference, re-open an URLConnection
     ** to the same file to check if the timestamp is different
     ** with the getLastModified() method.
     timestamp = uc.getLastModified();
catch (Exception e) {