SDSU Client-Server Programming
Spring Semester, 2005
Server Intro
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San Diego State University -- This page last updated 10-Feb-05

CS 580 Client-Server Programming Spring Semester, 2005 Doc 6 Server Intro


What is a Server?


Java TCP Sockets

A Simple Date Server

Simple Server Issues


Multi-homed Machines

Reusing a Port

End of Line

End of File

End of Message


Copyright ©, All rights reserved. 2005 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA. OpenContent ( license defines the copyright on this document.


Java Network Programming, Harold

VisualWorks Internet Client Developer's Guide & Socket. See

Reading AssignmentJava

Java Network Programming, Harold

Basic Network Concepts, Chapter 2

Java I/O Chapter 4

Sockets for Servers Chapter 10

What is a Server?


Basic algorithm:

   while (true) {
     Wait for an incoming request;
     Perform whatever actions are requested;

Example - Echo Server

| server |
server := SocketAccessor newTCPserverAtPort: 9009.
server listenFor: 5.
 [ | acceptedSocket |
   "wait for a new connection"
   acceptedSocket := server accept.
   "fork off processing of the new stream socket"
   [ | stream char |
      stream := acceptedSocket readAppendStream.
      stream lineEndTransparent.
      [ (char := stream next) isNil ] whileFalse: [
         stream nextPut: char; commit ].
      stream close. 
   ] forkAt: Processor userSchedulingPriority -1.
] repeat. 

Some Basic Server Issues


Streams verses Buffers

Both Java & Smalltalk provide access to socket data via

Stream access is easier

Buffer access can be faster

We will cover Stream access first

Java TCP Sockets

Main Classes


Used by servers to listen for clients


Used by clients to talk to servers Used by servers to talk to clients

ServerSocket basic methods

public ServerSocket(int port) //port = 0 gives random port
public ServerSocket(int port, int backlog)
public ServerSocket(int port, int backlog, InetAddress bindAddress)
public Socket accept() throws IOException
public void close() throws IOException
public int getLocalPort()

Socket basic methods

public InputStream getInputStream() throws IOException
public OutputStream getOutputStream() throws IOException

A Simple Date Server

public class DateServer {
   private static Logger log = Logger.getLogger("dateLogger");
    public static void main (String args[]) throws IOException {
      ProgramProperties flags = new ProgramProperties( args);
      int port = flags.getInt( "port" , 8765);
      new DateServer().run(port);
   public void run(int port) throws IOException {
      ServerSocket input = new ServerSocket( port );"Server running on port " + input.getLocalPort());
      while (true) {
         Socket client = input.accept();"Request from " + client.getInetAddress());

Processing Client Request

   void processRequest(InputStream in,OutputStream out)
      throws IOException {
      BufferedReader parsedInput = 
            new BufferedReader(new InputStreamReader(in));
      boolean autoflushOn = true;
      PrintWriter parsedOutput = new PrintWriter(out,autoflushOn);
      String inputLine = parsedInput.readLine();
      if (inputLine.startsWith("date")) {
         Date now = new Date();

Note: This server is just a first example. It needs a lot of work. We will be working on improving it in later lectures.

Running the Server

Sample run of DateServer.

(I typed everything appearing in bold font here.)

rohan 16-> java -jar DateServer.jar

Feb 19, 2004 10:56:59 AM DateServer run

INFO: Server running on port 8765

Al 13-> telnet 8765


Connected to

Escape character is '^]'.

Feb 19, 2004 10:57:34 AM DateServer run

INFO: Request from /


Thu Feb 19 10:57:39 PST 2004

Connection closed by foreign host.

Warning About telnet Usage

Using telnet to interact with a server is

Don’t design server assuming it interacts directly with a human!

Simple Server Issues

Using our SimpleDateServer

      Client A builds connection to server, 
      Client A goes to lunch 
      Client B builds connection to server and ... :-( 


Multiple connections need to be accepted concurrently.


TCP accepts connections before the server is ready

TCP keeps a backlog queue of connections server has not accepted

Java ServerSocket constructor

There is no reasonable way to find out:

Backlog Experiment

Start the SimpleDateServer

Connect to the server using telnet

While the server is waiting for you to type something

Connect to the server with a second telnet session

In the second session type “date” and return

What happens?

Now type “date” and return in the first session

What happens?

Multi-homed Machines

Some machines have two or more physical network interface

Each network interface has its own IP address

public ServerSocket(int port)
public ServerSocket(int port, int backlog, InetAddress bindAddress)

Reusing a Port

Closing TCP connections can remain for several minutes

TCP may block use of the port until the connection is gone

This can be annoying in development

ServerSocket method setReuseAddress(boolean on)

End of Line


End of Line Convention


Line Feed (LF)

Macintosh OS 9

Carriage return (CR)



LF is ASCII character 10

CR is ASCII character 13

A server should not make assumptions about a client’s platform

A client should not make assumptions about a server’s platform

Client-server protocol should specify which characters are used

End of Line & Smalltalk

Java and Smalltalk programs run on all major platforms

Smalltalk assumes files use platform’s end of line convention

Smalltalk input streams convert platforms end of line to CR

Smalltalk output streams convert CR to platform’s end of line

This makes writing cross platform programs easier

Don’t want this to happen socket streams

BufferedExternalStream>> lineEndTransparent

How does Java handle this?

End of File

On a stream connected to a socket

End of file indicates that the connection has been closed!

Don’t use end of file to determine when other end is done talking!

End of Message

How do we know when we are at the end of a message?

BufferedReader parsedInput = 
      new BufferedReader(new InputStreamReader(in));

char[] message = new char[500];
int sizeRead =, 0, 500);


We still may not have the entire message!


End of Message

A good client-server protocol specifies

Main methods used:


Java & Smalltalk streams are buffered

TCP buffers output before sending

A server cannot read bytes left in a client’s buffer

PrintWriter flush();

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