套接字输入流不会逐段读取

Question

I have problem in my app, I read from socket with ip=192.168.0.3 and port =502. Another program write messages to that port fragment by fragment ( mac fragmnet length is 205 bytes) and I need to read that messages fragment by fragment. But when I wrote this I get all fragments in one big.

boolean last=false;
                int numberFragment=0;
                while (((!last))  ) {

                    numberReceived = socketInputStream.read(buffer);
                    numberFragment++;
                    tempBuffer = new byte[numberReceived];
                    ByteBuffer baferce = ByteBuffer.wrap(tempBuffer);
                    baferce.put(buffer, 0, numberReceived);
                    //System.out.println("RECEIVED="+(new String(tempBuffer)));
                    last=ResponseFragmentCheck.IsLastFragment(tempBuffer, numberFragment);
                    System.out.println("LAST ="+last);

                    PrintBytesArray(tempBuffer);
                    received.add(tempBuffer);
                }

Is there any way i Java to read fragment by fragment ?

Answer 1

If you want Java to read fragment by fragment, there are two things you must do:

1. Use UDP, not TCP. You may already be doing this.
2. Use Socket.receive() to receive a DatagramPacket .

Any stream method may fail to respect the fragment boundaries.

Answer 2

There are no 'fragments' in TCP. It is a byte stream. If you want to read messages of a specific length, use DataInputStream.readFully(). If you want to read messages the same way you sent them you have to prefix a length word to each, or use a type-length-value protocol, or a self-describing protocol such as XML.

Answer 3

If you are using TCP, make sure your other program calls flush() (or equivalent in other languages) after writing one fragment. Otherwise your fragments may be collected by the sending TCP stack until enough data is there to fill one TCP packet (depending on implementation and maybe some settings).

Of course, you still can't be sure to receive the packets/fragments one by one, but you'll at least be sure to receive them as fast as possible.

If you need your packet structure, either use another protocol instead of TCP, or a protocol over TCP, which shows you when a packet starts and ends.