Using TCP sockets in Java

Created by

M Bateman, A Ruddle & C Allison

As part of the TCP View project

Overview

• TCP socket

• Client/Server

• Multithread server

• Thread pooling server

• Alternate thread pooling server

TCP Provides

• Process to process communication– Use tuple of IP address & port

• Reliable

• In order

• Socket is one end-point of a two way connection link

TCP Socket Operations

socket ()

listen ()

bind ()

accept ()

read ()

socket ()

connect ()

write ()

write ()read ()

Block waiting for a request

Process request

Server process

Client process

TCP in Java

ServerSocket ()

accept ()

read ()

Socket ()

write ()

write ()

read ()

Block waiting for a request

Process request

Client process

Socket ()

Server process

TCP in Java

• Implemented in java.net.*

• Two main classes– java.net.ServerSocket - for server– java.net.Socket - for client

• Provides abstractions over– Socket and connect operations– Bind, listen & accept

• Remember to close the socket at the end

ServerSocket

• Constructors– ServerSocket (port)– ServerSocket (port, backlog)– ServerSocket (port, backlog, bindAddress)

• Where– port = the TCP port to listen on– backlog = TCP queue length– bindAddress = interface to use (else all)

Socket

• Construction– Two important constructors– Socket () – random local port– Socket (host, port) – remote host and port

• Is the end point for communication– Used in both the client and server– Client connects to ServerSocket– ServerSocket returns Socket for

communications

Daytime serverimport java.net.*;import java.io.*public class DTServer {public static void main (String argv[]) { int dayTimePort = 13; try { ServerSocket dtserver = new ServerSocket (dayTimePort); while (Socket con = dtserver.accept ()) { PrintWriter out = new PrintWriter (con.getOutputStream (), true); Date now = new Date (); out.println (now.toString ()); con.close (); } } catch (Exception e) {} }}

Daytime clientimport java.net.*;import java.io.*public class DTClient {public static void main (String argv[]) { int dayTimePort = 13; try { Socket con = new Socket (argv[0], dayTimePort); InputStream is = con.getInputStream (); BufferedReader br = new BufferedReader (new InputStreamReader (is)); String time = br.readLine (); System.out.println (“The time at “ + argv[0] + “ is “ + time); con.close (); } catch (Exception e) {} }}

Issues

• Can only deal with one request at once

• Performance– Have to wait until the guy in front has finished

with the server

Solution Threads• Thread theory not covered here

– Basically allows server to do more than one thing at once (see operating system course to find out why this isn’t true)

• Two ways– Extend java.lang.Thread– Implement java.lang.Runnable

• Override public void run ()public class TThread extend Thread {

public void run () { for (int i = 0; i < 100; i++) { System.out.println (i); } }public static void main (String argv[]) { new TThread ().start (); }}

public class RThread implements Runnable {

public void run () { for (int i = 0; i < 100; i++) { System.out.println (i); } }public static void main (String argv[]) { new Thread (new TThread ()).start (); }}

Multithreaded Daytimeimport java.net.*;import java.io.*public class DTServer {public static void main (String argv[]) { int dayTimePort = 13; try { ServerSocket dtserver = new ServerSocket (dayTimePort); while (Socket con = dtserver.accept ()) { DTThread dtthread = new DTThread (); // create the new thread dtthread.setSocket (con); dtthread.start (); // start the thread now } } catch (Exception e) {} }}

Daytime Threadimport java.net.*;public class DTThread extends Thread {Socket con = null;

public void setSocket (Socket con) { this.con = con; }

public void run () { // Important work done here PrintWriter out = new PrintWriter (con.getOutputStream (), true); Date now = new Date (); out.println (now.toString ()); con.close (); }}

Issues

• Performance– Thread creation at client connection– Creating threads takes time

• Possible deadlock– Due to multithreaded

Thread Pools - Basicimport java.net.*;import java.io.*public class DTServer {public static void main (String argv[]) { int dayTimePort = 13; List list = Collections.synchronizedList(new LinkedList()); for (int i = 0; i < 10; i ++) { list.add (new DTThread ()); } // create 10 threads try { ServerSocket dtserver = new ServerSocket (dayTimePort); while (Socket con = dtserver.accept ()) { DTThread dtthread = (DTThread)list.remove (0); dtthread.setSocket (con); dtthread.start (); // start the thread now list.add (new DTThread ()); // create a thread to create the one we just used } } catch (Exception e) {} }}

Issues

• Possible deadlock

• Resource thrashing

• Concurrency errors

• Thread leakage

• Request overload

• Performance– Thread creation still has to be performed in the

main loop

Pool Alternative Implementation

• Create pool as before

• Create thread which makes sure the pool has n thread waiting

• On connection take thread from the pool– The above thread makes sure there are always

threads in the pool waiting to be used

Summary