What is the accurate way to read/download a file from the server (in Java)?

Question

In my client-server application I have used a command ( GET filename )to download a file into the client side. I have used the build in read() method to read the file. My teachers said it's not a very good practice to implement this read method. The reason is either it doesn't tell how exactly the file is reading from the server or it somehow is not able to download dynamic (large) file size. But at the moment I see that it's working fine. Since I am still in intermediate level in java, I need to learn the best way to do this job. How it could be improved in coding? That is I want to improve the while looping part in ClientSide.

I have pasted the relevent code:

ClientSide:

                         ............
                         ............
                        if (request.startsWith("GET")) {
                        File file = new File(request.substring(4));
                        is = socket.getInputStream();
                        fos = new FileOutputStream(file);

                        byte[] buffer = new byte[socket.getReceiveBufferSize()];
                        int bytesReceived = 0;

                        while ((bytesReceived = is.read(buffer)) >=0) {
                            //while ((bytesReceived = is.read(buffer))>=buffer) {
                            fos.write(buffer, 0, bytesReceived);
                        }
                        request = "";
                        fos.close();
                        is.close();
                    }
                       .................
                       .................

ServerSide:

                          .................
                          .................
else if (request.startsWith("GET")) {
                        System.out.println("");
                        String filename = request.substring(4);
                        File file = new File(System.getProperty("user.dir"));
                        File[] files = file.listFiles();

                        if (fileExists(files, filename)) {
                            file = new File(filename);
                            int fileSize = (int) file.length();
                            outputToClient.print("Status OK\r\n"
                                    + "Size " + fileSize + "KB" + "\r\n"
                                    + "\r\n"
                                    + "File " + filename + " Download was successfully\r\n");
                            outputToClient.flush();
                            // reading files
                            fis = new FileInputStream(file);
                            os = socket.getOutputStream();
                            byte[] buffer = new byte[2^7-1];
                            int bytesRead = 0;
                            while ((bytesRead = fis.read(buffer))!= -1) {
                                os.write(buffer, 0, bytesRead);
                            }
                            os.close();
                            fis.close();
                        } else {
                            outputToClient.print("Status 400\r\n"
                                    + "File " + filename + " not found\r\n"
                                    + "\r\n");
                            outputToClient.flush();
                        }
                    }
                    outputToClient.flush();
                }
                           .................
                           .................

Answer 1

You need to consume the rest of the HTTP response headers, by reading until you get a blank line, if you haven't already done that.

Apart from that, your code looks fine to me, except that I would use a much bigger buffer than 127, at least 8192, possibly a multiple of that.

Ask your teacher what (on earth) he's talking about.