Servlet:ServletContext对象:代表整个web应用，可以和程序的容器(服务器)来通信

1. ServletContext对象的获取

获取：
1. 通过request对象获取
request.getServletContext();
2. 通过HttpServlet获取
this.getServletContext();

package com.ServletContext;
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("/servletContextDemo1")
public class ServletContextDemo1 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.通过request对象获取
        ServletContext context1 = request.getServletContext();
        //2.通过HttpServlet获取
        ServletContext context2 = this.getServletContext();
        System.out.println(context1);//org.apache.catalina.core.ApplicationContextFacade@1f01a450        System.out.println(context2);//org.apache.catalina.core.ApplicationContextFacade@1f01a450
        System.out.println(context1==context2);//true
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}

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2. ServletContext对象的功能

功能1. 获取MIME类型：
*MIME类型:在互联网通信过程中定义的一种文件数据类型
*格式：大类型/小类型 text/html的扩展名是html或htm,
image/jpeg的扩展名是jpg
*获取：String getMimeType(String file)
代码1：

package com.ServletContext;
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("/servletContextDemo2")
public class ServletContextDemo2 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //(1)获取ServletContext对象：通过HttpServlet
        ServletContext context = this.getServletContext();
        //(2)定义文件名称
        String filename="a.jpg";
        //(3)获取MIME类型
        String mimeType=context.getMimeType(filename);
        System.out.println(mimeType);//image/jpeg 的扩展名是jpg
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}

---

功能2. 域对象：共享数据
1.setAttribute(String name,Object value)
2.getAttribute(String name)
3.removeAttribute(String name)
代码2:servletContextDemo3和servletContextDemo4共享

package com.ServletContext;
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("/servletContextDemo3")
public class ServletContextDemo3 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //(1)通过HttpServlet获取ServletContext对象
        ServletContext context = this.getServletContext();
        //设置数据
        context.setAttribute("msg","haha");
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}

package com.ServletContext;
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;
/**
 * 与ServletContextDemo3共享数据
 * */
@WebServlet("/servletContextDemo4")
public class ServletContextDemo4 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //获取共享数据
        Object msg=context.getAttribute("msg");
        System.out.println(msg);
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}

---

功能3.获取文件的真实(服务器)路径:
方法：String getRealPath(String path)

(1)String b = context.getRealPath("/b.txt");//web目录下资源访问System.out.println(b);
(2)String c = context.getRealPath("/WEB-INF/c.txt");//WEB-INF目录下的资源访问
System.out.println（c）;
(3)String a = context.getRealPath("/WEB-INF/classes/a.txt");//src目录下的资源访问
System.out.println(a);

代码3:

package com.ServletContext;
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.File;
import java.io.IOException;
@WebServlet("/servletContextDemo5")
public class ServletContextDemo5 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //通过HttpServlet获取
        ServletContext context = this.getServletContext();
        //获取文件的服务器路径
        String b= context.getRealPath("/b.txt");//web目录下资源访问
        System.out.println(b);
        //File file = new File(realPath);
        String c = context.getRealPath("/WEB-INF/c.txt");//WEB-INF目录下的资源访问
        System.out.println(c);
        String a = context.getRealPath("/WEB-INF/classes/a.txt");//src目录下的资源访问
        System.out.println(a);
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}
/**
 * 结果：E:\JAVA\JAVA1\IDEA程序\Servlet\out\artifacts\Servlet_war_exploded\b.txt
 * E:\JAVA\JAVA1\IDEA程序\Servlet\out\artifacts\Servlet_war_exploded\WEB-INF\c.txt
 * E:\JAVA\JAVA1\IDEA程序\Servlet\out\artifacts\Servlet_war_exploded\WEB-INF\classes\a.txt
 * */

---

3. 案例

1.文件下载需求：
(1). 页面显示超链接
(2). 点击超链接后弹出下载提示框
(3). 完成图片文件下载
2.分析：
(1). 超链接指向的资源如果能够被浏览器解析，则在浏览器中展示，如果不能解析，则弹出下载提示框。不满足需求
(2). 任何资源都必须弹出下载提示框
(3). 使用响应头设置资源的打开方式：
content-disposition:attachment;filename=xxx
3.步骤：
(1). 定义页面，编辑超链接href属性，指向Servlet，传递资源名称filename
(2). 定义Servlet

• 获取文件名称
• 使用字节输入流加载文件进内存
• 指定response的响应头： content-disposition:attachment;filename=xxx
• 将数据写出到response输出流
  4.问题：
  中文文件问题
  解决思路：(1). 获取客户端使用的浏览器版本信息
  (2). 根据不同的版本信息，设置filename的编码方式不同

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <a href="/day15/img/1.jpg">图片1</a>
    <a href="/day15/img/1.avi">视频</a>
    <hr>
    <a href="/day15/DownloadServlet?filename=九尾.jpg">图片1</a>
    <a href="/day15/DownloadServlet?filename=1.avi">视频</a>
</body>
</html>

package com.download;
import com.utils.DownLoadUtils;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;
@WebServlet("/DownloadServlet")
public class DownloadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取请求参数，文件名称
        String filename = request.getParameter("filename");
        //2.使用字节输入流加载文件进内存
        //2.1找到文件服务器路径
        ServletContext servletContext = this.getServletContext();
        String realPath = servletContext.getRealPath("/img/" + filename);
        //2.2用字节流关联
        FileInputStream fis = new FileInputStream(realPath);

        //3.设置response的响应头
        //3.1设置响应头类型：content-type
        String mimeType = servletContext.getMimeType(filename);//获取文件的mime类型
        response.setHeader("content-type",mimeType);
        
        //解决中文文件名问题
        //1.获取user-agent请求头、
        String agent = request.getHeader("user-agent");
        //2.使用工具类方法编码文件名即可
        filename = DownLoadUtils.getFileName(agent, filename);
        
        //3.2设置响应头打开方式:content-disposition
        response.setHeader("content-disposition","attachment;filename="+filename);
        //4.将输入流的数据写出到输出流中
        ServletOutputStream sos = response.getOutputStream();
        byte[] buff = new byte[1024 * 8];
        int len = 0;
        while((len = fis.read(buff)) != -1){
            sos.write(buff,0,len);
        }
        fis.close();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}

重要的DownLoadUtils包：各种浏览器

package com.utils;
import java.util.Base64;
import java.util.Base64.*;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
public class DownLoadUtils {
    public static String getFileName(String agent, String filename) throws UnsupportedEncodingException {
        if (agent.contains("MSIE")) {
            // IE浏览器
            filename = URLEncoder.encode(filename, "utf-8");
            filename = filename.replace("+", " ");
        } else if (agent.contains("Firefox")) {
            // 火狐浏览器
            Encoder base64Encoder = Base64.getEncoder();
            filename = "=?utf-8?B?" + base64Encoder.encode(filename.getBytes("utf-8")) + "?=";
        } else {
            // 其它浏览器
            filename = URLEncoder.encode(filename, "utf-8");
        }
        return filename;
    }
}

结果展示：