servlet里的javascript语句

在登录页面取值 用户名 和 密码，当不填写时或填写错误时，为什么不alert，而是直接跳转到登录页面了呢？
servlet里：

Java code:
package com.FAnimation.admin.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.text.SimpleDateFormat;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.FAnimation.admin.DB.AdminPro;
import com.FAnimation.admin.animation.Admin;

public class processAdmin extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html");
PrintWriter out = response.getWriter();
request.setCharacterEncoding("gb2312");
String name = request.getParameter("adname");
String password = request.getParameter("password");
Date d = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yy-MM-dd HH:mm:ss");
String date = sdf.format(d);
String ip = request.getRemoteAddr();
String servername = request.getServerName();
int serverport = request.g