JAVA: Socket Programming

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Web Design & Development CS506

Lesson 21

Socket Programming

Socket

A socket is one endpoint of a two-way communication link between two programs running

generally on a network.

A socket is a bi-directional communication channel between hosts. A computer on a network often

termed as host.

Socket Dynamics

As you have already worked with files, you know that file is an abstraction of your hard drive.

Similarly you can think of a socket as an abstraction of the network.

Each end has input stream (to send data) and output stream (to receive data) wired up to the other

host.

You store and retrieve data through files from hard drive, without knowing the actual dynamics of

the hard drive. Similarly you send and receive data to and from network through socket, without

actually going into underlying mechanics.

You read and write data from/to a file using streams. To read and write data to socket, you will also

use streams.

What is Port?

It is a transport address to which processes can listen for connections request.

There are different protocols available to communicate such as TCP and UDP. We will use TCP

for programming in this handout.

There are 64k ports available for TCP sockets and 64k ports available for UDP, so at least

theoretically we can open 128k simultaneous connections.

There are well-known ports which are

below 1024

provides standard services

Some well-known ports are:

-FTP works on port 21

-HTTP works on port 80 -TELNET works on port 23 etc.

How Client Server Communicate

Normally, a server runs on a specific computer and has a socket that is bound to a specific port

number.

The server just waits, listening to the socket for a client to make a connection request.

On the client side: The client knows the hostname of the machine on which the server is running

and the port number to which the server is connected.

On the server side, if the connection is accepted, a socket is successfully created and the client can

use the socket to communicate with the server.

Note that the socket on the client side is not bound to the port number used to make contact with

the server. Rather, the client is assigned a port number local to the machine on which the client is

running.

The client and server can now communicate by writing to or reading from their sockets

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. As soon as client creates a socket that socket attempts to connect to the specified server.

. The server listens through a special kind of socket, which is named as server socket.

. The sole purpose of the server socket is to listen for incoming request; it is not used for

communication.

. If every thing goes well, the server accepts the connection. Upon acceptance, the server gets a new

socket, a communication socket, bound to a different port number.

. The server needs a new socket (and consequently a different port number) so that it can continue to

listen through the original server socket for connection requests while tending to the needs of the

connected client. This scheme is helpful when two or more clients try to connect to a server

simultaneously (a very common scenario).

Steps To Make a Simple Client

To make a client, process can be split into 5 steps. These are:

1. Import required package

You have to import two packages

. java.net.*;

. java.io.*;

2. Connect / Open a Socket with Server

Create a client socket (communication socket)

Socket s = new Socket("serverName", serverPort) ;

Name or address of the server you wanted to connect such as

serverName :

http://www.google.com or 172.2.4.98 etc. For testing if you are

running client and server on the same machine then you can

specify "localhost" as the name of server

. serverPort :

Port number you want to connect to

The scheme is very similar to our home address and then phone number.

3. Get I/O Streams of Socket

Get input & output streams connected to your socket

. For reading data from socket As stated above, a socket has input stream attached to it.

InputStream is = s.getInputStream();

// now to convert byte oriented stream into character oriented buffered reader // we use intermediary

stream that helps in achieving above stated purpose

InputStreamReader isr= new InputStreamReader(is); BufferedReader br = new BufferedReader(isr);

. For writing data to socket

A socket has also output stream attached to it. Therefore,

OutputStream os = s.getOutputStream();

// now to convert byte oriented stream into character oriented print writer

// here we will not use any intermediary stream because PrintWriter constructor // directly accepts

an object of OutputStream

PrintWriter pw = new PrintWriter(os, true);

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Here notice that true is also passed to so that output buffer will flush.

4. Send / Receive Message

Once you have the streams, sending or receiving messages isn't a big task. It's very much similar to the

way you did with files

. To send messages

pw.println("hello world");

. To read messages

String recMsg = br.readLine();

5. Close Socket

Don't forget to close the socket, when you finished your work

s.close();

Steps To Make a Simple Server

To make a server, process can be split into 7 steps. Most of these are similar to steps used in making a

client. These are:

1. Import required package

You need the similar set of packages you have used in making of client

. java.net.*;

. java.io.*;

2. Create a Server Socket

In order to create a server socket, you will need to specify port no eventually on which server will listen

for client requests.

ServerSocket ss = new ServerSocket(serverPort) ;

. serverPort: port local to the server i.e. a free port on the server machine. This

is the same port number that is given in the client socket constructor

3. Wait for Incoming Connections

The job of the server socket is to listen for the incoming connections. This listening part is done

through the accept method.

Socket s = ss.accept();

The server program blocks ( stops ) at the accept method and waits for the incoming client connection

when a request for connection comes it opens a new communication socket (s) and use this socket to

communicate with the client.

4. Get I/O Streams of Socket

Once you have the communication socket, getting I/O streams from communication socket is similar to

the way did in making a client

1. For reading data from socket

InputStream is = s.getInputStream(); InputStreamReader isr= new

InputStreamReader(is); BufferedReader br = new BufferedReader(isr);

2. For writing data to socket

OutputStream os = s.getOutputStream();

PrintWriter pw = new PrintWriter(os, true);

5. Send / Receive Message

Sending and receiving messages is very similar as discussed in making of client

To send messages:

pw.println("hello world");

. To read messages

String recMsg = br.readLine();

6. Close Socket

s.close();

Example Code 21.1: Echo Server & Echo Client

The client will send its name to the server and server will append "hello" with the name send by the client.

After that, server will send back the name with appended "hello".

EchoServer.java

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Let's first see the code for the server

// step 1: importing required package

import java.net.*;

import java.io.*;

import javax.swing.*;

public class EchoServer

{

public static void main(String args[])

{

try

{

//step 2: create a server socket

ServerSocket ss = new ServerSocket(2222);

System.out.println("Server started...");

/* Loop back to the accept method of the serversocket and wait for a new connection request.

Soserver will continuously listen for requests

while(true) {

// step 3: wait for incoming connection

Socket s = ss.accept();

System.out.println("connection request recieved");

// step 4: Get I/O streams

InputStream is = s.getInputStream();InputStreamReader isr= new

InputStreamReader(is);BufferedReader br = new BufferedReader(isr);

OutputStream os = s.getOutputStream();

PrintWriter pw = new PrintWriter(os,true);

// step 5: Send / Receive message

// reading name sent by clientString name = br.readLine(); // appending "hello" with the

received name

String msg = "Hello " + name + " from Server"; // sending back to client

pw.println(msg);

// closing communication sockeys.close();

} // end while

}catch(Exception ex){

System.out.println(ex); } } } // end class

EchoClient.java

The code of the client is given below

// step 1: importing required package

import java.net.*;import java.io.*;import

javax.swing.*;

public class EchoClient{

public static void main(String args[]){

try {

//step 2: create a communication socket

// if your server will run on the same machine thenyou can pass "localhost" as server address */

/* Notice that port no is similar to one passedwhile creating server socket */server

Socket s = new Socket("localhost", 2222);

// step 3: Get I/O streams

InputStream is = s.getInputStream();InputStreamReader isr= new

InputStreamReader(is);BufferedReader br = new BufferedReader(isr);

OutputStream os = s.getOutputStream();PrintWriter pw = new

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PrintWriter(os,true);

// step 4: Send / Receive message

// asking use to enter his/her name

String msg = JOptionPane.showInputDialog("Enter your name");

// sending name to server

pw.println(msg);

// reading message (name appended with hello) from// servermsg = br.readLine();

// displaying received messageJOptionPane.showMessageDialog(null , msg);

// closing communication sockets.close();

}catch(Exception ex){ System.out.println(ex); } } }

// end class

Compile & Execute

After compiling both files, run EchoServer.java first, from the command prompt window. You'll see a

message of "server started" as shown in the figure below. Also notice that cursor is continuously blinking

since server is waiting for client request

Now, open another command prompt window and run EchoClient.java from it. Look at EchoServer

window; you'll see the message of "request received". Sooner, the EchoClient program will ask you to

enter name in input dialog box. After entering name press ok button, with in no time, a message dialog box

will pop up containing your name with appended "hello" from server. This whole process is illustrated

below in pictorial form:

sending name to server

response from server

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Notice that server is still running, you can run again EchoClient.java as many times untill server is running.

To have more fun, run the server on a different computer and client on a different. But before doing that

find the IP of the computer machine on which your EchoServer will eventually run. Replace "localhost"

with the new IP and start conversion over network ☺

References

Entire material for this handout is taken from the book JAVA A Lab Course by Umair Javed. This

material is available just for the use of VU students of the course Web Design and Development and not for

any other commercial purpose without the consent of author.

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