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

      20200528221013

      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
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原文地址：https://www.cnblogs.com/liyihua/p/14477501.html