记账本开发记录——第十三天（2020.1.31）

今天主要是学习了如何使用servletcontext来进行一个记录网站登录人数的案例。在实现这个案例之前，首先要学习相关的知识。

ServletContext:在服务器启动的时候，为每个web应用创建一个单独的ServletContext对象，我们可以使用这个对象存储数据。

那么我们实现这个案例的思路是如何呢？我们可以在登录的Servlet中初始一个变量count为0，然后将其存到ServletContext中，在进入页面时就读取ServletContext的内容。下面是相关代码：

package com.itheima.login;

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

import javax.servlet.ServletContext;
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;

import com.itheima.domain.User;
import com.itheima.utils.DataSourceUtils;

public class LoginServlet extends HttpServlet {
    
    @Override
    public void init() throws ServletException {
        //在Seveltcontext域中存一个数据count
        int count = 0;
        this.getServletContext().setAttribute("count", count);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        
        
        //username=zhangsan&password=123
        
        //1、获得用户名和密码
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        
        //2、从数据库中验证该用户名和密码是否正确
        QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
        String sql = "select * from user where username=? and password=?";
        User user = null;
        try {
            user = runner.query(sql, new BeanHandler<User>(User.class), username,password);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        
        //3、根据返回的结果给用户不同显示信息
        if(user!=null){
            //从servletcontext中取出count进行++运算
            ServletContext context = this.getServletContext();
            Integer count = (Integer) context.getAttribute("count");
            count++;
            //用户登录成功
            response.getWriter().write(user.toString()+"---you are success login person ："+count);
            context.setAttribute("count", count);
        }else{
            //用户登录失败
            response.getWriter().write("sorry your username or password is wrong");
        }
        
    }

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