	CS 580 Client-Server Programming Fall Semester, 2000 Simple Java Client
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Contents of Doc 7, Simple Java Client

References
Dr. Vinge's Java Networking lecture, Spring 2000, http://www-rohan.sdsu.edu/faculty/vinge/courses/spring00/cs580/notes/javanet/javanet.html

Java Network Programming 2nd Edition, Harold, 2000, O'Reilly, Chapter 10

Java.net.Socket, JDK 1.3, http://java.sun.com/j2se/1.3/docs/api/java/net/Socket.html


Doc 7, Simple Java Client Slide # 2

Building a Connection

Transport layer association is uniquely defined by the following:

The protocol (TCP or UDP)
The address of the local machine (the one building the connection)
The port number used by the local machine
The address of the remote machine (the one we are connecting to)
The port number used on the remote machine

Identify the parts:

    $ telnet sdsu.edu 13
    Trying 130.191.229.14...
    Connected to sdsu.edu.
    Escape character is '^]'.
    Thu Feb  3 22:30:35 2000
    Connection closed by foreign host.
    $


Doc 7, Simple Java Client Slide # 3

IP Connecting Parts
Information a client normally needs:

Information	Data from example
protocol	TCP (implied by the telnet program)
name of the server machine	sdsu.edu
port number on the server machine	13

The missing parts:

Information	Where does it come from?
address of the local machine	Automatically retrieved from the OS
port number on the local machine	OS supplies random open port


Doc 7, Simple Java Client Slide # 4

Simple Java Client

import java.net.*;
import java.io.*;
class GetTime {
   public static void main(String[] args)  {
      String      hostName;
      Socket   client = null;
      if ( args.length != 1 ) {
         System.out.println("Usage: java GetTime hostName");
         System.exit(1);
      }
      hostName = args[0];
      try {
         client = new Socket(hostName, 13);
      } catch (UnknownHostException hostError) {
         hostError.printStackTrace();
         System.exit(1);
      } catch (IOException genericError) {
         System.err.println(genericError.getMessage());
         System.exit(1);
      }
      try {
        BufferedReader in =
         new BufferedReader(
           new InputStreamReader(client.getInputStream()));
        String   inputLine;
        while ((inputLine = in.readLine()) != null) {
         System.out.println("Received: " + inputLine);
        }
      } catch (IOException IOError) {
         System.err.println(IOError.getMessage());
         System.exit(1);
      }
   }
}
Doc 7, Simple Java Client Slide # 5

java.net.Socket

Several constructors:

   Socket(String hostname, int portNumber) 
   Socket(InetAddress hostAddress, int portNumber)

The constructor builds the connection

Exceptions thrown by the constructors:

Exception	Reason it is raised
UnknownHostException	The specified host is not valid or cannot be looked up
SocketException	Something went wrong attempting to build a connection. Most common cause is "Connection refused" - no service on the specified port
IOException	Catchall


Doc 7, Simple Java Client Slide # 6

java.net.Socket Methods

   close(  )

Terminates an existing connection.

   getInputStream(  )
   getOutputStream(  )

These two provide access to IO through the connection.

NOTE network IO is always done in packets. This may produce strange results. Solution: use the Buffered IO classes.

   getLocalPort(  )


Doc 7, Simple Java Client Slide # 7

Domain Name Service (DNS)

On UNIX:

bind (Berkeley Internet Name Domain)

Standard UNIX DNS software

named (name daemon)

Program that provides the DNS service

DNS

Converts host and domain names into IP addresses
Converts IP addresses into host and domain names

Why DNS?

P addresses are hard to remember
Allows for

Abstraction
Easy migration
Maintenance


Doc 7, Simple Java Client Slide # 8

DNS through java.net.InetAddress
Use following static methods to create InetAddress

InetAddress  getByName(String host)
InetAddress[]  getAllByName(String host)
InetAddress getLocalHost(  )

Host can be machine name or IP address

Useful methods

String getHostName(  )
byte[] getAddress(  )


Doc 7, Simple Java Client Slide # 9

InetAddress Example

import java.net.InetAddress; 
import java.net.InetAddress;
import java.io.*;
class SimpleDNS {
   
   public static void main(String[] args)  {
      String   hostName;
      if ( args.length != 1 ) {
         System.out.println("Usage: java SimpleDNS hostName");
         System.exit(1);
      }
      hostName = args[0];
      try {
         InetAddress address = InetAddress.getByName(hostName);
         byte ip[] = address.getAddress();
   
         for (int octet=0; octet < ip.length; octet++) {
            System.out.print( ((int)ip[octet] & 0xff) + " " );
         }
         System.out.println();
      } catch (IOException DNSerror) {
         System.err.println(DNSerror);
         System.exit(1);
      }
   }
}


Doc 7, Simple Java Client Slide # 10

Running the Example

$ java SimpleDNS www.sdsu.edu
130 191 13 5 
$


Doc 7, Simple Java Client Slide # 11

Closing Sockets

A socket is closed when:

Program ends
One of its two streams is closed
It is garbage collected
You send the close message to it

Good practice to close socket when done with it

On a closed socket you can still get its

InetAddress
Port
Local address
Local port
Input and output stream

An exception is raised if you use a stream on a close socket

In JDK 1.3 you can close 1/2 of the socket


Doc 7, Simple Java Client Slide # 12

PortScanner From Java Network Programming chapter 10


import java.net.*;
import java.io.*;
public class PortScanner  {
   public static void main( String[] args) throws IOException  {
      String host = "localhost";
      Socket connection = null;
      try  {
         for (int port = 1; port < 65536; port++)  {
            try  {
               connection = new Socket(host, port);
               System.out.println("Server on port " + port )
            }
            catch (IOException error ) {// no server on this port 
            }
         }
      }
      catch (UnknownHostException error ) {
         System.err.println( error );
      }
      finally {
         if (connection != null ) connection.close();
      }
   }
}


Doc 7, Simple Java Client Slide # 13

Socket Options

You to change parameters controlling how native sockets send and receive data

TCP_NODELAY

public void setTcpNoDelay(boolean on) throws SocketException
public boolean getTcpNoDelay() throws SocketException

Disable/enable Nagle's algorithm

Do not pool small packets
Do not wait for acknowledgement of last packet before sending current packet

Exception is raised if underlying native sockets do not support this option


Doc 7, Simple Java Client Slide # 14

SO_LINGER

public void setSoLinger(boolean on, int lingerSeconds)
throws SocketException

public boolean getSoLinger() throws SocketException

What happens to waiting packets when socket is closed?

Option 1

Drop packets and close socket

Option 2

Wait N seconds to send packets and receive acknowledgements

lingerSeconds is number of seconds to wait

0 value = no wait

Max value is 65,535 seconds (~18 hours)

Best to use system default


Doc 7, Simple Java Client Slide # 15

SO_TIMEOUT


public void setSoTimeout(int timeout) throws SocketException
public boolean getSoTimeout() throws SocketException

How long does read() block on a socket?

Default is to block until there is data.

SO_TIMEOUT

specifies how long to wait in a read
In milliseconds
Default value is 0, meaning wait forever

An InterruptedException is thrown when time out occurs

Socket is still connected and usable


Doc 7, Simple Java Client Slide # 16

SO_RCVBUF & SO_SNDBUF

public void setReceiveBufferSize(int size) throws SocketException
public int getReceiveBufferSize()  throws SocketException

public void setSendBufferSize(int size) throws SocketException
public int getSendBufferSize() throws SocketException

Suggests the input/output buffer size used in the TCP stack

SO_KEEPALIVE

New in JDK 1.3

Have client send data on an idle connection to insure server is still alive

Default is off

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2000 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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