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Uploading and downloading of files

2022-07-27 18:50:00 【Fantasia of the cat】

File upload and download , Is a very common function . In a lot of systems , Or file upload and download are often used in software . such as ：QQ Head portrait , Upload... Is used . The mailbox also has the function of uploading and downloading attachments . OA Upload of approved attachment materials in the system

File upload Introduction ：

1. Want to have one form label ,method=post request

2.form Labeled encType Property value must be multipart / form-data value

3. stay form Use in Tags input type=file Add uploaded files

4. Write server code （Servlet Program ） receive , Processing uploaded data .

encType=multipart/form-data Represents submitted data , With multiple segments （ Each form item has a data segment ） In the form of splicing , It's then sent to the server as a binary stream .

1.2 commons-fileupload.jar Commonly used API explain

1.3 fileupload Use of class library ：

Form to upload file ：

<body>
<form action="http://localhost:8080/09_EL_JSTL/uploadServlet" method="post" enctype="multipart/form-data">
     user name ：<input type="text" name="username" /><br>
     Head portrait ：<input type="file" name="photo" /> <br>
    <input type="submit" value=" Upload ">
</form>

Code for parsing uploaded data

 */
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    // Judge whether the uploaded data is multi segment data ( It's just multi segment data , It's the files that are uploaded )
    if(ServletFileUpload.isMultipartContent(req)){
        // Create a tool class for parsing uploaded data 
        FileItemFactory fileItemFactory = new DiskFileItemFactory();
        // Create tool classes for parsing data 
        ServletFileUpload servletFileUpload = new ServletFileUpload(fileItemFactory);

        try {
            // Parsing uploaded data , Get every form item FileItem
            List<FileItem> list = servletFileUpload.parseRequest(req);

            // Circular judgement , Every form item , Is a normal cycle , Or uploaded files 
            for(FileItem fileItem : list){
                if(fileItem.isFormField()){
                    // Common form items 
                    System.out.println(" Of the form item name Property value ：" + fileItem.getFieldName());
                    // Parameters UTF-8 Solve the mess 
                    System.out.println(" Of the form item value Property value ：" + fileItem.getString("UTF-8") );
                }else{
                    // Upload to file 
                    System.out.println(" Of the form item name Property value ：" + fileItem.getFieldName());
                    System.out.println(" Uploaded file name ：" + fileItem.getName());
                    fileItem.write(new File("E://" + fileItem.getName()));
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }

    }

Download files ：

Download commonly used API explain ：

response.setHeader("Content-Disposition", "attachment; fileName=1.jpg");

This response header tells the browser . This is what you need to download . and attachment Means attachment , That is, a file downloaded .fileName= Back , Indicates the name of the downloaded file .

Complete the above two steps , There is no problem downloading files . But if the file we want to download is Chinese . You'll find that , The download cannot display the correct Chinese name correctly .

The reason is that in the response header , Cannot contain Chinese characters , Can only contain ASCII code .

package com.atguigu.servlet;

import org.apache.commons.io.IOUtils;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URLEncoder;

/**
 * @Description:
 * @Author : Jerry
 * @create : 2022-03-17 21:04
 */
public class Download extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //1. Get the name of the file to download 
        String downloadFileName = "d.jpg";
        //2. Read the contents of the file to be downloaded  ( adopt ServletContext Object can read )
        ServletContext servletContext = getServletContext();
            //  Get the file type to download 
        String mimeType = servletContext.getMimeType("/file/" + downloadFileName);
        System.out.println(" Type of file downloaded " + mimeType);
        //4. Before returning , The response header tells the client the type of data returned 
        resp.setContentType(mimeType);
        //5. Also tell the client that the data received is for download （ Or use the response header ）
            //Content-Disposition Response head , How to handle the received data 
            //attachment  Means attachment , It means to download and use 
            //filename=  Indicates the file name of the specified download 
            // url Coding is the conversion of Chinese characters into %xx%xx The format of 
        resp.setHeader("Content-Disposition","attachment;filename = " + URLEncoder.encode(" China .jpg","UTF-8"));


            //  /  The slash is parsed by the server to indicate that the address is http://ip:prot/ project name /   mapping   To code web Catalog 
        InputStream resourceAsStream = servletContext.getResourceAsStream("/file/" + downloadFileName);
            // Get the output stream of the response 
        OutputStream outputStream = resp.getOutputStream();
        //3. Send the downloaded file back to the client 
            // Read all the data in the input stream , Copy to the output stream , Output to the client 
        IOUtils.copy(resourceAsStream,outputStream);


    }
}

Attachment Chinese name garbled problem solution ：

Scheme 1 ：URLEncoder solve IE And Google browser Attachment Chinese name question .

BASE64 Encoding and decoding operations ：

Support all browser operations ：

Because it uses BASE64 Restore the Chinese characters in the response by encoding and decoding . So we need to use BASE64Encoder Class for encoding operations .

So how do we solve the above two different encoding and decoding methods . We just need to judge the request header User-Agent The browser information carried by the request header can determine what browser it is .