1. Java Sockets Tutorial
Rahul Malik
Nov 12, 2005

2. Outline What is a Socket? Benefits of Using Java Client Sockets
Server Sockets
Programming Example

3. What is a Socket?
Definition:  A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent.
Okay……so what does that mean? And how does it differ for the Client and Server?

4. What is a Socket ? : Client
The client knows the hostname of the machine on which the server is running and the port number to which the server is connected. To make a connection request, the client tries to rendezvous with the server on the server's machine and port.
If the connection is accepted, a socket is successfully created and the client can use the socket to communicate with the server

5. What is a Socket? : Server
The server accepts the connection.
Upon acceptance, the server gets a new socket bound to a different port.
It needs a new socket (and consequently a different port number) so that it can continue to listen to the original socket for connection requests while tending to the needs of the connected client.
The client and server can now communicate by writing to or reading from their sockets.

6. Java Sockets Pros/Cons
Classes are already in java.net package that make lower level Beej’s usage unnecessary.
There are separate classes for servers and clients.
Simplicity : It’s very functional and relatively easy to learn in a short amount of time.
Allows us to focus on the development and bugs of our program versus
Inner workings are similar to Beej’s Guide
Cons:
Similar to Beej’s Guide

7. Outline for Sockets Program:
Open a socket.
Open an input stream and output stream to the socket.
Read from and write to the stream according to the server's protocol.
Close the streams.
Close the socket.

8. Opening a Socket : Client
How do we create a socket?
Socket mySocket = new Socket(“hostname”, port);
Slightly easier than Beej’s right? This goes from defining a socket and connecting all in one line!!!
“Hostname” is the host you are trying to connect
The “port” is the port you wish to bind to.
When selecting a port number, you should note that port numbers between 0 and 1,023 are reserved for privileged users (that is, super user or root). )
Simple enough??
This is only for clients, servers actually use ServerSockets which listen on specific ports and handle connections versus making them.

9. Client : Open an input stream to the socket
How do I create an Input Stream??
Start with creating a socket
Socket mySocket = new Socket(“hostname”, port);
For input streams (to receive data from the server) we must utilize the DataInputStream class.
DataInputStream in = new DataStreamInput ( mySocket.getInputStream() );
The Data Input Stream Class allows you to read lines of text and Java primitive types. It contains methods like read, readChar, readInt, readDouble, and readLine, etc.

10. Client : Open an output stream
How do I create an Output Stream??
Continuing with the same socket
We must use the DataOutputStream or PrintStream Class to do this.
PrintStream Output = new PrintStream (mySocket.getOutputStream() );
PrintStream allows for .write() and .println() functions that are very useful.
DataOutputStream however prints one primitive type at a time and contains functions like .writebytes();

11. Closing
When we are done, we must make sure to close sockets AND input/output streams.
Java has made this very simple. Using the previous examples
Input and Output streams : input.close(); and output.close();
Socket : mySocket.close();

12. Server : Creating the Socket
On the Server side we have our own class, ServerSocket.
To open a Socket to listen on a specific port we would use :
ServerSocket myServer = new ServerSocket(“Port #”);
A server socket is useless without the input and output streams associated with it.

13. Server : I/O Stream’s
When implementing a server you also need to create a socket object from the ServerSocket in order to listen for and accept connections from clients.
Socket clientSocket = new Socket();
clientSocket = myServer.accept();
.accept() returns the socket that is connecting to the server. The advantage of that is that we can use .getInputStream() and .getOutputStream() from the socket class to handle our communication

14. Server : Closing Up
The fact that we interact with clients on individual sockets is good because :
We only close sockets that are connected to clients
The server is still listening and can receive connections.

15. Extra Notes : Required Libraries
You must use exception handling try{….}catch{……}finally{……}
try { ServerSocket ss = new ServerSocket(80); } catch (IOException e) {System.err.println(e);}
To do Java Sockets programming you must import(include) :
java.io.*
java.net.*

16. Practice Java Sockets : Our Task
Task: To create a Echo Client/Server model.
The Echo server simply receives data from its client and echoes it back. The Echo server is a well-known service that clients can rendezvous with on port 7.
Sun's Java Echo Client Example