Servlet案例5：用户登录失败信息回显

登录失败信息回显不会的新的一个页面，而是显示在登录页面

一种方法是：

登录页面表单中每个字段后添加<span>标签写入失败信息，利用ajax技术

通过改变<span>标签的display:none属性添加失败信息

这里用的类似的方法：

数据库准备略，

注意写好对应的user类供BeanHandler使用

sevlet：

package login;

import java.io.IOException;
import java.sql.SQLException;

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

import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanHandler;

public class LoginServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");
        // 获得用户名密码
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        // 在数据库中查询
        User login = null;
        try {
            login = login(username, password);
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        if (login != null) {
            // 成功,跳转首页
            response.sendRedirect(request.getContextPath());
        } else {
            // 错误
            // 在request域中存入错误信息
            request.setAttribute("loginInfo", "用户名或密码错误");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
        }

    }

    public User login(String username, String password) throws SQLException {
        QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
        String sql = "select * from user where username=? and password=?";
        User user = runner.query(sql, new BeanHandler<User>(User.class), username, password);
        return user;

    }

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

抽取连接池类：

package login;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import javax.sql.DataSource;

import com.mchange.v2.c3p0.ComboPooledDataSource;

public class DataSourceUtils {

    private static DataSource dataSource = new ComboPooledDataSource();

    private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>();

    // 直接可以获取一个连接池
    public static DataSource getDataSource() {
        return dataSource;
    }

    // 获取连接对象
    public static Connection getConnection() throws SQLException {

        Connection con = tl.get();
        if (con == null) {
            con = dataSource.getConnection();
            tl.set(con);
        }
        return con;
    }

    // 开启事务
    public static void startTransaction() throws SQLException {
        Connection con = getConnection();
        if (con != null) {
            con.setAutoCommit(false);
        }
    }

    // 事务回滚
    public static void rollback() throws SQLException {
        Connection con = getConnection();
        if (con != null) {
            con.rollback();
        }
    }

    // 提交并且 关闭资源及从ThreadLocall中释放
    public static void commitAndRelease() throws SQLException {
        Connection con = getConnection();
        if (con != null) {
            con.commit(); // 事务提交
            con.close();// 关闭资源
            tl.remove();// 从线程绑定中移除
        }
    }

    // 关闭资源方法
    public static void closeConnection() throws SQLException {
        Connection con = getConnection();
        if (con != null) {
            con.close();
        }
    }

    public static void closeStatement(Statement st) throws SQLException {
        if (st != null) {
            st.close();
        }
    }

    public static void closeResultSet(ResultSet rs) throws SQLException {
        if (rs != null) {
            rs.close();
        }
    }

}

注意写好c3p0-config.xml、web.xml配置文件

在登录页面加上这一句代码

<div><%=request.getAttribute("loginInfo")==null?"":request.getAttribute("loginInfo")%></div>

即可实现简单的信息回显