Question

I’m trying to write a server in Java. I know very little Java. I’ve found an example using Selector.

It looks good, but it behaves strangely. When I do my_socket_output_stream.writeBytes(“hello world”) in client code, the server reads this message one byte at a time. Shouldn’t I be notified only when the complete message is sent? Now I’d have to check my buffer after getting every byte to know if I can already work with it. Seems terribly inefficient.

I wonder if that’s due to Selector or is that just how sockets work (it’s been a long time since I used them). Could I make them wait for the full message somehow? Also, can I associate some objects with a channel? Right now all sockets use the same buffer. I’m sure you see how that is a problem..

The reason I want to use a Selector is that my server is only going to do io with a HashTable. Multiple threads would just be constantly waiting. And I only have one core anyway. Though maybe a combination of ThreadPoolExecutor and ConcurrentHashMap would be a good choice? It would surely enable me to have a buffer per socket..

I’d appreciate suggestions.

Answer 1

I faced the same problem a long time ago. I solved by first sending the number of bytes of the message, then sending the message itself byte by byte. Then I expanded it to line by line.

At the sender’s side:

// code at sender side
StreamConnectionNotifier service =  (StreamConnectionNotifier) Connector.open( url );
//System.out.println("opened");
StreamConnection con = (StreamConnection) service.acceptAndOpen();
OutputStream outputStream = con.openOutputStream();


// file to send
Scanner in = new Scanner(inFile);

//just count lines
String s=null;
int countLines=0;
while(in.hasNext()) {
    s=in.nextLine();
    countLines++;
}

//send num of lines
outputStream.write(Integer.toHexString(countLines).getBytes());
try{Thread.sleep(100);} catch(InterruptedException e){}

//send lines
in = new Scanner(inFile);
for(int i=0; i<countLines; i++) {
    s=in.nextLine()+"\n";
    outputStream.write(s.getBytes());
    Thread.sleep(100);
}

At the receiver’s side:

// code at receiver side
byte buffer[] = new byte[80];
int bytes_read = inputStream.read( buffer );
String received = new String(buffer, 0, bytes_read);
try{Thread.sleep(100);} catch(InterruptedException e){}
int receiveLines = Integer.parseInt(received);

PrintWriter out = new PrintWriter(new FileOutputStream("received.txt"));

for(int i=0; i<receiveLines; i++) {
  bytes_read = inputStream.read( buffer );
  received = new String(buffer, 0, bytes_read);
  out.println(received);    
  Thread.sleep(100);
} 

I hope this helps 🙂