Java networking

1. Java networking Jim Briggs based on notes by Amanda Peart based on Bell & Parr's bonus chapter http://www.shu.ac.uk/java/networkprogramming/starthere.html WECPP

2. Aims • To understand Java networking • To appreciate socket programming • Define the client socket • How to use sockets • Overview of server sockets WECPP

3. Review Internet basics • TCP/IP protocol (contra UDP/IP) • ensures reliable transmission of data • IP address • identifies Internet host (strictly interface) • Domain name • user-friendly way of identifying hosts • DNS maps names to IP addresses (and vice-versa) • Port • identifies particular service on a host (e.g. HTTP = 80) WECPP

4. Sockets • Sockets • abstract input-output devices • could apply to any sort of device, but most often referred to in the context of networking: • software mechanism for one program to talk to another • Two-way connection between two programs • full-duplex link • more than one socket can be associated with a port WECPP

5. Client and server sockets • Client socket and server socket need to be programmed in different ways • Client socket assumes the server is ready to accept connections • Server needs to listen for connection requests WECPP

6. How is a socket used? • A new socket is created by a program • The socket attempts to connect to a remote host • Once the connection has been established, the local machine and the remote machine send and receive data • When the transmission is complete, one or both machines closes the connection WECPP

7. Socket Programming • Java uses ‘class Sockets’ to interface to TCP sockets (import java.net.*) • The constructor method for the class: • creates a TCP socket • allows the user to specify a host and port • attempts to connect to the host Socket connection = new Socket("www.port.ac.uk", 80); Socket connection = new Socket(InetAddress, port); WECPP

8. When connection is established... • The client host port is chosen by the system at run time • The client port number can be accessed using:connection.getLocalPort() • A socket is closed when: • The socket is garbage collected • The program ends • The program explicitly closes it:connection.close() WECPP

9. Errors and exceptions • ConnectException • The connection is refused by the remote host. • Host could be busy or there might be no server on the port • NoRouteToHostException • The connection has timed out. WECPP

10. Input with sockets • For input it is most convenient to create a BufferedReader (java.io) as follows: BufferedReader in = new BufferedReader (new InputStreamReader(socket.getInputStream())); • This enables readLine to be used for string input. Data ends when null is returned. Example: String line; while ((line = in.readLine()) != null) { processLine(); } WECPP

11. Output with sockets • For output to a stream it is convenient to create a PrintWriter (java.io) object as follows: PrintWriter out = new PrintWriter(socket.getOutputStream(), true); • Use method println to output a line. out.println("string"); • Closing a socket closes all of the associated streams. WECPP

12. Socket crashes • When a program reads from a socket, the call on read blocks (waits) until the data has been obtained • But if the remote host crashes, the program is left hanging • An alternative is to call setSoTimeout(timeout) • Sets the maximum waiting time (in milliseconds) • When the time expires, an InterruptedException is thrown WECPP

13. Port scanner example • A port scanner is a program that hunts for working ports on an Internet host • Warning: can be impolite • PortScanner.java • Checks for unknown host • Attempts to connect to ports 1 to 256 of specified host • Ignores ports where no socket can be established WECPP

14. Get server time example • Connect to server's port 13 • Read a line of text (containing time and date) • GetRemoteTime.java • Of course it cannot connect if no service is running WECPP

15. Class Sockets WECPP

16. Class Sockets WECPP

17. Class Sockets WECPP

18. Class Sockets WECPP

19. Socket: Implemented by class socket A socket object is a connection to a certain host on a particular port Socket objects are used for data transfer A Socket object initiates a connection Server socket: Implemented by class ServerSocket A SocketServer object just waits on a particular port for incoming connections from anywhere A ServerSocket object waits for connections Sockets and Server Sockets WECPP

20. The algorithm for a server 1 • Create ServerSocket object on a particular port ServerSocket server = new ServerSocket(12345, 100); • ServerSocket object listens for any connection attempts Socket connection = server.accept(); • The server is blocked (waits) until a client attempts to connect • When a client attempts to connect, the server wakes up WECPP

21. The algorithm for a server 2 • When a client connects, method accept returns a new Socket object representing the connection between the server and the client • That socket object's getInputStream and/or getOutputStream methods are called to obtain streams which can be used to send/receive data • The server and the client interact using streams according to an agreed protocol • When finished, the server, the client, or both close the connection WECPP

22. Daytime server example • DaytimeServer.java • Listens on port 13 • When a connection is accepted it uses the socket's output stream to write the current date and time • Then closes the socket and listens for a new connection (forever) WECPP

23. More complex servers • A server dealing with one connection cannot respond to further connections • Normally, connection requests are queued • May be better to create a new thread to deal with each connection • Allows main thread to keep listening for new connections WECPP

24. Reading • Bell and Parr - bonus chapter http://www.shu.ac.uk/java/networkprogramming/starthere.html • Deitel, chapter 18 WECPP