Java Glossary : URI

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URI

née URL, URIs (Uniform Resource Identifier) are a platform-independent way to specify a file or resource somewhere on the web. Strictly speaking, every URL is also a URI, but not every URI is also a URL. Two RFCs specify the format of a URI: RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax, amended by RFC 2732: Format for Literal IPv6 Addresses in URIs There are many types of URI. Clever users can even invent new types on the fly, and register them in the java.protocol.handler.pkgs system property.

URIs must be constructed with the characters 0-9 a-z A-Z $-_.+!*'(), Other characters must be URI-encoded, e.g. space becomes %20 (because space = 0x20) or +, ;/?:@=& need be quoted only when they are not being used for their reserved purpose. For details see the Blooberry Essay. java.net.URLEncoder.encode will encode strings and java.net.URLDecoder.decode will decode strings. Earlier versions of Java were missing the decode and you had to roll your own.

You can specify IPv4 but not IPv6 numeric IP addresses in URIs.

Please report examples of other URI formats you see so I can add them to this list.

Types of URI
Example	Notes
AIM:	America Online Instant Messenger, an instant mail service.
dbaw://Gloria:8889	Symantec dbAnywhere. Gloria is the name of a local computer on the net with an IP in your HOSTS file. 8889 is the port.
dns://mynameserver.com/mindprod.com	JNDI specification of a site for which you want DNS (Domain Name Server) information. You feed these strange double-barrelled URLs to javax.naming.directory.Attributes.DirContext.getAttributes(). See JNDI for more details.
file:///C\|/mydir/myfile.txt	To get at a local file you must encode the name in a strange way. That example started out life as C:\mydir\myfile.txt . Use Netscape 4.79 to load a file and it will tell you the URL-encoded name on the top line. JNLP does not like this form.
file://localhost/C:/mydir/myfile.txt	This is a variant of the local file scheme above, popular in more recent browsers. This is the form JNLP likes.
file:////other/mydir/myfile.txt	Count'em four slashes after the file: Used to get at a file on remote LAN drive \\other\mydir\myfile.txt It is like the localhost form, with the host field and the drive field left blank.
file:/localhost//C\|/mydir/myfile.txt	This is the long form for accessing a local file. It could also be used to access a file on a computer attached via a LAN, by substituting localhost with the host name.
ftp://roedy:sesame@www.hans.org/downloads/getit.zip	for File Transfer Protocol downloads. roedy is the userid, and sesame is the password. For anonymous FTP, you can leave out the userid and password: ftp://www.hans.org/downloads/getit.zip FTP for Java client support
gopher://gopher.someplace.edu	Gopher protocol is an endangered species largely replaced by HTTP.
http://www.hans.org/index.html#FLUORIDE	Hypertext Transfer Protocol. It gives the site name and the document name within that site, and the spot within that document. Unfortunately, showDocument in older versions of Opera cannot handle a #FLOURIDE extension. html rendering
http://24.87.56.253:80/index.html	Hypertext Transfer Protocol, using an IP and port address 80. It gives the site name and the document name within that site.
myfile.txt	Relative URI. Usually this would get you a file on the mother web site from which the current page was loaded, relative to the web page being viewed. You can't use relative URIs directly. You must convert them to absolute URIs using the URI context constructor. An absolute URI must have a complete http: domain, filename and optional #REF. getDocumentBase and getCodeBase are useful in converting relative to absolute URIS. I know of no tools to go the other way to find the shortest relative URI to express a given absolute URI in a given context.
https://www.hans.org/index.html	Secure HTTP for SSL (encrypted secure socket) connections, only work inside Netscape. SSL: to learn how to add support in browsers that don't directly support https
icq:	ICQ instant messaging service. Also allows file transfer.
jar:file:///C\|/bar/baz.jar!/com/foo/Quux.class	Local jar file. In JDK1.2+ there are jar URIs for getting at the contents of the individual member of a local jar.
jar:http://www.foo.com/bar/baz.jar!/com/foo/Quux.class	Jar on server. In JDK1.2+ there are jar URIs for getting at the contents of the individual member of a remote jar.
jdbc:BorlandBroker://193.174.106.43:1600/sample,user,password	Typical JDBC connection.
mailto:someone@somedomain.com	Email
n2p:	Net To Phone. A scheme where you use the Internet to telephone people without Internet connections.
nap:	Napster MP3 file-sharing protocol. Allows peer-to-peer file transfers with a minimum of setup fuss.
news:comp.lang.java.programmer	Newsgroup. Needs a newsreader.
pnm:	Real Audio streaming format
rlogin:	remote login.
rmi://server:1099	rmi access the server.
rmi:Strawberry	rmi access an LDAP object.
rtsp:	Real Tme Streaming Protocol
telnet://melvyl.ucop.edu/	telnet
urn:isbn:096139210x	ISBN book number

In Java, the java.net.URI constructor will help you convert a relative URI to an absolute one given the context, e.g. in what webpage the URI link appears. For example, in the context of http://mindprod.com/index.html the relative uri jglossj.html#JAVACEXE is the same as absolute URI http://mindprod.com/jgloss/j.html#JAVACEXE .

However, I have not found any built-in way to convert an absolute URI to the most compact relative representation given a context. You would use this to correct HTML references to their most compact canonical form.

HTTP: URI Component Parts
http://roedy@www.mindprod.com:80/products/abc.html?type=all&colour=brown#DEF
Example Part	URI.method	Name
http://roedy@www.mindprod.com:80/products/abc.html?type=all&colour=brown#DEF	toString	url
http	getProtocol	protocol, scheme
roedy@www.mindprod.com:80	getAuthority	authority
roedy	getUserInfo	Userinfo, email address
www.mindprod.com	getHost	host
mindprod.com	-	domain
80	getPort	port, nearly always 80 for http.
/products/abc.html	getPath	path, URI: Uniform Resource Identifier
type=all&colour=brown	getQuery	query, used in CGI queries to pass data to the server.
DEF	getRef	ref, fragment, reference, target. Not technically part of the URI. Anchor in document to point to.

In windows 95/98/ME/NT/2000/XP, *.url files let you create short cuts (bookmarks) to web files on the Internet or your local disk. They are just little text files stored on your disk. If you peek inside one, they look something like this:

[DEFAULT]
BASEURL=file://E:\mindprod\jglosse.html
[InternetShortcut]
URL=file:///E:/mindprod/jglosse.html#ESSAYS
Modified=B0D4B81B0AADC001CF

When you double click them, they wake up a browser and display the web page. You create them inside Internet Explorer with right click | Save Target As .. .

URL ¤ URN

home	Canadian Mind Products
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