虽然登陆界面很简单，但是自己老是忘记，所以写下，以后看的时候能够比较清楚的理解。
1.写jsp页面
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<form action="${pageContext.request.contextPath}/LoginServlet" method="post" class="" role="form">
username<input type="username" name="username" class="" id="username" placeholder="请输入用户名...">
password<input type="password" name="password" class="" id="password" placeholder="请输入密码...">
<span style="color:red">${msg }</span>
<input type="submit">
</form>
</body>
</html>说明：（1）action后面跟着的就是这个页面要验证的servlet类的位置
（2）name=“username” name=“password”相当于一个标识在servlet中就是通过name值来对数据进行操作的
（3）${msg }表示获取内置对象中赋值给msg变量的值
2.写对应的servlet，这里的servlet的名字与login.jsp中action后面跟着的名字必须一致。
package demo.web.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LoginServlet extends HttpServlet {
static final String driverClassName = "com.mysql.jdbc.Driver";
static final String url = "jdbc:mysql://localhost:3306/test?characterEncoding=utf-8";
static final String mysqlUsername = "root";
static final String mysqlPassword = "slzslz";
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
Connection conn = null;
PreparedStatement pstmt = null;
ResultSet rs = null;
try {
Class.forName(driverClassName);
conn = DriverManager.getConnection(url, mysqlUsername, mysqlPassword);
String username = request.getParameter("username");
String password = request.getParameter("password");
String sql = "SELECT * FROM sign WHERE username=? and password=?";
pstmt = conn.prepareStatement(sql);
pstmt.setString(1, username);
pstmt.setString(2, password);
rs = pstmt.executeQuery();
System.out.println(username +", " + password);
if(rs.next()) {
request.setAttribute("username", rs.getString("username"));
request.getSession().setAttribute("user_name", username);
/*System.out.println(rs.getString("username"));*/
request.setAttribute("msg", "成功登陆！");
request.getRequestDispatcher("/success.jsp").forward(request, response);
} else {
request.setAttribute("msg", "用户名或密码不正确！");
request.getRequestDispatcher("/login.jsp").forward(request,
response);
}
} catch(Exception e) {
throw new RuntimeException(e);
} finally {
try {
if(conn != null) conn.close();
if(pstmt != null) pstmt.close();
if(conn != null) conn.close();
} catch(Exception e) {
throw new RuntimeException(e);
}
}
}
}说明：（1）首先获取数据库的连接
（2）request.setAttribute("msg", "成功登陆！")；意思就是把成功登陆这句话当作一个变量赋值给内置对象msg，这个内置对象可以通过jsp页面直接调用。
（3）在（2）的代码后面必须跟上request.getRequestDispatcher("/success.jsp").forward(request, response);表明我们所设置的内置对象值传给哪个jsp页面。
3.成功登陆跳转页面success.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
${msg }
</body>
</html>4.配置文件web.xml
<servlet>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>demo.web.servlet.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/LoginServlet</url-pattern>
</servlet-mapping>说明：（1）切记这里的<url-pattern>的值必须要与login.jsp页面的action后面跟的值的地址一致，否则一直会有报错的。
（2）<servlet-class>后面跟的是包名+类名
5.页面效果展示
用户名或者密码错误，会提示用户名或者密码错误
正确登陆会跳转到success.jsp
Tips:这里只需要一个架包 mysql-connector-java-5.1.7-bin.jar即可。