Class
URL
represents a Uniform Resource
Locator, a pointer to a "resource" on the World
Wide Web. A resource can be something as simple as a file or a
directory, or it can be a reference to a more complicated object,
such as a query to a database or to a search engine. More
information on the types of URLs and their formats can be found at:
http://www.socs.uts.edu.au/MosaicDocs-old/url-primer.html
In general, a URL can be broken into several parts. The previous
example of a URL indicates that the protocol to use is
http
(HyperText Transfer Protocol) and that the
information resides on a host machine named
www.socs.uts.edu.au
. The information on that host
machine is named /MosaicDocs-old/url-primer.html
. The exact
meaning of this name on the host machine is both protocol
dependent and host dependent. The information normally resides in
a file, but it could be generated on the fly. This component of
the URL is called the path component.
A URL can optionally specify a "port", which is the
port number to which the TCP connection is made on the remote host
machine. If the port is not specified, the default port for
the protocol is used instead. For example, the default port for
http
is 80
. An alternative port could be
specified as:
http://www.socs.uts.edu.au:80/MosaicDocs-old/url-primer.html
The syntax of URL
is defined by RFC 2396: Uniform
Resource Identifiers (URI): Generic Syntax, amended by RFC 2732: Format for
Literal IPv6 Addresses in URLs. The Literal IPv6 address format
also supports scope_ids. The syntax and usage of scope_ids is described
here.
A URL may have appended to it a "fragment", also known
as a "ref" or a "reference". The fragment is indicated by the sharp
sign character "#" followed by more characters. For example,
http://java.sun.com/index.html#chapter1
This fragment is not technically part of the URL. Rather, it
indicates that after the specified resource is retrieved, the
application is specifically interested in that part of the
document that has the tag chapter1
attached to it. The
meaning of a tag is resource specific.
An application can also specify a "relative URL",
which contains only enough information to reach the resource
relative to another URL. Relative URLs are frequently used within
HTML pages. For example, if the contents of the URL:
http://java.sun.com/index.html
contained within it the relative URL:
FAQ.html
it would be a shorthand for:
http://java.sun.com/FAQ.html
The relative URL need not specify all the components of a URL. If
the protocol, host name, or port number is missing, the value is
inherited from the fully specified URL. The file component must be
specified. The optional fragment is not inherited.
The URL class does not itself encode or decode any URL components
according to the escaping mechanism defined in RFC2396. It is the
responsibility of the caller to encode any fields, which need to be
escaped prior to calling URL, and also to decode any escaped fields,
that are returned from URL. Furthermore, because URL has no knowledge
of URL escaping, it does not recognise equivalence between the encoded
or decoded form of the same URL. For example, the two URLs:
http://foo.com/hello world/ and http://foo.com/hello%20world
would be considered not equal to each other.
Note, the URI
class does perform escaping of its
component fields in certain circumstances. The recommended way
to manage the encoding and decoding of URLs is to use URI
,
and to convert between these two classes using URL.toURI()
and
URI.toURL()
.
The URLEncoder
and URLDecoder
classes can also be
used, but only for HTML form encoding, which is not the same
as the encoding scheme defined in RFC2396.