Week 6: Basic Java Networking

Overview of Networking

Relatively easy in Java (compare with C, C++, etc)
URL - Uniform Resource Locator, the address of resource in the Internet
Use <APPLET> in the browser, browser did the undercover networking for you
Network classes in java.net package
Remember the JDK doc is at http://www.javasoft.com/products/jdk/1.1/docs/

Networking Basics

Application
(HTTP, ftp, telnet,etc)

Transport
(TCP, UDP, etc)

Network
(IP, etc)

Link
(Device driver)

Use classes in java.net to provide system-independent network communication
Java can use TCP or UDP
TCP (Transport Control Protocol) is a connection-based protocol that provides a reliable flow of data between two computers
Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Telnet are all examples of applications that use TCP
UDP (User Datagram Protocol) is a protocol that sends independent packets of data called datagrams, from one computer to another with no guarantees about arrival (it is not connection-based)
E.g. Clock server, ping command, etc

Ports

The use of ports let computer which application to forward the data to
32-bit IP address, 16-bit port number
TCP, UDP both use ports
In TCP, an application extablishes a connection by binding a socket to a port number
(socket is like a phone, and port is part of the number)
Binding registers the application with the system to receive all data destined for that port
Port numbers ranges from 0 to 65535
0-1023 are restricted as well known ports
(e.g. 80 for http, 21 for ftp, 23 for telnet, 7 for echo)
In Java, the URL, URLConnection, Socket, and SocketServer classes all use TCP
The DatagramPacket and DatagramServer classes use UDP

Working with URLs

URL is a reference (an address) to a resource on the Internet. URLs are the doorway to Internet and the World Wide Web
In Java, we have URL class from java.net. Once created, we can use its methods to find out its host name, filename, and other information
All URL has two components:
The protocol identifier
The resource name
For example: http://www.openloop.com
http is the protocol (Hypertext Transfer Protocol)
We can also have (File Transfer Protocol) ftp, gopher, File and News
The resouce name contains one or more of the following components:
1. Host name - the name of the machine the resource lives on
2. Filename - the pathname to the file on the machine
3. Port number - the port number connected to (Optional)
4. Reference - A reference to a named anchor within a resource (Optional)
For example:
http://www.openloop.com:80/index.html#bottom

Creating a URL

Can be created from String
Human string will be http://www.openloop.com

In Java:
	URL myObj = new URL("http://www.openloop.com/"); 
		// the / at the end has a default of /index.html

This is an absolute URL - contains all of the information necessary to reach the resource in question
A relative URL contains only enough information to reach the resource relative to (or in the context of) another URL

In HTML, if A.html, B.html and C.html is in the same directory on the server. A.html can refer to the others as follows:

	<A HREF="B.html">This is a title for B</A>
	<A HREF="C.html">This is a title for C</A>

In Java, we can create relative URL object as well

	URL myObj = new URL("http://www.openloop.com/");
	URL otherObj = new URL(myObj, "index2.html"); // index2.html is on site openloop

Two constructor of the URL class:

URL(String) // Absolute URL object
URL(URL, String) // relatvie URL object

The general forms are:

URL(String absoluteURL)
URL(URL baseURL, String relativeURL)

Other URL Constructors

These constructor are useful when working with hostname, filename, port number and references
URL myObj = new URL("http", "www.openloop.com", "/index.htm");
The above is equivalent to
URL("http://www.openloop.com/index.htm");
The final constructor will take a port as well:
URL myObj = new URL("http", "www.openloop.com", 80, "/index.htm");

MalformedURLException

Each of the four URl constructors will throws a MailformedURLException if there is a null argument or unknown protocol

// Catch and handle this exception by embedding your URL constructor
try
{
	URL myURL = new URL(...)
}
catch
{
	...
	// exception handler code here
	...
}			// We will talk about exception handling in detail later on

Parsing a URL

URL class methods:
- getProtocol()
- getHost()
- getPort()
- getFile() - Returns the filename component of the URL
- getRef - Returns the reference component of the URL
Yes, the URL class is somewhat HTTP centric

Example:
import java.net.*
import java.io.*

try 
{
	aURL = new URL("http://java.sun.com/");
	System.out.println("protocol = " + aURL.getProtocol());
}
catch (MalformedURLException e)
{
	System.out.println("MalformedURLException:" + e);
}

Reading Directly from a URL

Use the openStream method to get a stream from the content of the URL
openStream returns a java.io.InputStream

Example:
...
// This part of the java application reads the content from the input stream
// and then echo out the contents to the display
try 
{
	URL myObj = new URL("http://www.openloop.com/");
	BufferedReader dis = new BufferedReader(new InputStreamReader(myObj.openStream()));
	String inputLine;

	while ((inputLine = dis.readLine()) != null)
	{
		System.out.println(inputLine);
	}
	dis.close();
}
catch (MalformedURLException me)
{
	System.out.println("MalformedURLException: " + me);
}
catch (IOException ioe)
{
	System.out.println("IOException: " + ioe);
}

Lab

Write a Java application called Week6 that will

Create an URL object with http as protocol and www.yahoo.com as resource name.
Parse the newly created URL object and print out
- protocol,
- host,
- filename,
- port and
- reference
Then, read directly from the URL object and print out all the contents from the input stream to the display
Run the program by "java Week6 > tempfile", tempfile will hold all the output

Note: Remember to use the try catch block when working with URL and InputStream
Note: Our application *actually* get to the network and download information !!!