It seems like there is no good solution of this problem - logic of form login is very container-specific. I made a code that emulates user's sequence of actions by using jakarta commons HttpClient:

public class UserLogin {
 public static int PORT = 8080;

 public static void formLogin(HttpServletRequest request, String target,
 String username, String password) throws IOException {

 HttpClient client = new HttpClient();
 client.getState().setCookiePolicy(CookiePolicy.COMPATIBILITY);
 client.getHostConfiguration().setHost("localhost", PORT, "http");
 // copy cookies from request to pass correct jsessionid
 for(int i = 0; i < request.getCookies().length; i++) {
 client.getState().addCookie(new Cookie("localhost", request.getCookies().getName(),
 request.getCookies().getValue(), "/",
 request.getCookies().getMaxAge(),
 request.getCookies().getSecure()));
 }

 GetMethod m1 = new GetMethod(request.getContextPath() + target);
 m1.setFollowRedirects(true);
 client.executeMethod(m1);

 PostMethod m = new PostMethod(request.getContextPath() + "/j_security_check");
 NameValuePair userid = new NameValuePair("j_username", username);
 NameValuePair pwd = new NameValuePair("j_password", password);
 m.setRequestBody(new NameValuePair[] {userid, pwd});

 client.executeMethod(m);
 m.releaseConnection();
 }
}