I make a filter,in this filter,I check a request parameter,if this parameter equals a assigned name,I put a flag into a session,like follows:
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain) throws IOException,ServletException
 String name=request.getParameter("name");
 .....
 if(name.compareTo("John")==0){
  session.setAttribute("isLogin","true");
 }
 ...
}

But when I run above program,I will get a error,because session is not defined. I know HttpSession is get by HttpServletRequest.getSession(),
but the public void doFilter is only provide ServletRequest,not HttpServletRequest! I want to know whether there is a method which I can get HttpSession in Filter?

Any idea will be appreciated!

What you need to do is casting it to HttpServletRequest as following:

String name = request.getParameter( "name" );
if ( name.compareTo( "John" ) )
{
   if ( request instanceof HttpServletRequest )
   {
      httpRequest = (HttpServletRequest) request; // doing casting here
      httpRequest.setAttribute( "isLogin", "true" );
   }
}