What is an XML模式?
An XML模式 describes the structure of an XML document.
The XML模式 language is also referred to as XML模式 Definition (XSD).
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
The purpose of an XML模式 is to define the legal building blocks of an XML document:
- the elements and attributes that can appear in a document
- the number of (and order of) child elements
- data types for elements and attributes
- default and fixed values for elements and attributes
In the XML world, hundreds of standardized XML formats are in daily use.
Many of these XML standards are defined by XML模式s.
XML模式 is an XML-based (and more powerful) alternative to DTD.
XML模式s Support Data Types
One of the greatest strength of XML模式s is the support for data types.
- It is easier to describe allowable document content
- It is easier to validate the correctness of data
- It is easier to define data facets (restrictions on data)
- It is easier to define data patterns (data formats)
- It is easier to convert data between different data types
XML模式s use XML语法
Another great strength about XML模式s is that they are written in XML.
- You don't have to learn a new language
- You can use your XML editor to edit your Schema files
- You can use your XML parser to parse your Schema files
- You can manipulate your Schema with the XML DOM
- You can transform your Schema with XSLT
XML模式s are extensible, because they are written in XML.
With an extensible Schema definition you can:
- Reuse your Schema in other Schemas
- Create your own data types derived from the standard types
- 参考 multiple schemas in the same document
XML模式s Secure Data Communication
When sending data from a sender to a receiver, it is essential that both parts have the same "expectations" about the content.
With XML模式s, the sender can describe the data in a way that the receiver will understand.
A date like: "03-11-2004" will, in some countries, be interpreted as 3.November and in other countries as 11.March.
However, an XML element with a data type like this:
ensures a mutual understanding of the content, because the XML data type "date" requires the format "YYYY-MM-DD".
Well-Formed is Not Enough
A well-formed XML document is a document that conforms to the XML syntax rules, like:
- it must begin with the XML declaration
- it must have one unique root element
- start-tags must have matching end-tags
- elements are case sensitive
- all elements must be closed
- all elements must be properly nested
- all attribute values must be quoted
- entities must be used for special characters
Even if documents are well-formed they can still contain errors, and those errors can have serious consequences.
Think of the following situation: you order 5 gross of laser printers, instead of 5 laser printers. With XML模式s, most of these errors can be caught by your validating software.