Because the browser will store previously browsed data in the browser's cache, the browser will sometimes retrieve the data directly from the cache.
This is because after we modify the program, the following code will display the old data, This problem can be fixed
<%
if(request.getProtocol().compareTo("HTTP/1.0")==0)
request.setHeader("Pragma","no-cache");
if(request.getProtocol().compareTo("HTTP/1.1")==0)
request.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires",0);
%>
http://blog.csdn.net/daryl715/archive/2007/03/01/1517644.aspx