java applet qr code XML Schema and XML Namespaces in Java

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XML Schema and XML Namespaces
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We ll start with the basics of XML Schema and XML Namespaces. It s assumed that you already understand how to use basic XML elements and attributes. If you don t, you should probably read a primer on XML before proceeding. We recommend the book Learning XML (O Reilly). If you already understand how XML Schema and XML Namespaces work, skip ahead to the section on SOAP.
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An XML Schema is similar in purpose to a Document Type Definition (DTD), which validates the structure of an XML document. To illustrate some of the basic concepts of XML Schema, let s start with an XML document with address information:
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< xml version='1.0' encoding='UTF-8' standalone='yes' > <address> <street>800 Langdon Street</street> <city>Madison</city> <state>WI</state> <zip>53706</zip> </address>
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In order to ensure that the XML document contains the proper type of elements and data, the address information must be evaluated for correctness. You measure the correctness of an XML document by determining two criteria: whether the document is well formed and whether it is valid. To be well formed, an XML document must obey the syntactic rules of the XML markup language: it must use proper attribute declarations, the correct characters to denote the start and end of elements, and so on. Most XML parsers based on standards such as SAX and DOM automatically detect documents that aren t well formed. In addition to being well formed, the document must use the right types of elements and attributes in the correct order and structure. A document that meets these criteria is called valid. However, the criteria for validity have nothing to do with XML itself; they have more to do with the application in which the document is used. For example, the Address document would not be valid if it didn t include the zip code or state elements. In order to validate an XML document, you need a way to represent these application-specific constraints. The XML Schema for the Address XML document looks like this:
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< xml version='1.0' encoding='UTF-8' > <schema xmlns="http://www.w3.org/2001/XMLSchema"
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xmlns:titan="http://www.titan.com/Reservation" targetNamespace="http://www.titan.com/Reservation"> <element name="address" type="titan:AddressType"/> <complexType name="AddressType"> <sequence> <element name="street" type="string"/> <element name="city" type="string"/> <element name="state" type="string"/> <element name="zip" type="string"/> </sequence> </complexType> </schema>
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The first thing to focus on in this XML Schema is the <complexType> element, which declares a type of element in much the same way that a Java class declares a type of object. The <complexType> element explicitly declares the names, types, and order of elements that an AddressType element may contain. In this case, it may contain four elements of type string, in the following order: street, city, state, and zip. Validation is pretty strict, so any XML document that claims conformity to this XML Schema must contain exactly the right elements with the right data types, in the correct order. XML Schema automatically supports about two dozen simple data types, called builtin types. Built-in types are a part of the XML Schema language and are automatically supported by any XML Schema compliant parser. Table 20-1 provides a short list of some of the built-in types. It also includes Java types that correspond to each built-in type. (Table 20-1 presents only a subset of all the XML Schema [XSD] built-in types, but it s more than enough for this book.)
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Table 20-1. XML Schema built-in types and their corresponding Java types XML Schema built-in type
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byte boolean short int long float double string dateTime integer decimal
Java type
Byte, byte Boolean, boolean Short, short Integer, int Long, long Float, float Double, double java.lang.String java.util.Calendar java.math.BigInteger java.math.BigDecimal
By default, each element declared by a <complexType> must occur once in an XML document, but you can specify that an element is optional or that it must occur more than once by using the occurrence attributes. For example, we can say that the street element must occur once but may occur twice:
<complexType name="AddressType"> <sequence> <element name="street" type="string" maxOccurs="2" minOccurs="1" /> <element name="city" type="string"/> <element name="state" type="string"/> <element name="zip" type="string"/> </sequence> </complexType>
By default, the maxOccurs and minOccurs attributes are always 1, indicating that the element must occur exactly once. Setting maxOccurs to 2 allows an XML document to have either two street elements or just one. You can also set maxOccurs to unbounded, which means the element may occur as many times as needed. Setting minOccurs to 0 means the element is optional and can be omitted. The <element> declarations are nested under a <sequence> element, which indicates that the elements must occur in the order they are declared. You can also nest the elements under an <all> declaration, which allows the elements to appear in any order. The following shows the AddressType declared with an <all> element rather than a <sequence> element:
<complexType name="AddressType"> <all> <element name="street" type="string" maxOccurs="2" minOccurs="1" /> <element name="city" type="string"/> <element name="state" type="string"/> <element name="zip" type="string"/> </all> </complexType>
In addition to declaring elements of XSD built-in types, you can declare elements based on complex types. This is similar to how Java class types declare fields that are other Java class types. For example, we can define a CustomerType that uses the AddressType:
< xml version='1.0' encoding='UTF-8' > <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:titan="http://www.titan.com/Reservation" targetNamespace="http://www.titan.com/Reservation"> <element name="customer" type="titan:CustomerType"/> <complexType name="CustomerType"> <sequence> <element name="last-name" type="string"/> <element name="first-name" type="string"/> <element name="address" type="titan:AddressType"/>
</sequence> </complexType> <complexType name="AddressType"> <sequence> <element name="street" type="string" /> <element name="city" type="string"/> <element name="state" type="string"/> <element name="zip" type="string"/> </sequence> </complexType> </schema>
This XSD tells us that an element of CustomerType must contain a <last-name> and <first-name> element of the built-in type string, and an element of type AddressType. This is straightforward, except for the titan: prefix on AddressType. That prefix identifies the XML Namespace of the AddressType; we ll discuss namespaces later in the chapter. For now, just think of it as declaring that the AddressType is a custom type defined by Titan Cruises. It s not a standard XSD built-in type. An XML document that conforms to the Customer XSD would look like this:
< xml version='1.0' encoding='UTF-8' > <customer> <last-name>Jones</last-name> <first-name>Sara</first-name> <address> <street>3243 West 1st Ave.</street> <city>Madison</city> <state>WI</state> <zip>53591</zip> </address> </customer>
Building on what you ve learned so far, we can create a Reservation schema using the CustomerType, the AddressType, and a new CreditCardType:
< xml version='1.0' encoding='UTF-8' > <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:titan="http://www.titan.com/Reservation" targetNamespace="http://www.titan.com/Reservation"> <element name="reservation" type="titan:ReservationType"/> <complexType name="ReservationType"> <sequence> <element name="customer" type="titan:CustomerType"/> <element name="cruise-id" type="int"/> <element name="cabin-id" type="int"/> <element name="price-paid" type="double"/> </sequence> </complexType> <complexType name="CustomerType"> <sequence> <element name="last-name" type="string"/> <element name="first-name" type="string"/>
<element name="address" type="titan:AddressType"/> <element name="credit-card" type="titan:CreditCardType"/> </sequence> </complexType> <complexType name="CreditCardType"> <sequence> <element name="exp-date" type="dateTime"/> <element name="number" type="string"/> <element name="name" type="string"/> <element name="organization" type="string"/> </sequence> </complexType> <complexType name="AddressType"> <sequence> <element name="street" type="string"/> <element name="city" type="string"/> <element name="state" type="string"/> <element name="zip" type="string"/> </sequence> </complexType> </schema>
An XML document that conforms to the Reservation XSD would include information describing the customer (name and address), credit card information, and the identity of the cruise and cabin that are being reserved. This document might be sent to Titan Cruises from a travel agency that cannot access the TravelAgent EJB to make reservations. Here s an XML document that conforms to the Reservation XSD:
< xml version='1.0' encoding='UTF-8' > <reservation> <customer> <last-name>Jones</last-name> <first-name>Sara</first-name> <address> <street>3243 West 1st Ave.</street> <city>Madison</city> <state>WI</state> <zip>53591</zip> </address> <credit-card> <exp-date>09-2007/exp-date> <number>0394029302894028930</number> <name>Sara Jones</name> <organization>VISA</organization> </credit-card> </customer> <cruise-id>123</cruise-id> <cabin-id>333</cabin-id> <price-paid>6234.55</price-paid> </reservation>
At runtime, the XML parser compares the document to its schema, ensuring that the document conforms to the schema s rules. If the document doesn t adhere to the schema, it is considered invalid, and the parser produces error messages. An XML
Schema checks that XML documents received by your system are properly structured, so you won t encounter errors while parsing the documents and extracting the data. For example, if someone sent your application a Reservation document that omitted the credit card element, the XML parser could reject the document as invalid before your code even sees it; you don t have to worry about errors in your code caused by missing information in the document. This brief overview represents only the tip of the iceberg. XML Schema is a very rich XML typing system and can be given sufficient attention only in a text dedicated to the subject. For in-depth and insightful coverage of XML Schema, read XML Schema: The W3C s Object-Oriented Descriptions for XML (O Reilly), or read the XML Schema specification, starting with the primer at the World Wide Web Consortium (W3C) website (http://www.w3.org/TR/xmlschema-0).
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