URL : Java Glossary

URL

URL (Uniform Resource Locator) The old name for URI (Uniform Resource Indicator).RFC 4248 describes URL formats. RFC 3986 describes URI formats. java.net.URL has been replaced by java.net.URI. Strictly speaking, every URL is a URI, but not every URI is a URL. URI ’s are an extension and refinement of the URL. Yet, as classes, neither is a subclass of the other and they have no common base other than Object. java.net. URI is Java’s second attempt at doin a URL class and they started from scratch.

URL s never automatically armour their parameters, where URI s sometimes do.

If a bug report I got is correct, under some conditionsJava version 1.6 URL.openConnection returns null without throwing an IOException.

The term URL can also be an as a verb, as in “Please URL me that picture of the Windows Firewall”, meaning “Please email me the URL of that photograph”, in other words email the text: http://mindprod.com/image/jgloss/msfirewall.jpg.

The new URL ( URL , String ) constructor lets you construct a new URL using the original as a context to modify. You do the equivalent thing with URI .resolve( String );

Non-ASCII URL s

What if you wanted to embed the following URL in your HTML (Hypertext Markup Language)?

What do you do about that pesky non-ASCII ç character? HTML 4.01 says URLs should only contain ASCII (American Standard Code for Information Interchange) chars, but if non-ASCII chars appear they should be treated as UTF-8. In theory you should be able to encode it with ç, ç or ç, but in practice, you can’t count on this working. You must use the URL-Encoded %hh escaping mechanism of the equivalent UTF-8 chars like this:

+ is converted to %2B and space is converted to +.

The URL constructor neither encodes nor decodes. It just leaves your blanks as either blanks or as %20, however you had them.

URLs always use UTF-8 encoding, no matter what the encoding of the enclosing document.

Tips

The constructor URL ( String spec ) does not handle relative URLs. You need a lead http:// https:// ftp:// etc. To convert a relative URL to an absolute one, use URL ( URL context, String spec).

Component Parts

HTTP (Hypertext Transfer Protocol) : URI Component Parts
http://roedy@www.mindprod.com:80/products/abc.html?type=all&colour=brown#FOXES
Example Part	URI.method	Name
http://roedy@www.mindprod.com:80/products/abc.html?type=all&colour=brown#FOXES	toString	url
http	getProtocol	protocol, scheme
roedy@www.mindprod.com:80	getAuthority	authority
roedy	getUserInfo	Userinfo, email address
www.mindprod.com	getHost	host
http://www.mindprod.com	getProtocol() + ':' getHost()	home page
mindprod.com	n/a	domain
80	getPort	port, nearly always 80 for http. http://localhost:8080 accesses a local Tomcat server. http://localhost:4848 accesses a local Glassfish admin port.
/products/abc.html	getPath	path, URI : URI. This might be a file or it might be a directory. You can’t tell other than by the hint of a dot in the filename. It might be empty or just a single /.
/products	n/a	just the directory
abc.html	n/a	just the file
html	n/a	just the extension
type=all&colour=brown	getQuery	query, used in CGI (Common Gateway Interface) queries to pass data to the server.
/products/abc.html?type=all&colour=brown	getFile	path + query if any. It does not get you the file name!
FOXES	URL.getRef	ref, fragment, reference, target. Not technically part of the URI. Anchor in document to point to.
FOXES	URI.getFragment	URI equivalent ofURL.getRef. part after #, ref, fragment, reference, target. Not technically part of the URI. Anchor in document to point to.

Learning More

Oracle’s Javadoc on URL class : available:

on the web at Oracle.com
in the current JDK 1.7.0_02 or in the old JDK 1.6.0_30 on your local Windows J: drive.

Oracle’s Javadoc on URI class : available:

on the web at Oracle.com
in the current JDK 1.7.0_02 or in the old JDK 1.6.0_30 on your local Windows J: drive.

Oracle’s tutorial on URL Introductory

firewall
getResource to create URLs
IDN
makeashorterlink
protocol handler
SnipURL
TinyURL
URI
url-encoded
URLConnection
URN

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