<form action="LoginServlet" method="post">
   用户名:<input type="text" name="userName"/>
   密码：<input type="password" name="password"/>
   <input type="submit" value="提交"/>
  </form>

public class User {
 private String userName;
 private String password;
 public String getUserName() {
  return userName;
 }
 public void setUserName(String userName) {
  this.userName = userName;
 }
 public String getPassword() {
  return password;
 }
 public void setPassword(String password) {
  this.password = password;
 }
}

import java.io.IOException;

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

import com.test.dao.UserDao;
//创建时为Servlet而不是Class，需要在web.xml中进行配置，配置的代码Myeclipse将自动生成
public class LoginServlet extends HttpServlet {
 //创建UserDao的对象，以便于查询数据库
 UserDao userDao=new UserDao();
 //以下doGet方法和doPost方法分别对应form表单中的method="get"和method="post"
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
 }
 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  //利用getParameter方法获取到前台文本框中输入的值，其中括号内的内容为<input/>标签中的name属性
  String userName=request.getParameter("userName");
  String password=request.getParameter("password");
  //调用UserDao中的getSelect方法并获取到返回值
  boolean flag=userDao.getSelect(userName, password);
  //若用户名和密码存在则转发到index.jsp页面，否则重定向到error.jsp页面
  if (flag) {
   request.getRequestDispatcher("index.jsp").forward(request, response);
  }
  else
   response.sendRedirect("error.jsp");
 }

}

package com.test.dao;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class UserDao {
 //连接数据库的代码
 public Connection getCon() {
  //数据库连接名称
  String username="root";
  //数据库连接密码
  String password="";
  String driver="com.mysql.jdbc.Driver";
  //其中test为数据库名称
  String url="jdbc:mysql://localhost:3306/test";
  Connection conn=null;
  try{
   Class.forName(driver);
   conn=(Connection) DriverManager.getConnection(url,username,password);
  }catch(Exception e){
   e.printStackTrace();
  }
  return conn;
 }
 //进行查询的方法，若含有满足条件的数据则返回true
 public boolean getSelect(String userName,String password) { 
  boolean flag=false;
   String sql = "select * from user where userName='"+userName+"' and password='"+password+"'"; 
   Connection conn = getCon();
   PreparedStatement pst = null;
   try {
    pst = (PreparedStatement) conn.prepareStatement(sql);
    ResultSet rs = pst.executeQuery();
    if (rs.next()) {
     flag=true;
    }
   } catch (Exception e) {
   }
   return flag;
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
 <servlet>
 <servlet-name>LoginServlet</servlet-name>
 <servlet-class>com.test.servlet.LoginServlet</servlet-class>
 </servlet>

 <servlet-mapping>
 <servlet-name>LoginServlet</servlet-name>
 <url-pattern>/LoginServlet</url-pattern>
 </servlet-mapping>
</web-app>