ServletContext

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；

作用：

1）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

HelloServlet

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = "老尹";  // 数据
        context.setAttribute("username",username); // 将一个数据保存在ServletContext中
    }
}

getc

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        // 获得ServletContext中放置的uaername数据，强转为String
        String username = (String)context.getAttribute("username");
        resp.setContentType("text/html"); // 设置文本格式
        resp.setCharacterEncoding("utf-8");  // 设置字符格式
        resp.getWriter().print("name" + username);  // 输出信息到网页

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

2) 获取初始化参数

<context-param>
  <param-name>url</param-name>
  <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);

    }

3）请求转发

gp的代码（gp是ServletDemo03的缩写）

package com.kuang.servlet;

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

public class ServletDemo03 extends HelloServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}