<form action="LoginServlet" method="post">
  用戶名:<input type="text" name="userName"/>
  密碼：<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;
//創(chuàng)建時為Servlet而不是Class，需要在web.xml中進行配置，配置的代碼Myeclipse將自動生成
public class LoginServlet extends HttpServlet {
 //創(chuàng)建UserDao的對象，以便于查詢數(shù)據(jù)庫
 UserDao userDao=new UserDao();
 //以下doGet方法和doPost方法分別對應form表單中的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方法獲取到前臺文本框中輸入的值，其中括號內(nèi)的內(nèi)容為<input/>標簽中的name屬性
 String userName=request.getParameter("userName");
 String password=request.getParameter("password");
 //調用UserDao中的getSelect方法并獲取到返回值
 boolean flag=userDao.getSelect(userName, password);
 //若用戶名和密碼存在則轉發(fā)到index.jsp頁面，否則重定向到error.jsp頁面
 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 {
 //連接數(shù)據(jù)庫的代碼
 public Connection getCon() {
 //數(shù)據(jù)庫連接名稱
 String username="root";
 //數(shù)據(jù)庫連接密碼
 String password="";
 String driver="com.mysql.jdbc.Driver";
 //其中test為數(shù)據(jù)庫名稱
 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;
 }
 //進行查詢的方法，若含有滿足條件的數(shù)據(jù)則返回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>