如何用Java实现浏览器文件下载功能

￡神魔★判官ぃ 2023-09-30 09:08 21阅读 0赞

## 一、了解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. 概念 ###

ServletContext是Servlet中最大的一个接口，呈现了web应用的Servlet视图。服务器会为每一个工程创建一个对象，这个对象就是ServletContext对象。这个对象全局唯一，而且工程内部的所有servlet都共享这个对象。所以叫**全局应用程序共享对象**。

### 2. 获取 ###

#### 1. 通过request对象获取 ####

request.getServletContext();

#### 2. 通过HttpServlet获取 ####

this.getServletContext();

#### 3. 测试 ####

import javax.servlet.*;
    import javax.servlet.http.*;
    import javax.servlet.annotation.*;
    import java.io.IOException;
    
    @WebServlet(name = "servletContextDemo1", value = "/servletContextDemo1")
    public class servletContextDemo1 extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            ServletContext context1 = request.getServletContext();
            ServletContext context2 = this.getServletContext();               //httpServlet下的servletContext
            System.out.println(context1);
            System.out.println(context2);
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            this.doGet(request,response);
        }
    }

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16][]

### 3. 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 image/jpeg
 *  获取：String getMimeType(String file)

@WebServlet(name = "servletContextDemo2", value = "/servletContextDemo2")
    public class servletContextDemo2 extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            ServletContext context = this.getServletContext();               //httpServlet下的servletContext
            String filename = "a.jpg";
            String mimeType = context.getMimeType(filename);               //获取mime类型
            System.out.println(mimeType);                //mime类型为  image/jpeg
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            this.doGet(request,response);
        }
    }

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16 1][]

#### 2. 域对象：共享数据 ####

*  设置数据域：setAttribute(String name,Object value)

@WebServlet(name = "servletContextDemo3", value = "/servletContextDemo3")
    public class servletContextDemo3 extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            ServletContext context = this.getServletContext();               //httpServlet下的servletContext
            context.setAttribute("msg","ohmyga");            //设置共享数据,给整个服务器共享
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            this.doGet(request,response);
        }
    }

*  获取数据域：getAttribute(String name)

@WebServlet(name = "servletContextDemo4", value = "/servletContextDemo4")
    public class servletContextDemo4 extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            ServletContext context = this.getServletContext();               //httpServlet下的servletContext
            Object msg = context.getAttribute("msg");//设置共享数据
            System.out.println(msg);
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            this.doGet(request,response);
        }
    }

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16 2][]

## 二、如何获取文件真实路径 ##

> String getRealPath(String path)；

### 1. web目录下资源访问 ###

String a = context.getRealPath("/a.txt");
    System.out.println(a);

### 2. WEB-INF目录下的资源访问 ###

String b = context.getRealPath("/WEB-INF/b.txt");
    System.out.println(b);

### 3. src目录下的资源访问 ###

String c = context.getRealPath("/WEB-INF/classes/c.txt");
    System.out.println(c);

@WebServlet(name = "servletContextDemo5", value = "/servletContextDemo5")
    public class 获取文件真实路径 extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            ServletContext context = this.getServletContext();               //httpServlet下的servletContext
            String a = context.getRealPath("/WEB-INF/classes/a.txt");           //src目录下的a.txt
            System.out.println(a);
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            this.doGet(request,response);
        }
    }

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16 3][]

## 三、实现文件下载 ##

### 1. html文件 ###

#### 1. 编写一段简单的html代码，比如下载这个 1.jpg 文件 ####

<!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>下载</title>
    </head>
    <body>
    <a href="/Web/servletContextDemo6?filename=1.jpg">图片下载</a>
    </body>
    </html>

#### 2. 预览效果 ####

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16 4][]

### 2. web 服务器代码 ###

> 这里注意需要设置**response响应头的mime类型**和**响应头的打开方式**！！！
> 
> response.setHeader(String name, String value)；

import javax.servlet.*;
    import javax.servlet.http.*;
    import javax.servlet.annotation.*;
    import java.io.FileInputStream;
    import java.io.IOException;
    
    @WebServlet(name = "servletContextDemo6", value = "/servletContextDemo6")
    public class 文件下载 extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            String filename = request.getParameter("filename");           //获取请求参数
    
            //使用字节输入流加载文件进内存
            ServletContext servletContext = this.getServletContext();
            String realPath = servletContext.getRealPath("/WEB-INF/classes/servletContext/" + filename);  //找到文件的服务器路径
            FileInputStream fis = new FileInputStream(realPath);                    //使用字节输入流读取文件
    
            //设置response响应头
            String mimeType = servletContext.getMimeType(filename);                   //获取mime类型
            response.setHeader("content-type",mimeType);                              //设置响应头类型
            response.setHeader("content-disposition","attachment;filename=" + filename);    //设置响应头打开方式
    
            //将输入流的事件写出到输出事件
            ServletOutputStream sos = response.getOutputStream();
            byte[] buffer = new byte[1024 * 8];
            int length = 0;
            while((length = fis.read(buffer)) != -1){
                sos.write(buffer,0,length);
            }
            sos.close();
        }
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            this.doGet(request,response);
        }
    }

### 3. 预览效果 ###

点击下载

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16 5][]

文件就被默认（也可以用浏览器设置到其他地方）的下载到了这个位置

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5rap5Y2z5pivTnVsbA_size_20_color_FFFFFF_t_70_g_se_x_16 6][]

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