Java Tech - Chapter 13 : Networking Basics

Browsers obtain resources from the Web by specifying the Web addresses, which are referred to officially as Uniform Resource Locators (URL). For example,

http://www.ibm.com/

The formal description of the URL components goes as follows:

Protocol_ID://Host_IP_address:Port/directory/Filename#Target

where

Protocol_ID = HTTP, FTP, etc

Host_IP_address = Host name in either numerical IP or hostname format

Port = default is 80

Filename = Name of a hypertext web page or other type of file. Default index.htm[l]

Target = optional reference address within a Web page

In Java we can create an instance of the java.net.URL class. These specify the location of a file on the network. We have, for example, used URL objects to obtain the location of image files in applets as shown here:

getImage( getCodeBase(), filename);

The applet method getCodeBase() returns a URL object for the location of the applet file. Here it is assumed the image file resides in the same directory

The URL class provides several methods for obtaining URL information (after a program in the Sun Java Tutorial):

ParseAddress.java

import java.net.*;
import java.io.*;

/** Parse a URL address into its components. **/
public class ParseAddress
{
public static void main (String[] args) {
   if (arg.length !=1) {
     System.out.println ("Error: missing the url argument");
     System.out.println ("Usage: java ParseURL <url>");
     System.exit (0);
   }

   try {
     URL url = new URL (arg[0]);
    System.out.println ("Protocol = " + url.getProtocol ());
    System.out.println ("Host = " + url.getHost ());
    System.out.println ("File name = " + url.getFile ());
    System.out.println ("Port = " + url.getPort ());
    System.out.println ("Target = " + url.getRef ());
}
   catch (MalformedURLException e) {
      System.out.println ("Bad URL = " + arg[0]);
}
  } // main
} // class ParseAddress

You run the program as follows:

c:\> java ParseAddress http://www.myschool.edu:80/data/x.html#d

This program outputs:

  Protocol = http
  Host = www.myschool.edu
  File name = /data/x.html
  Port = 80
  Target = d

Also, URL's can be constructed from a URL string

URL kth = new URL ("http//www.kth.se/index.html");

or from separate protocol, host, file strings:

URL kth = new URL ("http","www.kth.se","/index.html");

When you attempt to create a URL object, the constructor will validate the components . If they are in error, it will throw a MalformedURLException. So you must catch this exception from the URL constructors.

References & Web Resources