Native servlet - upload & download of files

File upload and download

1. File upload details

1. Want to have one form label ,method-post request （ because get Limited length ）

2. form Attributes of the tag encType Value must be multipart/form-data

   Indicates that the submitted data is multi ended （ Each form item has a data segment ） In the form of splicing , It's then sent to the server as a binary stream

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

4. Write server code (Servlet receive ), Accept and process uploaded data

Upload files http Request information ：

Request header ：Content-Type: multipart/form-data; boundary=------WebKitFormBoundaryCd3g75eOt35olUs7

analysis ：

1. Content-Type Indicates the type of data submitted
2. multipart/form-data Means to submit the server in the form of a stream
3. boundary The separator for each piece of data , value ：----WebKitFormBoundarylXiF4fEzpo9c8L4p It is randomly generated by the browser every time , It is the delimiter of each piece of data . In each paragraph The first line is the description of the form item , Then there is a blank line , Here are the submitted values .

Request body ：

------WebKitFormBoundaryCd3g75eOt35olUs7
Content-Disposition: form-data; name="username"

zhu
------WebKitFormBoundaryCd3g75eOt35olUs7
Content-Disposition: form-data; name="photo"; filename="head.jpg"
Content-Type: image/jpeg

 File information （ For a long time, we have omitted ）
------WebKitFormBoundaryCd3g75eOt35olUs7--

Because the client submits by stream , So we need to get by stream , You can't do that ： req.getParameter("username");

Proper use ：

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    System.out.println(" Upload successful ");

    ServletInputStream inputStream = req.getInputStream(); //  First get   Byte input stream 
    byte[] buffer = new byte[1024]; //  buffer 
    int readCount = 0;
    while ((readCount = inputStream.read(buffer)) != -1) {
     //  All the request bodies above are printed out 
        System.out.println(new String(buffer, 0, readCount));
    }
}

2. Upload files

This kind of file upload （ Commonly used ） There's a lot of Third party offers good API We have used , It can help us parse the data we receive .

for example ：commons-fileupload-1.2.1.jar （ Depend on commons-io-1.4.jar）

1. Import two jar package

2. analysis

Key classes ：

• ServletFileUpload class ： Used to parse the uploaded data

• FileItem class ： Every form item

//ServletFileUpload The method in 
//  Judge whether the currently uploaded data is in multi terminal format , It's not that it can't be resolved 
public boolean ServletFileUpload.isMultipartContent(HttpServletRequest req) 
//  Parsing uploaded data ,FileItem Represents each form item 
public List<FileItem> parseRequest(HttpServletRequest req)

//FileItem Medium method 
//  Determine whether the current form item is a common form item or a file upload type ,true It means ordinary 
public boolean isFormField()    
//  Get form item name Property value 
public String getFieldName()
//  Get the value of the current form item 
public String getString() //  Passable character set , Prevent confusion code , In limine req.setCharacterEncoding("UTF-8"); It's OK 
//  Get the uploaded file name 
public String getName()
//  Write the uploaded file   Parameters file The disk location pointed to 
public void write(File file)

Servlet Upload file example ：

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    req.setCharacterEncoding("UTF-8"); //  Prevent confusion code 
    resp.setContentType("text/html; charset=utf-8");
    
    String savePath = getServletContext().getRealPath("/WEB-INF/uploadFile"); // Saved path 
    //  First, judge whether the uploaded data is multi segment data 
    if (ServletFileUpload.isMultipartContent(req)) {
    
        FileItemFactory fileItemFactory = new DiskFileItemFactory(); //  establish FileItemFactory Factory implementation class ,
        //  Create a tool class for parsing uploaded data 
        ServletFileUpload servletFileUpload = new ServletFileUpload(fileItemFactory); 
        try {
    
            List<FileItem> list = servletFileUpload.parseRequest(req); //  analysis , Get every form item 
            for (FileItem fileItem : list) {
    
                if (fileItem.isFormField()) {
     //  Common form items 
                    System.out.print(" Of the form item name Property value ：" + fileItem.getFieldName());
                    System.out.println("  Of the form item value：" + fileItem.getString("UTF-8"));
                } else {
     //  file type 
                    System.out.print(" Of documents name Property value ：" + fileItem.getFieldName());
                    System.out.println("  Uploaded file name ：" + fileItem.getName());
                    //  It is usually saved to a directory that users cannot access directly  File.separator Is the system default path separator ,win Next is  / 
                    // ( The following is saved to the deployment directory , Saved to the server )
                    //  It can be used UUID Ensure the uniqueness of the file name , Prevent file overlay .
                    //  Prevent too many single directory files from affecting the reading and writing speed , You can also use a directory generation algorithm to distribute storage 
                    fileItem.write(new File(savePath + File.separator + fileItem.getName()));
                    //fileItem.delete(); // close resource 
                }
            }
        } catch (Exception e) {
    
            e.printStackTrace();
        }
    }
}

3. File download

client ->( Send a request to tell the server what file I want to download ) -> The server

What the server does ：

1. Get the name of the file to download

2. Read the contents of the file to be downloaded

3. Tell the client what the returned data type is through the response header （ It is the same as the type to be downloaded ）

4. Tell the client that the data received is for downloading （ Or use the response header to set ）

5. Send the downloaded file content back to the client for downloading

This can also be used commons-io-1.4.jar Of IOUtils class ：

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    req.setCharacterEncoding("UTF-8"); //  Prevent confusion code 
    resp.setCharacterEncoding("UTF-8");

    //1. Get the file name and path name to download , And pass ServletContext Read read file 
    String downloadFileName = "head.jpg"; //  We are dead here 
    ServletContext servletContext = getServletContext();
    String savePath = servletContext.getRealPath("/WEB-INF/upload"); // The directory where the previously uploaded files are saved 
    String downloadPath = savePath + File.separator + downloadFileName;

    //2. Tell the client what type to return 
    String downloadType = servletContext.getMimeType(downloadPath); // Get the type of file to download  ( This is image/jpeg)
    resp.setContentType(downloadType); // ( Just like the type to download )
    //3. Tell the client that the data received is for download , Not directly displayed on the page 
    // Content-Disposition How to handle the received data ,attachment Indicates that the attachment is downloaded and used ,filename Indicates the name of the downloaded file 
    // filename Names can be different from local names , When there is Chinese, it will be garbled , because http Chinese is not supported when setting the protocol , Need to carry out url code 
    /resp.setHeader("Content-Disposition", "attachment;filename=" + downloadFileName);
    resp.setHeader("Content-Disposition", 
                   "attachment;filename=" + URLEncoder.encode(downloadFileName, "UTF-8"));

    InputStream resourceAsStream = servletContext.getResourceAsStream(downloadPath); 
    // getResourceAsStream()  Incoming file path , Read the file !!!!!!!!!!!!!

    // 4.commons-io-1.4.jar There is IOUtils We can use it directly , You don't have to be yourself read() write() 了 
    ServletOutputStream outputStream = resp.getOutputStream(); //  Get the output stream of the response 
    IOUtils.copy(resourceAsStream, outputStream); 
    //  Read the information from the input stream and copy it to the output stream , Output to the client , Pass in an input stream and an output stream  ( Byte character stream is OK )
}