[FIXED] Prevent client timing out while a servlet generates a large download

Issue

I have a Java servlet that generates some arbitrary report file and returns it as a download to the user's browser. The file is written directly to the servlet's output stream, so if it is very large then it can successfully download in chunks. However, sometimes the resulting data is not large enough to get split into chunks, and either the client connection times out, or it runs successfully but the download doesn't appear in the browser's UI until it's 100% done.

This is what my servlet code looks like:

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("application/pdf");
    response.setHeader("Content-Disposition", "attachment; filename=\"" + report.getFileName(params) + "\"");

    try(OutputStream outputStream = response.getOutputStream()) {
        // Send the first response ASAP to suppress client timeouts
        response.flushBuffer(); // This doesn't seem to change anything??

        // This calls some arbitrary function that writes data directly into the given stream
        generateFile(outputStream);
    }
}

I ran a particularly bad test where the file generation took 110,826ms. Once it got to the end, the client had downloaded a 0 byte file - I assume this is the result of a timeout. I am expecting this specific result to be somewhere between 10 and 30 KB - smaller than the servlet's buffer. When I ran a different test, it generated a lot of data quickly (up to 80MB total), so the download appeared in my browser after the first chunk was filled up.

Is there a way to force a downloaded file to appear in the browser (and prevent a timeout from occurring) before any actual data has been generated? Am I on the right track with that flushBuffer() call?

Solution

Well, it looks like shrinking the size of my output buffer with response.setBufferSize(1000); allowed my stress test file to download successfully. I still don't know why response.flushBuffer() didn't seem to do anything, but at least as long as I generate data quickly enough to fill that buffer size before timing out, the download will complete.

Answered By - John Brink