Basic Java Network Programming CS211 July 30 th , 2001 The Network - - PowerPoint PPT Presentation

Basic Java Network Programming

CS211 July 30th, 2001

The Network and OSI Model

IP Header TCP Header

TCP/IP: A Paradox

Connection Oriented and Connectionless Protocols

• TCP

– Reliable: no loss, or duplication, or out of order – End-to-end – Connection Oriented: congestion control, flow control

• UDP & IP

– Unreliable – Datagram – Connectionless

InetAddress Class

• represents an IP address.
• numeric addresses <---> host names

InetAddress utopia, duke; try { utopia=InetAddress.getByName("utopia.poly.edu"); duke=InetAddress.getByName("128.238.2.92"); } catch (UnknownHostException e) { System.err.println(e); } public static InetAddress[] getAllByName(String host) throws UnknownHostException public static InetAddress getLocalHost() throws UnknownHostException

URL class

• create new URLs
• parse the different parts of a URL, namely:

– the protocol – the authority

• user info: user name, password

– host name or address – port – the path, a.k.a. file – the ref, a.k.a. section or anchor – the query string

• get an input stream from a URL so you can read data

from a server

• get content from the server as a Java object

Webcat.java: Reading data from a URL

public class Webcat { public static void main(String[] args) { for (int i = 0; i < args.length; i++) { try { URL u = new URL(args[i]); BufferedReader br = new BufferedReader( new InputStreamReader (new InputStream( u.openStream()))); String theLine; while ((theLine = br.readLine()) != null) { System.out.println(theLine); } } catch (IOException e) { System.err.println(e); } } } }

Socket Communication

Client Server Paradigm

TCP Sockets

• Server socket class: ServerSocket

– wait for requests from clients. – after a request is received, a client socket is generated.

• Client socket class: Socket

– an endpoint for communication between two apps/applets. – obtained by contacting a server – generated by the server socket

• Communication is handled by

input/output streams.

– Socket provides an input and an output stream.

TCP Sockets cont.

• public Socket(String host, int port,

InetAddress localAddr, int localPort)

• public Socket(InetAddress address, int port,

InetAddress localAddr, int localPort)

• public ServerSocket(int port, int backlog,

InetAddress bindAddr)

Summary: TCP Server

• 1. Create the server socket and begin listening.
• 2. Call the accept() method to get new connections.
• 3. Create input and output streams for the returned

socket.

• 4. Conduct the conversation based on the agreed protocol.
• 5. Close the client streams and socket.
• 6. Go back to step 2 or continue to step 7.
• 7. Close the server socket

Datagram Sockets

• same class is used for both client and server halves
• public DatagramSocket (int port, InetAddress laddr)

DatagramSocket serverSocket = new DatagramSocket( 4545 ); DatagramSocket clientSocket = new DatagramSocket();

• public DatagramPacket(byte[] buf, int offset, int length,

InetAddress address, int port) DatagramPacket sendPacket = new DatagramPacket( sendbuf, sendbuf.length, addr, port);

Summary: UDP Server

• 1. Create the datagram socket on a specific port.
• 2. Call receive() to wait for incoming packets.
• 3. Respond to received packets according to the

agreed protocol.

• 4. Go back to step 2 or continue to step 5.
• 5. Close the datagram socket.

Multicast

A multicast address is a class D address in the range 224.0.0.1 to 239.255.255.255, inclusive

Multicast Sockets

• Multicast Sockets use Datagram Packets
• BIG difference lies in Multicast's capability to

transmit to all listening hosts simultaneously.

• Downside is the lack of support for multicast
• routing. Most routers still don’t forward

multicast packets.

• Notion of Groups: Join and leave groups.

Applets

public void init(): To initialize the applet each time it's loaded (or reloaded). public void start(): To start the applet's execution, such as when the applet's loaded or when the user revisits a page that contains the applet. public void stop(): To stop the applet's execution, such as when the user leaves the applet's page or quits the browser. public void destroy(): To perform a final cleanup in preparation for unloading.

Applet Example

import java.applet.*; import javax.swing.*; public class HTMLLabelApplet extends JApplet { public void init() { JLabel theText = new JLabel( "<html>Hello! This is a multiline label with <b>bold</b> " + "and <i>italic</i> text. <P> " + "It can use paragraphs, horizontal lines, <hr> " + "<font color=red>colors</font> " + "and most of the other basic features of HTML 3.2</html>"); this.getContentPane().add(theText); } }

Applet Tags

<APPLET CODE=AppletSubclass.class WIDTH=anInt HEIGHT=anInt> <PARAM NAME=parameter1Name VALUE=aValue> <PARAM NAME=parameter2Name VALUE=anotherValue> </APPLET> e.g. <APPLET CODE="Animator.class" WIDTH=460 HEIGHT=160> <PARAM NAME="imageSource" VALUE="images/Beans"> <PARAM NAME="backgroundColor" VALUE="0xc0c0c0"> <PARAM NAME="endImage" VALUE=10> <PARAM NAME="soundSource" VALUE="audio"> <PARAM NAME="soundtrack" VALUE="spacemusic.au"> <PARAM NAME="sounds" VALUE="1.au|2.au|3.au|4.au|5.au|6.au|7.au|8au|9.au|0.au" > <PARAM NAME="pause" VALUE=200> . . . </APPLET>

Applet Example

Applet Security

• Can open a socket only to the host name

from which it was loaded. Other connections results in a SecurityException.

• Datagram sockets don't open connections. Hence

when an inbound packet is received, the host name is checked.

• Multicast sockets are disallowed.