EXCEPTION HANDLING AND TEXT I/O - Introduction to Java Programming - page 551

14.13 Reading Data from the Web
Just like you can read data from a file on your computer, you can read data from a file
on the Web.

Key
Point

In addition to reading data from a local file on a computer or file server, you can also access
data from a file that is on the Web if you know the file's URL (Uniform Resource Locator—
the unique address for a file on the Web). For example, www.google.com/index.html is the URL
for the file index.html located on the Google Web server. When you enter the URL in a Web
browser, the Web server sends the data to your browser, which renders the data graphically.
Figure 14.11 illustrates how this process works.

Client Server

Web
Browser

Web
Server

Internet

Application
Program

Local files

F IGURE 14.11

The client retrieves files from a Web server.

For an application program to read data from a URL, you first need to create a URL object
using the java.net.URL class with this constructor:

public URL(String spec) throws MalformedURLException

For example, the following statement creates a URL object for http://www.google
. com/index.html .

1 try {
2 URL url = new URL( "http://www.google.com/index.html ");
3 }
4 catch (MalformedURLException ex) {
5 ex.printStackTrace();
6 }

A MalformedURLException is thrown if the URL string has a syntax error. For example,
the URL string “ http: www.google.com/index.html ” would cause a MalformedURLException
runtime error because two slashes ( // ) are required after the colon ( : ). Note that the
http:// prefix is required for the URL class to recognize a valid URL. It would be wrong if
you replace line 2 with the following code:

/

URL url = new URL( "www.google.com/index.html" );

After a URL object is created, you can use the openStream() method defined in the URL
class to open an input stream and use this stream to create a Scanner object as follows:

Scanner input = new Scanner(url.openStream());

Now you can read the data from the input stream just like from a local file. The example in
Listing 14.17 prompts the user to enter a URL and displays the size of the file.

L ISTING 14.17 ReadFileFromURL.java
1 import java.util.Scanner;
2