URLs and URIs - Java Network Programming - page 121

or how to get that resource. In the physical world, it's the difference between the title
“Harry Potter and The Deathly Hallows” and the library location “Room 312, Row 28,
Shelf 7”. In Java, it's the difference between the java.net.URI class that only identifies
resources and the java.net.URL class that can both identify and retrieve resources.
The network location in a URL usually includes the protocol used to access a server
(e.g., FTP, HTTP), the hostname or IP address of the server, and the path to the resource
on that server. A typical URL looks like http://www.ibiblio.org/javafaq/javatutori‐
al.html . This specifies that there is a file called javatutorial.html in a directory called
javafaq on the server www.ibiblio.org , and that this file can be accessed via the HTTP
protocol.
The syntax of a URL is:
protocol :// userInfo @ host : port / path ? query # fragment
Here the protocol is another word for what was called the scheme of the URI.
( Scheme is the word used in the URI RFC. Protocol is the word used in the Java docu‐
mentation.) In a URL, the protocol part can be file , ftp , http , https , magnet , telnet , or
various other strings (though not urn ).
The host part of a URL is the name of the server that provides the resource you want.
It can be a hostname such as www.oreilly.com or utopia.poly.edu or an IP address, such
as 204.148.40.9 or 128.238.3.21.
The userInfo is optional login information for the server. If present, it contains a user‐
name and, rarely, a password.
The port number is also optional. It's not necessary if the service is running on its default
port (port 80 for HTTP servers).
Together, the userInfo, host, and port constitute the authority .
The path points to a particular resource on the specified server. It often looks like a
filesystem path such as /forum/index.php . However, it may or may not actually map to
a filesystem on the server. If it does map to a filesystem, the path is relative to the
document root of the server, not necessarily to the root of the filesystem on the server.
As a rule, servers that are open to the public do not show their entire filesystem to clients.
Rather, they show only the contents of a specified directory. This directory is called the
document root, and all paths and filenames are relative to it. Thus, on a Unix server, all
files that are available to the public might be in /var/public/html , but to somebody con‐
necting from a remote machine, this directory looks like the root of the filesystem.
The query string provides additional arguments for the server. It's commonly used only
in http URLs, where it contains form data for input to programs running on the server.
Finally, the fragment references a particular part of the remote resource. If the remote
resource is HTML, the fragment identifier names an anchor in the HTML document.