What is the meaning of DOCTYPE in xml file?

In hibernate we use configuration and mapping xml files. In xml the first line will be version and then we specify DOCTYPE DTD line. Example:

<!DOCTYPE hibernate-mapping PUBLIC  "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

Can anybody please explain what is the meaning of this? I know that DTD is document type definition which is like defining grammar for the xml.

I want to know the attributes in开发者_开发知识库 this statement.

The line you refer to is the document type declaration.

It is documented in the W3C's XML Recommendation here: http://www.w3.org/TR/xml/#dt-doctype

It specifies a DTD that is to be used when processing the document. Two mechanisms are available for specifying this

1. an optional PUBLIC identifier (rare these days) which is, in your example, " -//Hibernate/Hibernate Mapping DTD 3.0//EN". The mechanism by which this is resolved to a DTD resource is application-specific.

2. A SYSTEM identifier, which in your example is "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd". This is typically interpreted as a URL from where the DTD can be retrieved.

A Doctype describes which DTD the XML file follows.

This:

1. Describes what elements are allowed inside what other elements (for the purpose of validation)
2. Describes what named character references are available (beyond the 5 XML built-ins of &, <, >, ' and ").