ServletContext

ServletContext概述

ServletContext：是一个全局对象，上下文对象。
服务器为每一个应用(项目)都创建了一个ServletContext对象。
ServletContext是属于整个应用的，不局限于某个Servlet。
ServletContext的作用：
- 作为域对象存取数据，让Servlet共享
- 获得文件MIME类型(文件下载)
- 获得全局初始化参数
- 获取web资源路径，可以将Web资源转换成字节输入流

ServletContext的功能

作为域对象存取值

API
- getAttribute(String name)：向ServletContext对象的map取数据
- setAttribute(String name, Object object)：从ServletContext对象的map中添加数据
- removeAttribute(String name)：根据name去移除数据

案例代码

需求：在ServletDemo02中获取servletDemo01中的一个变量的值

ServletDemo01

package servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
  
@WebServlet("/demo01")
public class ServletDemo01 extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String name = "周杰棍";
        //1.获取ServletContext对象
        ServletContext servletContext = getServletContext();
        //2.往容器ServletContext中存值
        servletContext.setAttribute("name",name);
    }
}

ServletDemo02

package servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
  
@WebServlet("/demo02")
public class ServletDemo02 extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. 获取那个容器ServletContext
        ServletContext servletContext = getServletContext();
        //2. 从容器ServletContext中获取数据
        String name = (String) servletContext.getAttribute("name");
        System.out.println("在ServletDemo02中获取的name是:" + name);
    }
}

发布项目，浏览器跳转到welcome页面，此时我们访问/demo01路径，此时ServletDemo01往容器ServletContext中存值，再访问/demo02路径，可以看到控制台输出：在ServletDemo02中获取的name是:周杰棍

获得文件mime-type

API：getMimeType(String file)

案例代码

需求：根据文件名获得文件的mime类型

ServletDemo03

package servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
  
@WebServlet("/demo03")
public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //根据文件名获得文件的mime类型
        // 1.获得ServletContext
        // 2.调用getMimeType()方法
        String file01 = "a.mp3";
        String file02 = "b.png";
        String mimeType01 = getServletContext().getMimeType(file01);
        String mimeType02 = getServletContext().getMimeType(file02);
        String value = getServletContext().getInitParameter("bkey");
        response.getWriter().print("ServletDemo03...:file01:"+mimeType01+";file02:"+mimeType02+";bkey:"+value+";");
    }
}

发布项目，浏览器跳转到welcome页面，此时我们访问/demo03路径，可以看到页面输出：ServletDemo03...:file01:audio/mpeg;file02:image/png;bkey:null;

获得全局初始化参数

API
- String getInitParameter(String name)：根据配置文件中的key得到value

案例代码

需求：通过ServletContext来获得全局的初始化参数

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
    <context-param>
        <param-name>bkey</param-name>
        <param-value>bbb</param-value>
    </context-param>
</web-app>

ServletDemo04

package servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
  
@WebServlet("/demo04")
public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String value = getServletContext().getInitParameter("bkey");
        System.out.println("value=" + value);
    }
}

发布项目，浏览器跳转到welcome页面，此时我们访问/demo04路径，可以看到控制台输出：value=bbb

获取web资源路径

API
- String getRealPath(String path)：根据资源名称得到资源的绝对路径
- getResourceAsStream(String path)：返回制定路径文件的流
注意：filepath直接从项目的根目录开始写

案例代码

需求：使用ServletContext获取web里面的资源的真实路径

ServletDemo05

package servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
  
/**
 * 使用ServletContext获取web里面的资源文件的真实路径
 * 目标: 使用字节输入流，读取test.jpg这张图片
 * 在web项目中，将文件转换成流，有两种方式
 * 	1.如果文件在resources里面，使用类加载器
 *  2.如果文件在web里面，使用ServletContext
 */
@WebServlet("/demo05")
public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //使用ServletContext可以获取web里面的资源的真实路径
        ServletContext servletContext = getServletContext();
        InputStream is = servletContext.getResourceAsStream("test.jpg");
        System.out.println(is);
    }
}

发布项目，浏览器跳转到welcome页面，此时我们访问/demo05路径，可以看到控制台输出：java.io.ByteArrayInputStream@7ce7f87d

案例：统计网站被访问的总次数

需求：在页面中显示网站被访问的总次数

思路分析

代码实现

CountServlet

package servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
  
@WebServlet("/count")
public class CountServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. 获取容器中计数器原本的次数
        ServletContext servletContext = getServletContext();
        Object count = servletContext.getAttribute("count");
        //判断ServletContext中是否有计数器
        if (count == null) {
            //说明当前是第一次访问
            //就往ServletContext中添加一个计数器
            servletContext.setAttribute("count",1);
        }else {
            //说明我不是第一次访问，那么就要对原来的计数器+1，再存进去
            int number = ((int) count) + 1;
            servletContext.setAttribute("count",number);
        }
        //2. 向客户端响应WelCome
        response.getWriter().write("WelCome!!!");
    }
}

ShowServlet

package com.itheima.servlet;
  
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
  
@WebServlet("/show")
public class ShowServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
  
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //从容器中获取计数器的值
        ServletContext servletContext = getServletContext();
        int count = (int) servletContext.getAttribute("count");
        //将count响应给客户端
        response.getWriter().print("access count is:"+count);
    }
}

发布项目，浏览器跳转到welcome页面，此时我们访问/count路径五次，然后访问/show路径，可以看到页面输出：access count is:5

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