About
A Uniform Resource Identifier (URI) is a string of characters that Identify a resource.
Because a resource can be created dynamically, an URI is also a request.
Example
URL and template
http://example.com/product/id?sort=asc
- As template
http://{host}/product/{id}{?sort}
ISBN
The ISBN system for uniquely identifying books provides a typical example of the use of URNs. ISBN 0486275574 (urn:isbn:0-486-27557-4) cites, unambiguously, a specific edition of Shakespeare's play Romeo and Juliet.
To get this book, a location is needed: a URL address. A typical URL for this book on a Unix-like operating system would be a file path such as file:///home/username/RomeoAndJuliet.pdf, identifying the electronic book saved in a file on a local hard disk.
Address Mail
To open the email client application and send an email, the mailto scheme
<a href="mailto:[email protected]">link</a>
FTP
ftp://ftp.is.co.za/rfc/rfc1808.txt
Ldap
ldap://[2001:db8::7]/c=GB?objectClass?one
Telephone
tel:+1-816-555-1212
URN example
urn:oasis:names:specification:docbook:dtd:xml:4.1.2
Telnet
telnet://192.0.2.16:80/
News Room
news:comp.infosystems.www.servers.unix
Type (URI, URL and URN)
URIs can be of the type:
- names
- locators
- or both.
| Type | Description | Goal | Example |
|---|---|---|---|
| Names | Uniform resource name (URN) | To defines an item's identity | A person's name |
| Locators | Uniform resource locator (URL) | To provide a method for finding an item | person's street address |
Classification
Opaque
An opaque URI is an absolute URI whose scheme-specific part does not begin with a slash character ('/').
Opaque URIs are not subject to further parsing. Some examples of opaque URIs are:
- news:comp.lang.java
- urn:isbn:096139210x
Hierarchical
A hierarchical URI is subject to further parsing according to the syntax
URI = scheme ":" (/|//) [authority] path [ "?" query ] [ "#" fragment ]
where:
- the characters ,, /,?, and # stand for themselves.
- the scheme is required
- the path is required but may be empty
- /|// means that when authority is:
- present, the path must either be empty or begin with a slash (/) character.
- not present, the path cannot begin with two slash characters (//).
Type
Absolute
A URI is absolute if, and only if, it has a scheme component.
Absolute URI = URI without fragment identifier
An absolute hierarchical URI reference follows the pattern: ://?#
absolute-URI = scheme ":" hier-part [ "?" query ]
More see Absolute URI
Relative
A URI that is not absolute is said to be relative
Relative URI references (which are always hierarchical) follow one of two patterns: ?# or //?#
Opaque
An opaque URI follows this pattern: :#
Token
Components of a parsed URL, which is the result of processing a URL through the URL parser
Scheme
Authority
Path
See URI - Path
Query
A query string is the part of a URI that defines key/value data.
Fragment
Interpretation and Access
Interpretation is independent of access. “http://localhost/” has the same interpretation for every user of that reference, even though the network interface corresponding to “localhost” may be different for each end-user:
However, an action made on the basis of that reference will take place in relation to the end-user's context.
Global vs user context
An action intended to refer to a globally unique thing must use a URI that distinguishes that resource from all other things. URIs that identify in relation to the end-user's local context should only be used when the context itself is a defining aspect of the resource, such as when an on-line help manual refers to a file on the end-user's file system (e.g., "file:///etc/hosts").
Management
Encoding
Because an URL uses characters to delimit its component such as the /, ?, #, if you want to use this character in a URL as value, you need to encode them.
Compare
Example: Simple String Comparison
Template
You can create templates of URI and expand them to create URI with variables.
Specifications
For URI:
- The W3C URL specification defines the term url, various algorithms for dealing with URLs, and an API for constructing, parsing, and resolving URLs.