XSD : Java Glossary

XSD (XML Scheme Definition). W3C’s XML (extensible Markup Language) Schema that is itself a form of XML. It is often simply called XML Schema. It offers much finer control of XML document content than the older DTD (Document Type Definition)-style schema borrowed from HTML. XSD has a schema written in XSD used to validate other schemas. You can download it. It is 88k. Unfortunately, by default, Opera treats it as raw text. IE nicely lists it with colours.

The two main advantages of XSD over DTD for specifying an XML grammar are:

The XSD grammar of the schema is simpler than DTD. XSD is just a flavour of XML.
XSD lets you restrict in much more detail just what constitutes a valid file.

Sample XSD Schemas	Extracting data from an XML file
Understanding an XSD Schema	Learning More
Validating an XML file conforming to an XSD schema	Links

Sample XSD Schemas

The academics who wrote the XSD spec were more interested in impressing you than informing you. Therefore there are no examples or even anything remotely like English language descriptions of what the various grammatical elements are for. Your only hope of making sense of it is to find example documents. Even the primer fairly tough slogging. Keep looking at the example XML and XSD to clarify the text. People can learn languages from a set of examples, heavily commented gradually adding features much more easily than from descriptions of the grammar in some esoteric formal

Note how it allows forward and backward references to permit a top-down description of the document. In typical XML fashion, it is revoltingly verbose. Oddly, you specify the attributes on a tag after you describe all the nested tags that tag may enclose, even though when you write the actual XML the schema describes, the attributes come first. NMTOKEN is an atomic string without spaces, often the name of an enumeration value. The 2-letter country codes would be NMTOKEN. XML lets you specify the types of the fields with a rich set of built-in types which included bounded integers, float, double, fixed decimal, dates, times, strings, urls, hex, Boolean, durations… You can set up enumerated types where you give a list of the legal values of a field. There is even a pattern scheme, similar to Perl regex, for describing legal string values. XSD also allows you to enforce ordering of fields. Complete list of types.

XSD allows you to specify the minimum and maximum number of times a field may appear with the minOccurs="0" maxOccurs="unbounded".

You can specify the types of fields with: type="xsd:positiveInteger" type="xsd:string" type="xsd:anyURI" options.

You can specify the allowable low and high bounds on a numeric field with: mininclusive and maxinclusive.

There is a scheme to insist a data value be unique.

Sometimes the files are peppered with xs: and sometimes with xsd:. This is an arbitary string to abbreviate the xmlns name space defined at http://www.w3.org/2001/XMLSchema. You can make it anything you like so long as it you use it consistently. It lets the parser know that a word is a keyword. This way you can accidentally use keywords for field names without confusion.

Understanding an XSD Schema

Understanding the keywords used in schemas, and comparing a schema with a known valid compliant XML/jnlp file will be almost all you need to make sense of the schema. After you read the XSD tutorial, this list will refresh your memory.

all: you must supply exactly one of each of a group of tags.
attribute: a keyword="value" modifier on a tag.
choice: A group of possible tags. You can specify only one of them.
complexType: A tag that contains other tags nested inside it.
dateTime: A date/time in the form 2009-12-13T12:25:00.0000000-08:00
default: Specifies the default value for an attribute.
element: a tag that must appear in a particular order within a sequence group.
enumeration: describes one possible value of an attribute that has only a limited set of legal values.
length: the precise length for this field in characters. More commonly you use minLength and maxLength.
maxLength: the maximum length for this field in characters.
maxOccurs: the maximum number of times this tag can appear, "unbounded" for no upper limit. Oddly, must go on the ref not the target.
minLength: the minimum length for this field in characters.
minOccurs: the minimum number of times this tag can appear, "0" for optional. Oddly, must go on the ref not the target.
NCName: describes a field that can have pretty well any character, including spaces, except a colon.
NMTOKEN: describes an enumerated field that can only have a value selected from a list. Letters, digits, period, colons, hyphens but no commas or spaces. You can have enumerated values based on strings, which do allow spaces.
restriction: describes an attribute whose value is restricted in some way.
sequence: a group of tags that must appear in a particular order.
simpleType: usually describes an attribute that has restrictions on it.
string: a field that can include any Unicode-16 character.
use: either "required" or "optional". It applies to attributes, not tags..

Stylus Studio has a wizard that will take a well formed XML document and compose an approximate XSD schema for it. You can then fine tune it. This greatly speeds up the work of composing schemas. You can keep validating your schema as you work. As you type it shows you multiple choices for what you most likely want to type next.

Validating an XML File Conforming to an XSD Schema

Here is an example of validating XML with an XSD schema. This schema describes a valid JNLP (Java Network Launching Protocol) 1.0 XML file. You can check that your JNLP file is correctly formed using an XSD Schema originally from Vampqh. You must copy the JNLP 1.0 XSD schema posted below into the current directory as file jnlp1.xsd or use the JNLP 6.0 XSD jnlp6.xsd then run the Java validation posted below with:

rem Validate a JNLP file with CMP ValidateJNLP utility
java.exe ValidateJNLP jnlp6.xsd C:\mydir\myapp.jnlp

The above validator is not user friendly. If all is ok, it prints nothing. If there is problem, you get a cryptic exception. You can get a good idea what it is looking for by reading the XSD file. I have composed three schemas for it jnlp1.xsd, jnlp5.xsd and jnlp6.xsd. Use the one that corresponds to the version of your JNLP. XSDs are a bit like a BNF (Backus-Naur Form) description of JNLP, written by someone with a terrible lexical stutter.

Extracting Data From an XML File

Extracting Data from an xml file with an XSD schema is verbose undertaking. Unfortunately, the ranges, defaults etc. in the XSD schema, are all ignored when you extract information from a conforming xml file. They are just used for validating. Here are some of the classes you will

There are dozens of classes in other packages with identical or similar names. You have to make sure you use matching classes. Your IDE (Integrated Development Environment) might automatically import the wrong classes if you are not careful.

I am in the process of writing an HtmlReflow utility (to reflow/tidy HTML (Hypertext Markup Language)) that uses an XSD schema to describe an xml configuration file. You can have a look at my code in progress:

Java Program to validate the XML and extract the data from it.
XSD schema which is an xml file itself.
XML config file that conforms to the schema

Learning More

Oracle’s Javadoc on Schema class : available:

on the web at Oracle.com
in the current JDK 1.7.0_04 or in the old JDK 1.6.0_32 on your local Windows J: drive.

Oracle’s Javadoc on SchemaFactory class : available:

on the web at Oracle.com
in the current JDK 1.7.0_04 or in the old JDK 1.6.0_32 on your local Windows J: drive.

Oracle’s Javadoc on Validator class : available:

on the web at Oracle.com
in the current JDK 1.7.0_04 or in the old JDK 1.6.0_32 on your local Windows J: drive.

Oracle’s Javadoc on XMLConstants class : available:

on the web at Oracle.com
in the current JDK 1.7.0_04 or in the old JDK 1.6.0_32 on your local Windows J: drive.

Oracle’s Javadoc on SAXParser class : available:

on the web at Oracle.com
in the current JDK 1.7.0_04 or in the old JDK 1.6.0_32 on your local Windows J: drive.

DOM
DTD
HitSoftware: DTD ⇒ XSD
JAXP
JNLP
Liquid Technologies XSD validator
online DTD → XSD conversion
RELAX NG
schema
Schematron
Stylus Studio: a commercial schema generator
Various schemas for the XSD schema language itself
WSDL
Xerces: parses and does schema validation
XML
xmlns
XSD datatypes
XSD primer
XSD structures
XSD Tools: e.g. DTD ⇒ XSD
XSD tutorial

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