how to write bytes to server socket

Question

I\'m writing a java socket program to read data from server, I\'ve no control to server, below is protocol agreed,

• 2-bytes: magic number
• 2-bytes: data l

Answer 1

According to the specification you must build a packet shaped in the following way

| 2 | 2 | N ........ |

Now this could be quite easy and there are multiple ways to do it, I suggest you one:

import java.nio.ByteBuffer;
import java.nio.ByteOrder;

static byte[] buildPacket(int magicNumber, String payload) throws UnsupportedEncodingException
{
  // 4 bytes for header + payload
  ByteBuffer buffer = ByteBuffer.allocate(2 + 2 + payload.length());
  // we set that we want big endian numbers
  buffer.order(ByteOrder.BIG_ENDIAN);

  buffer.putShort((short)magicNumber);
  buffer.putShort((short)payload.length());
  buffer.put(payload.getBytes("US-ASCII"));
  return buffer.array();
}

public static void main (String[] args) throws java.lang.Exception
{
    try
    {
        byte[] bytes = buildPacket(0xFF10, "foobar");
        for (byte b : bytes)
          System.out.printf("0x%02X ", b);
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }
}

Mind that if you declare the method to accept a short magic number directly, you won't be able to pass a literal magic number > 32767 because short is signed in Java.

Answer 2

Watch out for the big endinanness!

DataxxxStream - although it is quite handy - does not offer a full support for both little and big endian numbers and arbitrary String encodings.

See my post at How to Read Byte Stream from Socket

Answer 3

Use a DataOutputStream around a BufferedOutputStream around the `Socket.getOutputStream(). Then you can use:

• writeShort() for the magic number
• writeShort() for the length word
• write() for the payload.

Similarly you can use DataInputStream and the corresponding readXXX() methods to read the response.

NB You are writing to a socket here, not a server socket.