Basic Networking

Uniform Resource Identifiers and Locators

URI

Uniform Resource Identifier

A URI is an identifier of a specific resource. Like a page, or book, or a document.

It identifies a resource unambiguously

example: ISBN 0-486-27557-4

URN

Uniform Resource Name

URI defined to name a resource. a URN is also a URI.

example: urn:isbn:0-486-27557-4

URL

Uniform Resource Locator

Reference to a web resource that specifies its location on a computer network and a mechanism for retrieving this resource.

A URL is also a URI. A URL is special type of identifier that also tells you how to access it, such as HTTPs, FTP

example: https://www.google.com

URI/URL Syntax

scheme:[//[user:password@]host[:port]][/]path[?query][#fragment]

• scheme: defines a kind of uri (http, https, ...)
• host: host name (e.g bytehub.nl) or IPv4/IPv6 address (e.g., 192.168.0.1)
• port: 16-bit number that points to a communication endpoint (e.g., 8080)
• path: separated relative path on host (e.g, directory/file)
• query: usually contains user input to display a resource in a custom way.
• fragment: a subsection of a document

Java URI/URL

Example: Reading a URL

public class WebCat {
    public static void main(String[] args) {
        for (String arg : args) {
            try (BufferedReader br = new BufferedReader(new InputStreamReader(new URL(arg).openStream()))) {
                String line;
                while ((line = br.readLine()) != null) {
                    System.out.println(line);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

review module1 networks for information on networks.

Java IP addressing

• InetAddress encapsulates an IP address
  • Supports name lookup (convert host name to IP address)
  • and reverse lookup (convert address to host-name)
  • has a factory method getByName which supports domain or ip.

Sockets

A socket is a common abstraction of the transport layer in either TCP or UDP.

A socket provides communication between a Client and a Server, a Server can create a socket at a port, if this port is available on the internet a client can connect with the address of the server followed by the port number.

• Server listens with a socket on a port for connections.

• Client tries to connect with the socket.

• Java classes

  • Socket
  • ServerSocket (TCP)
  • DatagramSocket (UDP)

ServerSocket

Listens to a fixed port for incoming connections. + creates a connection on a port for each incoming request. + provides a Socket object for each created connection.

the constructor is ServerSocket(int port). If 0 is passed as port then a free port is chosen.

Socket accept()

• blocks, waiting for a client's attempt to establish a connecton.
• returns a Socket if the attempt is successful.

void close()

• like for streams, deallocates resources.
• Threads waiting on accept() calls throw a SocketException.

InetAddress getInetAddress()

• Returns the local IP-address of this ServerSocket.

int getLocalPort()

• returns the port on which this ServerSocket listens.

Socket

• provides access to TCP/IP streams
  • BI-directional communication between sender and receiver.
• Socket(String remoteHost, int port)
  • Constructor, starts a connection with the remote host at a port.
• InputStream getInputStream()
  • Allows data from the other party to be received
• OutputStream getOutputStream()
  • Allows data to be sent to the other party

Date Server Example

don't get exited this is not a romantic date... jokes aside this is a simple server that sends the current date to clients that connect. the server listens on port 32007. Important: This server deals with one client at a time. other clients have to wait until the sendDate method closes the client before another client can be accepted.

public class DateServer {
    public static final int LISTENING_PORT = 32007;

    public static void main(String[] args) {
        ServerSocket listener;
        Socket connections;
        try {
            listener = new ServerSocket(LISTENING_PORT);
            System.out.printf("Server listening on port: %d", LISTENING_PORT);
            while (true) {
                connection = listener.accept();
                sendDate(connection)
            }
        } catch (Exception e) {
            System.out.println("Server has shutdown");
            return;
        }
    }

    private void sendDate(Socket client) {
        try {
            System.out.printf("Connection from %s", client.getInetAddress().toString());
            Date now = new Date();
            PrintWriter outgoing = new PrintWriter(client.getOutputStream());
            outgoing.println(now.toString());
            outgoing.flush();
            client.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Date Client Example

public class DateClient {
    public class final int LISTENING_PORT = 32007

    public static void main(String[] args) {
        String hostName;
        Socket connection;
        BufferedReader incoming;

        if (args.length > 0) {
            hostname = args[0];
        } else {
            hostname = localhost;
        }

        try {
            connection = new Socket(hostName, LISTENING_PORT);
            incoming = new BufferedReader(new InputStreamReader(connection.getInputStream());
            String lineFromServer = incoming.readLine();
            if (lineFromServer == null) {
                throw new IOException("no data!");
            }
            System.out.println(lineFromServer);
            incoming.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}