Cannot extract correct information from message received through SocketChannel

Question

I am sending a string which has been converted into bytes using a DataOutput stream

// creates a client socket which connects to the first successor's server.
Socket clientSocket = new Socket(host, succ1_port);

// send the output_string to the first successor.
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
outToServer.writeBytes(output_string);

and then receiving them through a SocketChannel:

// accept connection
SocketChannel connectionSocket = tcpserver.accept(); // tcpserver is a ServerSocketChannel

ByteBuffer buf = ByteBuffer.allocate(48);

// store and print no. of bytes read
int bytes_read = connectionSocket.read(buf);
System.out.println("bytes read = " +bytes_read);
String from_client_string = new String(buf.array(), Charset.forName("UTF-8"));

The messages received are always of the format, "XXXX XXX" where X is any digit from 0-9.

I then try and separate these messages into two parts:

Pattern p = Pattern.compile("([0-9]{4}) ([0-9]{3})"); 
Matcher m = p.matcher(from_client_string);

if (m.matches()){   
  int filename = Integer.parseInt(m.group(1));
  int hash = Integer.parseInt(m.group(2));
  System.out.println("filename = " +filename);
  System.out.println("hash = " +hash);
else
  System.out.println("no match");

The problem is, sometimes when I print out the string converted from the bytebuffer, its value changes... Usually it is correct like "1234 210" but on other occasions it may cut out a digit and display "1234 21". However even when it is correct, I don't get a match? I also found the number of bytes being read always changes...

Would anyone know what the problem here is?

Thanks for your help.

Connr · Answer 1 · 2013-04-11T02:58:49.250

you should keep reading from the socket in a loop until it is closed. if there are no bytes available the read will block so make sure you are closing the connection on the other end.

so on the client side

// creates a client socket which connects to the first successor's server.
Socket clientSocket = new Socket(host, succ1_port);

// send the output_string to the first successor.
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
outToServer.writeBytes(output_string);
outToServer.close();
clientSocket.close();

on the server side:

// accept connection
SocketChannel connectionSocket = tcpserver.accept(); // tcpserver is a ServerSocketChannel

ByteBuffer buf = ByteBuffer.allocate(48);

// store and print no. of bytes read
String result="";
while(connectionSocket.isOpen()){
  int bytes_read = connectionSocket.read(buf);
  System.out.println("bytes read = " +bytes_read);
  String from_client_string = new String(buf.array(),0,bytes_read Charset.forName("UTF-8"));
  reulst+= from_client_string;

} }

Hi user2242863, Thank you for that. I made the changes you suggested and it appears to work partially. However, the connection never closes and so it continues to print out "bytes read = -1". If I close the connection on the server side, I get the correct answer once and then the results I get are similar to those in the OP. The server accepting the TCP connection is part of a Selector, if that makes a difference? — , Apr 11 '13 at 06:26

score 1 · Answer 2 · answered Apr 11 '13 at 07:39

When you read with ByteBuffer, you get the number of byte which are available which might not be all the byte you have written. What DataInputStream.readUTF() does is to wait for the number of bytes needed to decode the length (a 16 bit unsigned value).

I suggest you use DataOutputStream.writeUTF(String) and String DataInputStream.readUTF(). I suggest you also consider using buffering.

score 0 · Accepted Answer · answered Apr 11 '13 at 10:38

Thank you Peter! You're suggestion appears to be working :) I would vote you up but I don't have sufficient reputation.

Sending the data:

// creates a client socket which connects to the first successor's server.
Socket clientSocket = new Socket(host, succ1_port);

// send the output_string to the first successor.
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
outToServer.writeUTF(output_string);

outToServer.close();    
clientSocket.close();

Server Side (receiving the data)

// accept connection
SocketChannel connectionSocket = tcpserver.accept(); // tcpserver is a ServerSocketChannel

DataInputStream inToServer = new DataInputStream(connectionSocket.socket().getInputStream());
String from_client_string = inToServer.readUTF();

The pattern matching stuff is the same as in the OP...except now it actually seems to work :)

Cannot extract correct information from message received through SocketChannel

3 Answers3