Java Spring - dynamically generated csv file download response is hanging

Question

On my company's site we have some tables that we need to export to a csv file.
There are some varying parameters, so the csv file needs to be dynamically created on request.

My problem is that after clicking to download, the response hangs, and waits for the whole file to be created (which can take some time) and only then downloads the entire file in one instant.

I'm using AngularJS, so I'm using window.location = <url_for_file_download> In order to make the browser download the file.

On the server side I'm using Java Spring and I've followed all the instructions I could find on the web in order to create a file download controller.

My controller code is something like this:

@RequestMapping(value = "http://yada.yada.yada/csv/myFile.csv", method = RequestMethod.GET)
public @ResponseBody
void getCustomers(HttpServletResponse response,
                         @RequestParam(required = false) String someParameters) 
                         throws NotAuthorizedException, IOException {  
// set headers
setHeaders(response);
// generate writer
CSVWriter write = generateWriter(response);
// get data
List<String[]> data = getData();
// write and flush and all that
.
.
.
}

My code for setting the response headers are:

response.setContentType("text/csv;charset=utf-8");
response.setHeader("Content-Disposition","attachment; filename=\"" + fileName + ".csv\"");

I've also tried adding the following headers:

response.setHeader("Transfer-Encoding", "Chunked");
response.setHeader("Content-Description", "File Transfer");

and I've also tried setting the Content-type to "application/octet-stream".

Notice that I don't add a Content-length header, since the file doesn't exist yet, and is being written on the fly.

For writing the csv file I'm using OpenCSV and my code is as follows:

OutputStream resOs = response.getOutputStream();
OutputStream buffOs = new BufferedOutputStream(resOs);
OutputStreamWriter outputWriter = new OutputStreamWriter(buffOs,"UTF-8");
CSVWriter writer = new CSVWriter(outputWriter);

I iterate over the data and write it like so:

for (String[] row: data) {
    writer.writeNext(line);
}

(It's not exactly the code - but this is more or else what happens in the code)

And at the end I flush and close:

writer.flush();
writer.close();

I also tried flushing after each line I write.

So why isn't the file being transferred before it has all been written? Why is my browser (Google chrome) downloading the file in one instant after waiting a long time? And how can I fix this.

I hope I've added enough code, if there's something missing just please tell me and I'll try to add it here.

Thank you so much in advance.

Answer 1

Can you try returning a null value in your java

return null ;

Or you can try below code also 1. Jquery code upon clicking the submit button

$(document).ready( function() {
     $('#buttonName').click(function(e){
    $("#formName").submit();
     //alert("The file ready to be downloaded");

});
});

Your controller code

@RequestMapping(value="/name",method=RequestMethod.POST)                


public ModelAndView downloadCSV(ModelMap model,HttpSession session,@ModelAttribute(value="Pojo") Pojo pojo
            ,HttpServletRequest request,HttpServletResponse response){

----------------some code----------------

response.setContentType("application/csv");   
("application/unknown"); 
response.setHeader("content-disposition","attachment;filename =filename.csv"); 
ServletOutputStream  writer = response.getOutputStream();   
            logger.info("downloading contents to csv");

            writer.print("A");
            writer.print(',');
            writer.println("B");            

        for(int i=0;i<limit;i++){

                writer.print(""+pojo.get(i).getA());
                writer.print(',');
                writer.print(pojo.get(i).getB()); 
                writer.println();   
        }

        writer.flush();
        writer.close();

---------------some code-----------
return null;
}

Hope this helps

Answer 2

The Controller will wait for the response to be written before the response is send back to the client.

Here is a nice post with multiple approaches / options outlined

Downloading a file from spring controllers

This post talks about flushing the output periodically to help fasten the download.

how to download large files without memory issues in java

If all you are trying to do is let the user know that the file download is in progress and due soon, I think an Ajax progress status indicaor might be your solution.

1. Trigger the ajax call to the back-end to generate the file

2. Show progress indicator to the user while file is being generated server side

3. once response is available, file is presented to the user.

I think something similar is being explored here download file with ajax() POST Request via Spring MVC

Hope this helps!

Thanks, Paul

Answer 3

I faced the same issue. The code that didn't work for me was

@RequestMapping(value = "/test")
 public void test(HttpServletResponse response)
            throws IOException, InterruptedException {
    response.getOutputStream().println("Hello");
    response.getOutputStream().flush();
    Thread.sleep(2000);
    response.getOutputStream().println("How");
    response.getOutputStream().flush();
    Thread.sleep(2000);
    response.getOutputStream().println("are");
    response.getOutputStream().flush();
    Thread.sleep(2000);
    response.getOutputStream().println("you");
    response.getOutputStream().flush();
}

The culprit was ShallowEtagHeaderFilter. When this filter is enabled the response is sent in one chunk. When this filter is diabled the response is send in multiple chunks.

From this thread Tomcat does not flush the response buffer it looks like another possible culprit can be GzipFilter