web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用

• 共享数据 : 我在这个Servlet中保存的数据，可以在另外一个servlet中拿到 ;

  
  servletContext
  

  放置数据类

package com.karl.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        this.getInitParameter();//初始化参数
//        this.getServletConfig();//servlet配置
//        this.getServletContext();//servlet上下文

        ServletContext context = this.getServletContext();
        String username = "karl";//数据
        context.setAttribute("username", username);//将一个数据保存到了servletContext中
    }
}

读取数据类

package com.karl.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = (String)context.getAttribute("username");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().println("名字:"+username);
    }
}

---

<!--可以配置一些web应用初始化参数-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//读取初始化参数
        ServletContext context = this.getServletContext();
        String url = (String)context.getInitParameter("url");
        resp.getWriter().println(url);
    }

---

转发和重定向

doGet(){
...
//        RequestDispatcher requestDispatcher = context.getRequestDispatcher("");//转发的路径
//        requestDispatcher.forward(req,resp);//实现转发
        context.getRequestDispatcher("").forward(req, resp);
...
}

转发(上)和重定向(下)

---

读取资源文件

poperties

classpath路径
读取资源文件

思路: 需要一个文件流;