servlets - Form filling validation in Java -

i check in servlet, works.

how validation of form filling? example, re-send user .jsp-file, if username registered?

sorry bad english.

public void doget(httpservletrequest request, httpservletresponse response)     throws servletexception, ioexception {      // fetch username sent in request     string username = request.getparameter("username");      // todo: verify if username taken in database      // based on results set value     request.setattribute("isusernametaken", "true");      requestdispatcher dispatcher = getservletcontext().getrequestdispatcher("/register.jsp");      dispatcher.forward(request, response);       }