var formData= $('form[name="mainFrm"]').serialize();
$.ajax({
   type : 'POST',
   url : 'action1.do',
   data : formData+ "&param1=123456&param2=654321",
   async: false,
   contentType: 'application/x-www-form-urlencoded;charset=UTF-8'
});

String paramForm = "параметр_формы";
String[] attribForm = ( String[] ) request.getParameterMap().get(paramForm);

request.getParameterMap()

....
public void doFilter(final ServletRequest request, final ServletResponse response, final FilterChain next)
					throws IOException, ServletException {
   request.setCharacterEncoding("Cp1251");
   next.doFilter(request, response);
}
...