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;
    }
}

public class UserDAO {
    public static final String DRIVER = "com.mysql.cj.jdbc.Driver";
    public static final String DBURL = "jdbc:mysql://localhost:3306/mydb?serverTimezone=UTC";//在这里将3306改为自己的数据库端口号
    public static final String DBUSER = "root";//在这里更改为自己数据库的用户名
    public static final String DBPASS = "123456";//在这里更改为自己数据库的密码
    private Connection conn = null;
    private PreparedStatement pStat = null;
    private ResultSet rs = null;
    public Connection getConnectionn() {
        try {
            Class.forName(DRIVER);
            return DriverManager.getConnection(DBURL, DBUSER, DBPASS);
        } catch (Exception e) {
            return null;
        }
    }
    public void close() {
        try {
            if (rs != null)
                rs.close();
            if (pStat != null)
                pStat.close();
            if (conn != null)
                conn.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    } // end close
    public boolean isUsernameExists(String username) {
        conn = getConnectionn();
        try {
            pStat = conn.prepareStatement("select * from users where username=?");
            pStat.setString(1, username);
            rs = pStat.executeQuery();
            if (rs.next())
                return true;
            else
                return false;
        } catch (Exception e) {
            return false;
        } finally {
            close();
        }
    } // end isUsernameExists
    public boolean addUser(User user) {
        conn = getConnectionn();
        try {
            pStat = conn.prepareStatement("insert into users values(null,?,?)");
            pStat.setString(1, user.getUsername());
            pStat.setString(2, user.getPassword());
            int cnt = pStat.executeUpdate();
            if (cnt > 0)
                return true;
            else
                return false;
        } catch (Exception e) {
            return false;
        } finally {
            close();
        }
    } // end add
    public boolean findUser(String username, String password) {
        conn = getConnectionn();
        try {
            pStat = conn.prepareStatement("select * from users where username=? and password=?");
            pStat.setString(1, username);
            pStat.setString(2, password);
            rs = pStat.executeQuery();
            if (rs.next())
                return true;
            else
                return false;
        } catch (Exception e) {
            return false;
        } finally {
            close();
        }
    } // end findUse
}//end class

// TODO Auto-generated method stub
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        UserDAO userdao = new UserDAO();
        boolean flag = userdao.findUser(username, password);
        if (flag) {
            request.getSession().setAttribute("username", username);
            response.sendRedirect("welcome.jsp");
        } else {
            request.getSession().setAttribute("err", "用户名或密码不正确！");
            response.sendRedirect("login.jsp");
        }

// TODO Auto-generated method stub
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        User user = new User();
        user.setUsername(username);
        user.setPassword(password);
        UserDAO userdao = new UserDAO();
        if (userdao.isUsernameExists(username)) {
            request.getSession().setAttribute("err", "用户名已存在");
            response.sendRedirect("register.jsp");
        } else {
            boolean flag = userdao.addUser(user);
            if (flag) {
                request.getSession().setAttribute("username", username);
                response.sendRedirect("welcome.jsp");
            } else {
                request.getSession().setAttribute("err", "注册失败");
                response.sendRedirect("register.jsp");
            }
        }

<%
String errMsg=(String)session.getAttribute("err"); 
if( errMsg!=null ) { %>
<div style="color:red;"><%=errMsg %></div>
<% session.removeAttribute("err");
} %>
<form action="LoginServlet"> 
用户名<input type="text" name="username"><br/>
密码<input type="password" name="password"><br/>
<input type="submit" value="登录">
<input type="reset" value="重填">
</form>

<%
String errMsg=(String)session.getAttribute("err"); 
if( errMsg!=null ) { %>
<div style="color:red;"><%=errMsg %></div>
<% session.removeAttribute("err");
}%>
<form action="RegServlet">
用户名<input type="text" name="username"><br/>
密码<input type="password" name="password"><br/>
<input type="submit" value="注册">
<input type="reset" value="重填">
</form>

<%
String username=(String)session.getAttribute("username");
if(username!=null&&!username.isEmpty()) { %>
你已经登录，欢迎<%=session.getAttribute("username") %>
<% } 
else{
out.print("你还没有登录，3秒后自动跳往登录页面");
response.setHeader("refresh", "3;url=login.jsp");
} %>