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Programming Section 9

James King12 August 2003

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Internet and Network programming in TCP/IP

Networking conceptsHello World in TCPHello World in UDPSending objects across the networkAccepting multiple concurrent clients

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Network programming

Network conceptsNetworks allow several machines to communicate together. Within a room or department Internet

Many different hardware and software technologies Ethernet, ATM, modem, ADSL, TCP/IP, IPX

etc..Enabling sharing of resources (files, printers etc)See computer networks modules for more detailWe will look at how to write programs that transfer data using Internet Protocol – the internet standard

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Internet ProtocolsIP provides the protocols to connect the internet together

The only internet protocol Most common network protocol in small networks too... Relatively reliable and proven technology

Provides two types of communication short packets called datagrams (UDP)

maximum packet size 64Kbytes packets may not arrive in order some packets may not arrive Some packets may be duplicated

Streams of bytes (TCP) send any amount of data data is sequenced (in order no duplicates) and reliable

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Network programming Implementing network connections

Each computer on network or internet has unique IP address which is made up of 4 numbers separated by . e.g. 100.12.11.13 127.0.0.1 always means yourself

Each computer has a number of connection points available called ports e.g. port 6666 applications listen to one or more ports for

data applications can not share the same port

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Machine 192.12.21.7

Network programming Implementing network connections

Network/Internet

Actual Flow of Data

Operating System

Application

Network Layer

socket 1234

Machine 192.12.21.6

Operating System

Application

Network Layer

socket 665

Apparent Flow of Data

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TCP/IP to send Data across a network

Networking Section

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Simple TCP Networked Hello World

Aim send the string “Hello World” from the client to the server using TCP/IPServer must be accepting on the port before the client can sendClient must connect to the server before it can sendClient and server should close the connection before exiting

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Network programming

Implementation of the TCP Server

ServerSocket sv=new ServerSocket(6666);

System.out.println("waitng for client");

Socket s=sv.accept();

InputStream i=s.getInputStream();

DataInputStream oi=new DataInputStream(i);

String h=oi.readUTF();

System.out.println(h);

port to listen to connects on

wait for a connect.

Conversation with client will

use a temporary socket s

read the one line message from the temporary socket

display message on console

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Implementation of the TCP Client

Socket sv=new Socket("localhost",6666); OutputStream i=sv.getOutputStream();

DataOutputStream oi=new DataOutputStream(i); oi.writeUTF("Hello World");

i.flush(); oi.flush();

force sending the message by flushing

any buffers

write the message to the socket Java takes care of converting the string into

something that can be send across a network

port to connect to

machine to connect to

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What happens in what order

Server creates main socket and blocks inside accept() waiting for a connection from the client Client creates socket and connects to serverServer accept() returns with a temporary socket to talk to client onServer calls readUTF() and blocksClient sends a message using writeUTF()Server receives message and readUTF() returns the message as stringServer prints message on console

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UDP to send Data across a network

Networking Section

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Simple UDP Networked Hello World

Aim send the string “Hello World” from the client to the server using UDP/IPthere are no streams in UDP instead we have to make a UDP packetUDP packets can only contain an array of bytes so we have to convert the string to bytes before we can put it in a UDP packetTo read the contents of a UDP packet we have to reverse this processUDP in C or C++ is much easier than in Java – fault of Java and not UDP or the internet

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Implementation of a UDP server

byte [] bytes=new byte[65536];

DatagramSocket sv=new DatagramSocket(6666);

DatagramPacket p=new DatagramPacket(bytes,bytes.length);

sv.receive(p);

bytes=p.getData();

String h=new String(bytes,0,p.getLength());

System.out.println(h);

create storage for the incoming data

create a UDP socket on port

6666

Build a datagram packet and attach the

storage to it

receive a UDP packet and copy it

into our DatagramPacket extract the bytes

from the datagram

convert the bytes into a string

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Implementation of a UDP Client

byte [] bytes="Hello World".getBytes();

DatagramSocket sv=new DatagramSocket();

DatagramPacket p=new DatagramPacket(bytes, bytes.length, InetAddress.getByName("localhost"), 6666);

sv.send(p);

convert the string hello

world into an array of bytescreate a

datagram socket

create a datagram packet, attach the data we want to send and the destination

address

send the packet

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What happens in what order

Server creates socket Server creates a byte array to store the incoming dataServer builds a datagram packet to store the incoming datagramserver calls receive and blocks waits for a datagramClient creates socketclient marshals message into a byte array and puts the byte array into a Datagram packetClient sends message to server using sendServer receives message and receive unblocksServer unmarshals the data from the datagram packet and converts it into StringServer prints String on console

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Serialisation to send Objects across a network

Networking Section

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Sending Instances of Classes across a network

You can send your own instances of classes across a network using Java serialisationserialisation is the process of making a copy of the attributes that can be handled in a streamThe class must implement SerializableThe class must have a constructor with no parametersBy default all attributes in the class are sent across the network You can implement your own read and write methods

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Hello World Network Class

public class Hello implements Serializable{private String hello="";public Hello(String h){ hello=h;}public Hello(){ hello="";}public String message(){return hello;}}

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Sending Objects across the net using TCP Server Code

ServerSocket sv=new ServerSocket(6666,2);

Socket s=sv.accept();

InputStream i=s.getInputStream();

ObjectInputStream oi=new ObjectInputStream(i);

Hello h=(Hello)oi.readObject();

System.out.println(h.message());

We use a ObjectInputStream

instead of a DataInputStream

We read an object using readObject and type cast

it into what was sent instead of readUTFwe ask the object to

return its message

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Sending Objects across the net using TCP Client Code

Socket sv=new Socket("localhost",6666); OutputStream i=sv.getOutputStream(); ObjectOutputStream oi=new

ObjectOutputStream(i); Hello h=new Hello("Hello World"); oi.writeObject(h);

We use a ObjectOutputStream instead of a

DataOutputStream

We use writeObject instead of writeUTF