Java and Spring development

Introduction

This tutorial demonstrates how to generate an XSD schema from a sample XML document and how to improve the schema before generating JABX classes from it.

The sample document used throughout this tutorial contains an order element as you can see here:

<order>
	<date>2010-01-01</date>
	<id>12345</id>
	<item name="Effective Java" type="BOOK" quantity="1" />
	<item name="Spring in Action" type="BOOK" quantity="1" />
	<item name="Gladiator" type="DVD" quantity="1" />
</order>

The motivation for creating a sample document first, is that it’s more simple and less verbose to write than an XSD schema. You can also generate an XSD schema from JAXB classes, but this tutorial generates JAXB classes from an XSD schema. This is because an XSD schema is language neutral and more feature rich for describing a data contract. For example the ability to add restrictions with regular expressions like this:

<xs:attribute name="number" use="required">
	simpleType>
		<xs:restriction base="xs:string">
			<xs:pattern value="\d{10}" />
		</xs:restriction>
	</xs:simpleType>
</xs:attribute>

Generate XSD schema from XML sample document with Trang

The Trang project lives here: http://code.google.com/p/jing-trang/

You can execute Trang’s Driver class with the XML sample document’s path and the path for the schema that will be generated to generate the schema. Another and probably more common way to generate is with Ant like this target:

<target name="generate-order-schema" description="Generates schema from XML with Trang">
	<mkdir dir="${xsd-folder}" />

	<java jar="${lib-generate-folder}/trang.jar" fork="true">
		<arg value="${xml-example-folder}/order.xml" />
		<arg value="${xsd-folder}/order-generated.xsd" />
	</java>

	<echo message="${xsd-folder}/order-generated.xsd was generated" />
</target>

After you have executed this target, remember to update the folder with the generated file if you’re using Eclipse.

Tweaking the generated XSD schema

I only specified the input file and the path to the generated file when I executed Trang. This results in a very simple XSD without namespace for example. See Trang’s manual for more advanced use. The generated schema looks like this:

<?xml version="1.0" encoding="UTF-8"?>
xmlns:xs="http://www.w3.org/2001/XMLSchema"
	elementFormDefault="qualified">
	<xs:element name="order">
		<xs:complexType>
			<xs:sequence>
				<xs:element ref="date" />
				<xs:element ref="id" />
				<xs:element maxOccurs="unbounded" ref="item" />
			</xs:sequence>
		</xs:complexType>
	</xs:element>
	NMTOKEN" />
	<xs:element name="id" type="xs:integer" />
	<xs:element name="item">
		<xs:complexType>
			<xs:attribute name="name" use="required" />
			<xs:attribute name="quantity" use="required" type="xs:integer" />
			NCName" />
		</xs:complexType>
	</xs:element>
</xs:schema>

The generated schema is an excellent start, but you often want to add namespace, restrictions and often to change the type of some of the fields Trang guessed on. After some manual modifications, my schema looks like this:

<?xml version="1.0" encoding="UTF-8"?>
xmlns:xs="http://www.w3.org/2001/XMLSchema"
	elementFormDefault="qualified" targetNamespace="http://www.redpill-linpro.com/order"
	xmlns:o="http://www.redpill-linpro.com/order">

	<xs:element name="order">
		<xs:complexType>
			<xs:sequence>
				<xs:element name="date" type="xs:date" />
				<xs:element name="id" type="xs:int" />
				<xs:element name="item" maxOccurs="unbounded">
					<xs:complexType>
						<xs:attribute name="name" use="required" type="xs:string" />
						<xs:attribute name="quantity" use="required" type="xs:int" />
						itemType" />
					</xs:complexType>
				</xs:element>
			</xs:sequence>
		</xs:complexType>
	</xs:element>

	<xs:simpleType name="itemType">
		<xs:restriction base="xs:string">
			<xs:enumeration value="BOOK" />
			<xs:enumeration value="DVD" />
			<xs:enumeration value="CD" />
		</xs:restriction>
	</xs:simpleType>
</xs:schema>

Another difference is that it’s only one xs:element in the tweaked version. This is because id or date doesn’t give any value alone, but only together as part of the order element.

A JAXB tips is to create an xs:element with a nested type element inside it. This enables JAXB to add an @XmlRoot annotation above the generated class. This annotation is necessary for JAXB to marshall your objects.

<?xml version="1.0" encoding="UTF-8"?>
xmlns:o="http://www.redpill-linpro.com/order"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.redpill-linpro.com/order order.xsd ">
	<o:date>2001-01-01</o:date>
	<o:id>0</o:id>
	<o:item name="Effective Java" quantity="3" type="BOOK" />
	<o:item name="Spring in Action" quantity="1" type="BOOK" />
</o:order>

public class OrderJaxbTest {

	@Test
	public void testOrder() throws Exception {
		Order order = createJaxbOrder();

		String xmlDocument = marshallOrderToXml(order);
		assertTrue(xmlDocument.contains("type=\"CD\" quantity=\"2\" name=\"Effective Java\""));

		Order unmarshalledOrder = unmarshallToOrderFromXml(xmlDocument);
		assertEquals(order.getId(), unmarshalledOrder.getId());
	}

	private Order unmarshallToOrderFromXml(String xmlDocument)
			throws JAXBException {
		JAXBContext context = JAXBContext.newInstance(Order.class);
		Unmarshaller unmarshaller = context.createUnmarshaller();
		StringReader reader = new StringReader(xmlDocument);
		Order unmarshalledOrder = (Order) unmarshaller.unmarshal(reader);
		return unmarshalledOrder;
	}

	private Order createJaxbOrder() throws DatatypeConfigurationException {
		Order order = new ObjectFactory().createOrder();
		order.setDate(DatatypeFactory.newInstance()
				.newXMLGregorianCalendar(2010, 5, 5, 12, 0, 0, 0, 1));
		order.setId(1);

		Item item = new Item();
		item.setName("Effective Java");
		item.setQuantity(2);
		item.setType(ItemType.CD);
		order.getItem().add(item);

		return order;
	}

	public String marshallOrderToXml(Order order) throws JAXBException {
		JAXBContext context = JAXBContext.newInstance(Order.class);
		Marshaller marshaller =context.createMarshaller();
		StringWriter writer = new StringWriter();
		marshaller.marshal(order, writer);

		return writer.toString();
	}
}