What is the difference between a URI, a URL and a URN
跟web端经常打交道，难免遇到一些难懂的名词。
今天就来看一些什么是uri url urn。
看看uri:
A URI can be further classified as a locator, a name, or both. The term "Uniform Resource Locator" (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location"). The term "Uniform Resource Name" (URN) has been used historically to refer to both URIs under the "urn" scheme [RFC2141], which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name.URI – Uniform Resource Identifier
URIs are a standard for identifying documents using a short string of numbers, letters, and symbols. They are defined by RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax. URLs, URNs, and URCs are all types of URI.
URL – Uniform Resource Locator
Contains information about how to fetch a resource from its location. For example:
http://example.com/mypage.html
ftp://example.com/download.zip
mailto:user@example.com
file:///home/user/file.txt
tel:1-888-555-5555
http://example.com/resource?foo=bar#fragment
/other/link.html (A relative URL, only useful in the context of another URL)URLs always start with a protocol (http) and usually contain information such as the network host name (example.com) and often a document path (/foo/mypage.html). URLs may have query parameters and fragment identifiers.
URN – Uniform Resource Name
Identifies a resource by a unique and persistent name, but doesn’t necessarily tell you how to locate it on the internet. It usually starts with the prefix urn: For example:
urn:isbn:0451450523 to identify a book by its ISBN number.
urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66 a globally unique identifier
urn:publishing:book - An XML namespace that identifies the document as a type of book.URNs can identify ideas and concepts. They are not restricted to identifying documents. When a URN does represent a document, it can be translated into a URL by a “resolver”. The document can then be downloaded from the URL.
Difference between URL and URI
A URI is an identifier for some resource, but a URL gives you specific information as to obtain that resource. A URI is a URL and as one commenter pointed out, it is now considered incorrect to use URL when describing applications. Generally, if the URL describes both the location and name of a resource, the term to use is URI. Since this is generally the case most of us encounter everyday, URI is the correct term.
URI，是uniform resource identifier，统一资源标识符，用来唯一的标识一个资源。
而URL是uniform resource locator，统一资源定位器，它是一种具体的URI，即URL可以用来标识一个资源，而且还指明了如何locate这个资源。
URN，uniform resource name，统一资源命名，是通过名字来标识资源，比如mailto:[email protected]。也就是说，URI是以一种抽象的，高层次概念定义统一资源标识，而URL和URN则是具体的资源标识的方式。URL和URN都是一种URI。
从HttpServletRequest的javadoc中可以看出，getRequestURI返回一个String，“the part of this request’s URL from the protocol name up to the query string in the first line of the HTTP request”，比如“POST /some/path.html?a=b HTTP/1.1”，则返回的值为”/some/path.html”。现在可以明白为什么是getRequestURI而不是getRequestURL了，因为此处返回的是相对的路径。而getRequestURL返回一个StringBuffer，“The returned URL contains a protocol, server name, port number, and server path, but it does not include query string parameters.”，完整的请求资源路径，不包括querystring。
URL是一种具体的URI，它不仅唯一标识资源，而且还提供了定位该资源的信息。URI是一种语义上的抽象概念，可以是绝对的，也可以是相对的，而URL则必须提供足够的信息来定位，所以，是绝对的，而通常说的relative URL，则是针对另一个absolute URL，本质上还是绝对的。