What does xmlns in XML mean

Question

I saw the following line in an XML file   xmlns android  http   schemas android com apk res android    I have also seen xmlns in many other XML files that I ve come across   What is it

User · Answer

You have name spaces so you can have globally unique elements. However, 99% of the time this doesn't really matter, but when you put it in the perspective of The Semantic Web, it starts to become important.

For example, you could make an XML mash-up of different schemes just by using the appropriate xmlns. For example, mash up friend of a friend with vCard, etc.

User · Answer

It means XML namespace   Basically  every element  or attribute  in XML belongs to a namespace  a way of  qualifying  the name of the element   Imagine you and I both invent our own XML  You invent XML to describe people  I invent mine to describe cities  Both of us include an element called name  Yours refers to the person   s name  and mine to the city name   OK  it   s a little bit contrived    lt person gt       lt name gt Rob lt  name gt       lt age gt 37 lt  age gt       lt homecity gt           lt name gt London lt  name gt           lt lat gt 123 000 lt  lat gt           lt long gt 0 00 lt  long gt       lt  homecity gt   lt  person gt    If our two XMLs were combined into a single document  how would we tell the two names apart  As you can see above  there are two name elements  but they both have different meanings   The answer is that you and I would both assign a namespace to our XML  which we would make unique    lt personxml person xmlns personxml  http   www your example com xml person                    xmlns cityxml  http   www my example com xml cities  gt       lt personxml name gt Rob lt  personxml name gt       lt personxml age gt 37 lt  personxml age gt       lt cityxml homecity gt           lt cityxml name gt London lt  cityxml name gt           lt cityxml lat gt 123 000 lt  cityxml lat gt           lt cityxml long gt 0 00 lt  cityxml long gt       lt  cityxml homecity gt   lt  personxml person gt    Now we   ve fully qualified our XML  there is no ambiguity as to what each name element means  All of the tags that start with personxml  are tags belonging to your XML  all the ones that start with cityxml  are mine   There are a few points to note    If you exclude any namespace declarations  things are considered to be in the default namespace  If you declare a namespace without the identifier  that is  xmlns  http   somenamespace   rather than xmlns rob  somenamespace   it specifies the default namespace for the document  The actual namespace itself  often a IRI  is of no real consequence  It should be unique  so people tend to choose a IRI URI that they own  but it has no greater meaning than that  Sometimes people will place the schema  definition  for the XML at the specified IRI  but that is a convention of some people only  The prefix is of no consequence either  The only thing that matters is what namespace the prefix is defined as  Several tags beginning with different prefixes  all of which map to the same namespace are considered to be the same   For instance  if the prefixes personxml and mycityxml both mapped to the same namespace  as in the snippet below   then it wouldn t matter if you prefixed a given element with personxml or mycityxml  they d both be treated as the same thing by an XML parser  The point is that an XML parser doesn t care what you ve chosen as the prefix  only the namespace it maps too  The prefix is just an indirection pointing to the namespace    lt personxml person       xmlns personxml  http   example com same url       xmlns mycityxml  http   example com same url    gt   Attributes can be qualified but are generally not  They also do not inherit their namespace from the element they are on  as opposed to elements  see below     Also  element namespaces are inherited from the parent element  In other words I could equally have written the above XML as   lt person xmlns  http   www your example com xml person  gt       lt name gt Rob lt  name gt       lt age gt 37 lt  age gt       lt homecity xmlns  http   www my example com xml cities  gt           lt name gt London lt  name gt           lt lat gt 123 000 lt  lat gt           lt long gt 0 00 lt  long gt       lt  homecity gt   lt  person gt

User · Answer

I think the biggest confusion is that xml namespace is pointing to some kind of URL that doesn t have any information  But the truth is that the person who invented below namespace   xmlns android  http   schemas android com apk res android    could also call it like that   xmlns android  asjkl fhgaslifujhaslkfjhliuqwhrqwjlrknqwljk rho il    This is just a unique identifier  However it is established that you should put there URL that is unique and can potentially point to the specification of used tags attributes in that namespace  It s not required tho   Why it should be unique  Because namespaces purpose is to have them unique so the attribute for example called background from your namespace can be distinguished from the background from another namespace   Because of that uniqueness you do not need to worry that if you create your custom attribute you gonna have name collision

User · Answer

xmlns - xml namespace  It s just a method to avoid element name conflicts  For example    lt config xmlns rnc  URI1  xmlns bsc  URI2  gt     lt rnc node gt         lt rnc rncId gt 5 lt  rnc rncId gt     lt  rnc node gt      lt bsc node gt         lt bsc cId gt 5 lt  bsc cId gt     lt  bsc node gt   lt  config gt    Two different node elements in one xml file  Without namespaces this file would not be valid

User · Answer

It defines an XML Namespace     In your example  the Namespace Prefix is  android  and the Namespace URI is  http   schemas android com apk res android   In the document  you see elements like   lt android foo   gt   Think of the namespace prefix as a variable with a short name alias for the full namespace URI   It is the equivalent of writing  lt http   schemas android com apk res android foo   gt  with regards to what it  means  when an XML parser reads the document    NOTE  You cannot actually use the full namespace URI in place of the namespace prefix in an XML instance document   Check out this tutorial on namespaces  http   www sitepoint com xml-namespaces-explained

[xml] What does "xmlns" in XML mean?

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