Download URL - Joensuu

Uniform Resource Locator: URL
 Hierarchy
of Classes
 Methods of the Class
 Examples of Using the Class
Hierarchy of Classes
DatagramPacket
DatagramSocket
InetAddress
Object
ServerSocket
Socket
URL
URLConnection
URLEncode
r
URL Class
Class
URL is a description of a resource location on the
Internet.
Complete URL: http://www.cs.joensuu.fi:1547/~john/pub/index.html
-- protocol : http://
-- host : www.cs.joensuu.fi
-- port : 1547
-- path : ~smith/pub/index.html
Java
provides a class—java.net.URL—to manipulate
URLs.
Methods of URL Class
Main
methods of the class URL:

URL(String url): create an object using the string parameter
(exception MalformedURLException).

URL(String, String, String): protocol, host, path of
the
resource.

String toString(): return the URL as a strings.

String getHost(): return the name of the host linked to this URL.

String getProtocol(): return le protocol of this URL.

String getPort(): return the number of the associate port.
Methods of URL Class

InputStream openStream(): realize the connection to the
URL previously instantiated.

An InputStream object is returned and permits to retrieve
information specified into the URL.

If the connection fails, the exception IOException is raised.
Creating a URL Instance
The following statement creates a Java URL object:
String str = "http://www.sun.com”;
try
{
URL location = new URL(str);
}
catch(MalformedURLException ex)
{ }
Example 18.6: Retrieving Remote Files
Rather than reading the file from the local system, this example
reads the file from a Web server.
private void showFile() {
URL url = null;
try {
url = new URL(urlString);
InputStream is = url.openStream();
infile = new BufferedReader(
new InputStreamReader(is));
}
catch (FileNotFoundException e) { ... }
catch (IOException e) { ... }
Viewing HTML Pages

Given the URL of the page, a Web browser can view an
HTML page—for example, http://www.sun.com.

HTTP (Hypertext Transfer Protocol) is the common
standard used for communication between a Web
server and the Internet. You can open a URL and
view a Web page in a Java applet.
Example 18.5 Viewing HTML Pages from Java
The following figure shows the process by which
an applet/application reads the files on the Web
server:
Web Server
Local file
Internet
Web Browser
Application reads the file
Applet reads the file
Example 18.5 Viewing HTML Pages from Java
The following figure shows the process by which
an applet/application reads the files on the Web
server:
Web Server
Local file
Internet
Web Browser
Application reads the file
Applet reads the file