ServletContext对象

ServletContext是什么？

ServletContext对象也就是Servlet上下文对象，它是用来管理我们所有的Servlet的

ServletContext对象的使用

1.共享数据

就是我们可以在一个Servlet中通过ServletContext对象存入数据，再通过另一个Servlet拿到数据

ServletContext servletContext = this.getServletContext();//获得上下文对象
servletContext.setAttribute("username","hhhhhhh");//往对象中设置信息，键值对
servletContext.getAttribute("username");//在对象中拿到数据

2.获得初始化参数

我们在项目的web.xml中设置初始化参数
  <context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql//localhost::3306/school</param-value>
  </context-param>

拿到参数
ServletContext servletContext = this.getServletContext();//获得上下文对象
servletContext.getInitParameter("url");

3.请求转发

这里我们注册了两个Servlet程序
<!-- 注册Servlet -->
  <servlet>
    <servlet-name>firstServlet</servlet-name><!--  注册一个Servlet程序名字为XXX  -->
    <servlet-class>com.mrlibo.Servlet.HelloServlet</servlet-class><!--  Servlet程序的路径  -->
  </servlet>
  <servlet>
    <servlet-name>TwoServlet</servlet-name><!--  注册一个Servlet程序名字为XXX  -->
    <servlet-class>com.mrlibo.Servlet.TwoServlet</servlet-class><!--  Servlet程序的路径  -->
  </servlet>

  <servlet-mapping>
    <servlet-name>firstServlet</servlet-name><!--  去找哪一个Servlet  -->
    <url-pattern>/firstServlet</url-pattern><!--  在web服务器上的请求地址  -->
  </servlet-mapping>

  <servlet-mapping>
    <servlet-name>TwoServlet</servlet-name><!--  去找哪一个Servlet  -->
    <url-pattern>/TwoServlet</url-pattern><!--  在web服务器上的请求地址  -->
  </servlet-mapping>

这里通过上下文对象转发请求

package com.mrlibo.Servlet;

import jakarta.servlet.ServletContext;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;
import java.io.PrintWriter;

public class TwoServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();//获得上下文对象
        servletContext.getRequestDispatcher("/firstServlet").forward(req,resp);
        //forward表示转发，并将请求对象和响应对象都传过去
    }

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

4.通过web浏览器拿到资源

我在resources文件夹下创建了一个db.properties文件

username=root
password=123456

这里要注意如果你在java文件夹下创建的话会出现导出不了的问题，解决方法需要在pom.xml中配置resources，参考https://blog.youkuaiyun.com/APCSZDDXM/article/details/122587940?spm=1001.2014.3001.5501

然后我们通过ServletContext对象去获取这个资源流

package com.mrlibo.Servlet;

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

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Properties;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties properties = new Properties();
        properties.load(is);

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter pw = resp.getWriter();
        pw.println(properties.getProperty("username"));
        pw.println(properties.getProperty("password"));
    }

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