ServletContext 对象的学习笔记

ServletContext对象概述

ServletContext用来存放全局变量，每个Java虚拟机每个Web项目只有一个ServletContext，这个ServletContext是由Web服务器创建的，来保证它的唯一性。由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象通讯。ServletContext对象通常也被称之为Context域对象。

• 获取方式

  • 通过request对象获取

    request.getServletContext();
    

  • 通过HttpServlet获取

    this.getServletContext();
    

    
    

获取ServletContext对象

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 ServletContextDemo01 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 通过 Request 对象获取 ServletContext 对象
        ServletContext requestServletContext = request.getServletContext();
        // 通过 HttpServlet 获取 ServletContext 对象
        ServletContext servletContext = this.getServletContext();
        System.out.println(requestServletContext);
        System.out.println(servletContext);
        System.out.println(requestServletContext == servletContext);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

启动服务器，浏览器访问：http://localhost:8080/ServletContext_war_exploded/demo01

控制台输出：

org.apache.catalina.core.ApplicationContextFacade@86cb2b8
org.apache.catalina.core.ApplicationContextFacade@86cb2b8
true

ServletContext对象功能

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

     如：大类型/小类型 text/html image/jpeg

   • 获取方法：

     String getMimeType(String file) 
     

2. 域对象：共享数据

   setAttribute(String name,Object value)
   getAttribute(String name)
   removeAttribute(String name)
   

3. 获取文件的真实(服务器)路径

   String getRealPath(String path)
   

获取mime 类型

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 ServletContextDemo02 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 通过 HttpServlet 获取 ServletContext 对象
        ServletContext servletContext = this.getServletContext();
        // 定义文件名称
        String fileName01 = "a.png";
        String fileName02 = "b.jpeg";
        // 获取 MIME 类型
        String mimeType01 = servletContext.getMimeType(fileName01);
        System.out.println(mimeType01);
        String mimeType02 = servletContext.getMimeType(fileName02);
        System.out.println(mimeType02);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

启动服务器，浏览器访问：http://localhost:8080/ServletContext_war_exploded/demo02

控制台输出：

image/png
image/jpeg

域对象：共享数据

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 ServletContextDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 通过 HttpServlet 获取 ServletContext 对象
        ServletContext servletContext = this.getServletContext();
        // 设置数据
        servletContext.setAttribute("demo03's data", "我是demo03这里设置的数据！！！");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

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 ServletContextDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 通过 HttpServlet 获取 ServletContext 对象
        ServletContext servletContext = this.getServletContext();
        // 获取demo03中设置的数据
        System.out.println(servletContext.getAttribute("demo03's data"));
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

启动服务器，浏览器中访问：http://localhost:8080/ServletContext_war_exploded/demo03，目的是，访问demo03，设置数据。

然后，浏览器中访问：http://localhost:8080/ServletContext_war_exploded/demo04。控制台输出：

我是demo03这里设置的数据！！！

获取文件的真实(服务器)路径

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("/demo05")
public class ServletContextDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 通过 HttpServlet 获取 ServletContext 对象
        ServletContext servletContext = this.getServletContext();
        // 获取文件的服务器路径
        System.out.println(servletContext.getRealPath("/file01.txt"));
        System.out.println(servletContext.getRealPath("/WEB-INF/file02.txt"));
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

启动服务器，浏览器中访问：http://localhost:8080/ServletContext_war_exploded/demo05

控制台输出：

/Users/liyihua/IdeaProjects/ServletContext/out/artifacts/ServletContext_war_exploded/file01.txt
/Users/liyihua/IdeaProjects/ServletContext/out/artifacts/ServletContext_war_exploded/WEB-INF/file02.txt

案例

• 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

• 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

• 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
     1. 获取文件名称
     2. 使用字节输入流加载文件进内存
     3. 指定response的响应头： content-disposition:attachment;filename=xxx
     4. 将数据写出到response输出流
  • 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

• 实现：

  • java

    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 {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            // 获取用户请求参数（文件名称）
            String fileName = request.getParameter("filename");
            // 获取 ServletContext 对象
            ServletContext servletContext = request.getServletContext();
    
            // 根据文件名称，找到服务器中文件的路径
            String filePath = servletContext.getRealPath("/downloadFile/" + fileName);
            // 根据文件名称，获取文件类型
            String mimeType = servletContext.getMimeType(fileName);
    
            // 设置 response 的响应头 —————— 响应头类型：content-type
            response.setHeader("content-type", mimeType);
            // 响应头的打开方式：content-disposition
            response.setHeader("content-disposition", "attachment;filename=" + fileName);
    
            // 使用字节输入流加载文件进内存
            FileInputStream fileInputStream = new FileInputStream(filePath);
            // 将字节输入流的数据写出到输出流中
            ServletOutputStream responseOutputStream = response.getOutputStream();
            byte[] bytes = new byte[1024 * 8];
            int len = 0;
            while ((len = fileInputStream.read(bytes)) != -1) {
                responseOutputStream.write(bytes, 0, len);
            }
            fileInputStream.close();
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            this.doGet(request, response);
        }
    }
    

  • html

    <!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Title</title>
    </head>
    <body>
        <!-- 虚拟路径：/ResponseDownload_war_exploded -->
        <!-- /downloadServlet 为Servlet的访问路径 -->
        <!-- 资源名称：image06.png、view.avi -->
        <a href="/ResponseDownload_war_exploded/downloadServlet?filename=image06.png">下载图片</a>
        <a href="/ResponseDownload_war_exploded/downloadServlet?filename=view.avi">下载视频</a>
    
    </body>
    </html>
    

  • 启动服务器，浏览器中访问：http://localhost:8080/ResponseDownload_war_exploded/download.html

    

    点击下载图片，会下载： image06.png

    点击下载视频，会下载：view.avi

    
    

参考文献

1. ServletContext对象详解