What is on the first line of an XML document?

问题

So before we write the XML Schema, a lot of tutorials use this:

 <?xml version='1.0'?>

or

 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 <data-set xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

My question is, what is this part for? What is that website specifically and why do we use it? Are there other methods to do this?

If it helps, I am doing this to convert an excel worksheet into XML.

回答1:

XML Declaration

<?xml version='1.0'?> is an XML declaration and is not particular to XSDs but to XML documents in general.

[Definition: XML documents should begin with an XML declaration which specifies the version of XML being used.]

As an XSD is an XML document, it may also have an XML declaration.

Here is the BNF of an XML declaration (XMLDecl) with links to the definitions of its constituent parts:

XMLDecl ::= '<?xml'VersionInfo EncodingDecl? SDDecl? S? '?>'

Note: Only one XML declaration is permitted in well-formed XML, and it must be at the top if anywhere. If you violate this requirement, you'll see an error such as

The processing instruction target matching "[xX][mM][lL]" is not allowed.

and you'll have to fix the problem before your XML will be well-formed.

---

XML Schema Instance Namespace

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" is a namespace declaration for the special XML Schema instance namespace. As a namespace URI, it's purpose is to facilitate control of grouping of component name. An XML namespace URI does not have to be retrievable.

See also

• What are XML namespaces for regarding the purpose of namespaces in general.
• Schema-Related Markup in Documents Being Validated for XSD attributes that use xsi in particular. (xsi:type, xsi:nil, xsi:schemaLocation, and xsi:noNamespaceSchemaLocation)

回答2:

XML declaration. See https://www.w3.org/TR/2006/REC-xml-20060816/#sec-prolog-dtd for more information.

回答3:

<?xml version='1.0'?>

1.0 is the current version of XML. And the number might change for future versions. This is a mandatory field and it indicates which version of XML standard this file conforms to.

encoding="UTF-8"

This mean the file is encoded in UTF-8. This is optional since this is the default character encoding in XML.

standalone="yes"

standalone indicates whether if the current XML document depends on an external markup declaration. This is also optional.

<data-set xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

This is the XML Schema Instance namespace. After having this declared you can use attributes such as schemaLocation.

See similar answer: Is xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" a special case in XML?

来源：https://stackoverflow.com/questions/35835870/what-is-on-the-first-line-of-an-xml-document