Return file from Spring @Controller having OutputS

问题:

I want to return a file from a Spring controller. I already have API that can give me any implementation of OutputStream and then I need to send it to a user.

So the flow is something like that:

getting outputstream -> service passes this outputstream to controller -> controller has to send it to a user

I think I need inputstream to do it and I have also found Apache Commons api feature that looks like this:

IOUtils.copy(InputStream is, OutputStream os)

but the problem is, it converts it to the other side -> not from os to is, but from is to os.

Edit

to be clear, because I see the answers are not hitting right thing:
I use Dropbox api and recieve file in OutputStream and I want this output stream to be sent to user while entering some URL

FileOutputStream outputStream = new FileOutputStream(); //can be any instance of OutputStream
DbxEntry.File downloadedFile = client.getFile("/fileName.mp3", null, outputStream);

Thats why i was talking about converting outputstream to inputstream, but have no idea how to do it. Furthermore, I suppose that there is better way to solve this (maybe return byte array somehow from outputstream)

I was trying to pass servlet outputstream [response.getOutputstream()] through parameter to the method that downloads file from dropbox, but it didnt work, at all

Edit 2

The "flow" of my app is something like this: @Joeblade

1. User enters url: /download/{file_name}

2. Spring Controller captures the url and calls the @Service layer to download the file and pass it to that controller:

   @RequestMapping(value = "download/{name}", method = RequestMethod.GET)
   public void getFileByName(@PathVariable("name") final String name, HttpServletResponse response) throws IOException {
       response.setContentType("audio/mpeg3");
       response.setHeader("Content-Disposition", "attachment; filename=" + name);
       service.callSomeMethodAndRecieveDownloadedFileInSomeForm(name); // <- and this file(InputStream/OutputStream/byte[] array/File object/MultipartFile I dont really know..) has to be sent to the user
   }
   

3. Now the @Service calls Dropbox API and downloads the file by specified file_name, and puts it all to the OutputStream, and then passes it (in some form.. maybe OutputStream, byte[] array or any other object - I dont know which is better to use) to the controller:

   public SomeObjectThatContainsFileForExamplePipedInputStream callSomeMethodAndRecieveDownloadedFileInSomeForm(final String name) throws IOException {
       //here any instance of OutputStream - it needs to be passed to client.getFile lower (for now it is PipedOutputStream)
       PipedInputStream inputStream = new PipedInputStream(); // for now
       PipedOutputStream outputStream = new PipedOutputStream(inputStream);
   
   
       //some dropbox client object
       DbxClient client = new DbxClient();
       try {
           //important part - Dropbox API downloads the file from Dropbox servers to the outputstream object passed as the third parameter
           client.getFile("/" + name, null, outputStream);
       } catch (DbxException e){
           e.printStackTrace();
       } finally {
           outputStream.close();
       }
       return inputStream;
   }
   

4. Controler recieves the file (I dont know, at all, in which form as I said upper) and passes it then to the user

So the thing is to recieve OutputStream with the downloaded file by calling dropboxClient.getFile() method and then this OutputStream that contains the downloaded file, has to be sent to the user, how to do this?

回答1:

You could use the ByteArrayOutputStream and ByteArrayInputStream. Example:

// A ByteArrayOutputStream holds the content in memory
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();

// Do stuff with your OutputStream

// To convert it to a byte[] - simply use
final byte[] bytes = outputStream.toByteArray();

// To convert bytes to an InputStream, use a ByteArrayInputStream
ByteArrayInputStream inputStream = new ByteArrayInputStream(bytes);

You can do the same with other stream pairs. E.g. the file streams:

// Create a FileOutputStream
FileOutputStream fos = new FileOutputStream("filename.txt");

// Write contents to file

// Always close the stream, preferably in a try-with-resources block
fos.close();

// The, convert the file contents to an input stream
final InputStream fileInputStream = new FileInputStream("filename.txt");

And, when using Spring MVC you can definitely return a byte[] that contains your file. Just make sure that you annotate your response with @ResponseBody. Something like this:

@ResponseBody
@RequestMapping("/myurl/{filename:.*}")
public byte[] serveFile(@PathVariable("file"} String file) throws IOException {
    ByteArrayOutputStream outputStream = new ByteArrayOutputStream(); 
    DbxEntry.File downloadedFile = client.getFile("/" + filename, null, outputStream);
    return outputStream.toByteArray();
} 

回答2:

Get the OutputStream from the HttpServletResponse and write the file to it (in this example using IOUtils from Apache Commons)

@RequestMapping(value = "/download", method = RequestMethod.GET)
public void download(HttpServletResponse response) {
    ...
    InputStream inputStream = new FileInputStream(new File(PATH_TO_FILE)); //load the file
    IOUtils.copy(inputStream, response.getOutputStream());
    response.flushBuffer();
    ...
}

Make sure you use a try/catch to close the streams in case of an exception.

回答3:

The most preferable solution is to use InputStreamResource with ResponseEntity. All you need is set Content-Length manually:

@RequestMapping(value = "/download", method = RequestMethod.GET)
public ResponseEntity download() throws IOException {
    String filePath = "PATH_HERE";
    InputStream inputStream = new FileInputStream(new File(filePath));
    InputStreamResource inputStreamResource = new InputStreamResource(inputStream);
    HttpHeaders headers = new HttpHeaders();
    headers.setContentLength(Files.size(Paths.get(filePath));
    return new ResponseEntity(inputStreamResource, headers, HttpStatus.OK);
}

回答4:

I recommend reading this answer

@ResponseBody
@RequestMapping("/photo2", method = RequestMethod.GET, produces = MediaType.IMAGE_JPEG_VALUE)
public byte[] testphoto() throws IOException {
    InputStream in = servletContext.getResourceAsStream("/images/no_image.jpg");
    return IOUtils.toByteArray(in);
}

answered by michal.kreuzman

I was going to write something similar myself but ofcourse it's already been answered.

If you want to just pass the stream instead of first getting everything in memory you could use this answer I haven't tested this (not at work) but it looks legit :)

@RequestMapping(value = "report1", method = RequestMethod.GET, produces = "application/pdf")
@ResponseBody
public void getReport1(OutputStream out) {
    InputStream in; // retrieve this from wherever you are receiving your stream
    byte[] buffer = new byte[1024];
    int len;
    while ((len = in.read(buffer)) != -1) {
        out.write(buffer, 0, len);
    }
    in.close();
    out.flush(); // out.close? 
}

The thing is, this is pretty much the same as IOUtils.copy / IOUtils.copyLarge does. line: 2128 Which you say copies the wrong direction.

However first make sure you understand what you ask. If you want to read from an outputstream(object for writing) and write to an input stream (object to read from) then I think what you really want is to write to an object that also supplies a read option.

for that you could use a PipedInputStream and PipedOutputStream. These are connected together so that bytes written to the outputstream are available to be read from the corresponding input stream.

so in the location where you are receiving the bytes I assume you are writing bytes to an outputstream. there do this:

// set up the input/output stream so that bytes written to writeToHere are available to be read from readFromhere
PipedInputStream readFromHere = new PipedInputStream();
PipedOutputStream writeToHere = new PipedOutputStream(readFromHere);

// write to the outputstream as you like
writeToHere.write(...)

// or pass it as an outputstream to an external method
someMather(writeToHere);

// when you're done close this end.
writeToHere.close();


// then whenever you like, read from the inputstream
IOUtils.copy(readFromHere, out, new byte[1024]); 

If you use IOUtils.copy it will continue to read until the outputstream is closed. so make sure that it is already closed before starting (if you run write/read on the same thread) or use another thread to write to the output buffer and close it at the end.

If this is still not what you're looking for then you'll have to refine your question.

回答5:

One thing to keep in mind when writing to the response outputstream is that it is a very good idea to call flush() on whatever writer that you've wrapped it with periodically. The reason for this is that a broken connection (for example caused by a user canceling a download) may not end up throwing an exception for a long time, if ever. This can effectively be a resource leak on your container.

回答6:

The most memory-efficient solution in your case would be to pass the response OutputStream right to the Dropbox API:

@GetMapping(value = "download/{name}")
public void getFileByName(@PathVariable("name") final String name, HttpServletResponse response)
        throws IOException, DbxException {
    response.setContentType("audio/mpeg3");
    response.setHeader(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + name + "\"");
    response.setContentLength(filesize); // if you know size of the file in advance

    new DbxClient().getFile("/" + name, null, response.getOutputStream());
}

Data read by the API will be sent directly to the user. No additional byte buffer of any type is required.

---

As for PipedInputStream/PipedOutputStream, they are intended for the blocking communication between 2 threads. PipedOutputStream blocks writing thread after 1024 bytes (by default) until some other thread start reading from the end of the pipe (PipedInputStream).