Create a text file for download on the fly with Java

Question

I have a Java Servlet that generates randomly thousands of Strings every time is called. I want the user to be able to get them in a file when he calls the Servlet. I don't want to write first the file on disk or memory. Is there a way to write the file on the fly when the user calls the servlet? Thanks

Answer 1

Any text that you generate in the Servlet can simply be written to the OutputStream returned by ServletResponse.getOutputStream().

If you want the output to be downloadable as a file, you can follow the approach in this answer - https://stackoverflow.com/a/11772700/1372207

The difference would be, that the Content-type would be text/plain and instead of reading from another inputstream, you would just write the String objects directly to the ServletOutputStream using the print(String) method.

Answer 2

If you use the idea to write content to HttpServletResponse's output stream while offering download service, rather than saving the content locally and then reading the file as FileInputStream, you can just convert the file content to InputStream by InputStream stream = new ByteArrayInputStream(exampleString.getBytes("UTF-8"));.

The following code partially references https://www.codejava.net/java-ee/servlet/java-servlet-download-file-example.

    public void doDownload(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String fileName = "xxx.txt";
        String fileContent = "";

        // get absolute path of the application
        ServletContext context = request.getServletContext();

        // get MIME type of the file
        String mimeType = context.getMimeType(fileName);
        if (mimeType == null) {
            // set to binary type if MIME mapping not found
            mimeType = "application/octet-stream";
        }

        setResponseHeader(response, fileName, mimeType, (int) fileContent.length());

        InputStream inputStream = new ByteArrayInputStream(fileContent.getBytes("UTF-8"));
        // get output stream of the response
        OutputStream outStream = response.getOutputStream();

        byte[] buffer = new byte[4096];
        int bytesRead = -1;

        // write bytes read from the input stream into the output stream
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outStream.write(buffer, 0, bytesRead);
        }

        inputStream.close();
        outStream.close();
    }

    private void setResponseHeader(HttpServletResponse response, String fileName, String mimeType, Integer fileLength) {
        response.setContentType(mimeType);
        response.setContentLength(fileLength);
        response.setContentType("application/octet-stream; charset=UTF-8");

        String headerKey = "Content-Disposition";
        String headerValue = String.format("attachment; filename=\"%s\"", fileName);
        response.setHeader(headerKey, headerValue);

        response.addHeader("Pargam", "no-cache");
        response.addHeader("Cache-Control", "no-cache");
    }