Problems with ArrayList and file transfer

Question

I'm creating a TFTP server. I've got it tranfering files fine but most of the files wont open when they arrive at the other end. This is because the output of the ArrayList im using to store file bytes from every packet received adds a load of bytes to the start of the file. eg. "¬í sr java.util.ArrayListxÒ™Ça I sizexp w ur [B¬óøTà xp ü!". The reason for using the List in the first place is that the server im creating has no way to tell the file size of the file which is being received. Therefore as far as I can tell I cant use a byte[] as this needs to be initialised with a set length. Is there any way round this?

            WRQ WRQ = new WRQ();
            ACK ACK = new ACK();
            DatagramPacket outPacket;
            byte[] bytes;
            byte[] fileOut;
            List fileBytes = new ArrayList();
            outPacket = WRQ.firstPacket(packet);
            socket.send(outPacket);

            socket.receive(packet);

            while (packet.getLength() == 516){

            bytes = WRQ.doWRQ(packet);
            fileBytes.add(bytes);

            outPacket = ACK.doACK(packet);
            socket.send(outPacket);

            socket.receive(packet); 

            }

            bytes = WRQ.doWRQ(packet);
            fileBytes.add(bytes);

            outPacket = ACK.doACK(packet);
            socket.send(outPacket);

            ObjectOutputStream os;

            ByteArrayOutputStream byteStream = new ByteArrayOutputStream(5000);
            os = new ObjectOutputStream(new BufferedOutputStream(byteStream));
            os.flush();
            os.writeObject(fileBytes);
            os.flush();
            byte[] outFile = byteStream.toByteArray();
            os.close();

            FileOutputStream foStream = new FileOutputStream(filename);
            foStream.write(outFile);
            foStream.close();

Answer 1

You store byte arrays in an ArrayList, and then you write the whole ArrayList to a ByteArrayOutputStream wrapped in an ObjectOutputStream, using the writeObject() method.

This uses the native Object serialization mechanism to save the ArrayList object. It doesn't write every byte array in the list one after the other. To make it clear: it writes the class name, and the internal structure of the ArrayList, using the object serialization protocol.

You don't need an ArrayList. Write directly to a ByteArrayOutputStream, or even directly to a FileOutputStream. As is, you're trying to

• write bytes to a list
• write the bytes in the list to a byte array
• write the byte array to a file.

It would be much more straightforward (and efficient) to write directly to the output file (wrapped in a BufferedOutputStream for buffering)