Marshalling XML with Spring WS and JAXB
The ‘object-relational impedance mismatch’ is a well documented set of
conceptual and technical difficulties that are often encountered when a
relational database management system is being used by a program
written in an object-oriented programming language. A similar impedance
mismatch exists when XML is used by a program written in an
object-oriented programming language.
Many popular ‘Object-Relational Mapping’ (ORM) frameworks exist that
address the object-relational impedance mismatch and no doubt helped to
inspire the evolution of ‘Object-XML Mapping’ (OXM) frameworks to
address the object-xml impedance mismatch.
For the Java community, there are a number of OXM frameworks from which to choose (Castor, XStream, JiBX, JAXB) with each having particular strengths and weaknesses. The standard OXM framework for Java is JAXB.
In the following I work through a simple example that demonstrates object to XML marshalling (and demarshalling) using Spring, Spring WS and JAXB (and later JAXB Introductions!)
I have provided Maven projects for download so you can follow along:
The model object that I have used for this example is a simple object
representing a ‘Person’. The class contains only simple attributes.
Listing 1: Person.java
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| package net.thoughtforge.model;import java.io.Serializable;import java.math.BigDecimal;import java.util.Calendar;import javax.xml.bind.annotation.XmlAccessType;import javax.xml.bind.annotation.XmlAccessorType;import javax.xml.bind.annotation.XmlElement;import javax.xml.bind.annotation.XmlRootElement;import javax.xml.bind.annotation.XmlType;@XmlRootElement( name="Person",@XmlAccessorType(XmlAccessType.FIELD)@XmlTypepublic class Person implements Serializable { private static final long serialVersionUID = 8465162879793776395L; private Calendar dateOfBirth; private String firstName; private BigDecimal height; private String lastName; private BigDecimal weight; public Calendar getDateOfBirth() { return dateOfBirth; } public void setDateOfBirth(Calendar dateOfBirth) { this.dateOfBirth = dateOfBirth; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public BigDecimal getHeight() { return height; } public void setHeight(BigDecimal height) { this.height = height; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } public BigDecimal getWeight() { return weight; } public void setWeight(BigDecimal weight) { this.weight = weight; }} |
As you can see from the listing, the class is annotated with JAXB
annotations that are fairly self explanatory. I will leave it to you to
look up the precise definition and consequence of these annotations
(which can be found on the JAXB Reference Implementation website).
The XML Schema (XSD)
JAXB does not require an XML schema (a default schema can be
generated), but for this example (and always on commercial projects) I
have explicitly specified a schema in order to include type
restrictions. It is important to remember that the XSD forms the
contract between XML producers and consumers (marshaller and
unmarshaller) and should be as detailed as possible.
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| attributeFormDefault="unqualified" elementFormDefault="qualified" version="1.0"> <xsd:element name="Person" type="tns:Person"/> <xsd:complexType name="Person"> <xsd:sequence> <xsd:element name="dateOfBirth" type="xsd:dateTime"/> <xsd:element name="firstName"> <xsd:simpleType> <xsd:restriction base="xsd:string"> <xsd:maxLength value="50"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="height"> <xsd:simpleType> <xsd:restriction base="xsd:decimal"> <xsd:fractionDigits value="2"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="lastName"> <xsd:simpleType> <xsd:restriction base="xsd:string"> <xsd:maxLength value="50"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="weight"> <xsd:simpleType> <xsd:restriction base="xsd:decimal"> <xsd:fractionDigits value="2"/> </xsd:restriction> </xsd:simpleType> </xsd:element> </xsd:sequence> </xsd:complexType></xsd:schema> |
The Marshaller Abstraction
Rather than use the Spring Marshaller/Unmarshaller interface directly, I
often use an abstraction for the Marshaller. The abstraction I used
here is very simple and of course would not be practical for use with
large XML files as it does not support streaming etc.
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| package net.thoughtforge.marshaller;public interface Marshaller { String marshal(Object object); Object unmarshal(String string);} |
The implementation of the marshaller delegates to a Spring JAXB marshaller.
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| package net.thoughtforge.marshaller;import java.io.StringWriter;import javax.xml.transform.stream.StreamResult;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.beans.factory.annotation.Qualifier;import org.springframework.stereotype.Component;import org.springframework.xml.transform.StringSource;@Component(value="marshaller")public class Jaxb2Marshaller implements Marshaller { @Autowired @Qualifier(value="jaxb2Marshaller") private org.springframework.oxm.jaxb.Jaxb2Marshaller marshaller; public String marshal(Object object) { final StringWriter out = new StringWriter(); marshaller.marshal(object, new StreamResult(out)); return out.toString(); } public Object unmarshal(String string) { return marshaller.unmarshal(new StringSource(string)); }} |
The Configuration
Using Spring to define a JAXB marshaller is relatively straight forward
as the listing shows. I have specified the classes to be bound and
the XML schema. If the XML schema is specified, the marshaller will
instruct the XML parser to validate XML against the schema.
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| <?xml version="1.0" encoding="UTF-8"?> xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:component-scan base-package="net.thoughtforge.marshaller"/> <bean id="jaxb2Marshaller" class="org.springframework.oxm.jaxb.Jaxb2Marshaller"> <property name="classesToBeBound"> <list> <value>net.thoughtforge.model.Person</value> </list> </property> <property name="schema" value="classpath:schema/person.xsd"/> </bean></beans> |
The Test
The following test is not exhaustive but demonstrates the marshalling
and unmarshalling of a Person object. It also demonstrates XML schema
validation occurring during the marshalling and unmarshalling process.
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| package net.thoughtforge.marshaller;import java.math.BigDecimal;import java.util.Calendar;import net.thoughtforge.model.Person;import org.apache.commons.lang.text.StrBuilder;import org.junit.Assert;import org.junit.BeforeClass;import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.beans.factory.annotation.Qualifier;import org.springframework.oxm.MarshallingFailureException;import org.springframework.test.context.ContextConfiguration;import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = { "classpath:applicationContext/applicationContext-*.xml"})public class Jaxb2MarshallerTest { private static final String MARSHALLED_PERSON = "1965-01-01T00:00:00ZJoe1.85Bloggs12.2"; private static Calendar dateOfBirth; private static String firstName; private static BigDecimal height; private static String lastName; private static BigDecimal weight; @Autowired @Qualifier(value="marshaller") private Jaxb2Marshaller marshaller; @BeforeClass public static void beforeClass() { dateOfBirth = Calendar.getInstance(); dateOfBirth.clear(); dateOfBirth.set(Calendar.DATE, 1); dateOfBirth.set(Calendar.MONTH, Calendar.JANUARY); dateOfBirth.set(Calendar.YEAR, 1965); firstName = "Joe"; height = new BigDecimal("1.85"); lastName = "Bloggs"; weight = new BigDecimal("12.2"); } @Test public void marshallPerson() { Person person = new Person(); person.setDateOfBirth(dateOfBirth); person.setFirstName(firstName); person.setHeight(height); person.setLastName(lastName); person.setWeight(weight); String xml = marshaller.marshal(person); Assert.assertNotNull(xml); Assert.assertEquals(MARSHALLED_PERSON, xml); } @Test public void marshallPersonInvalidFirstName() { Person person = new Person(); person.setDateOfBirth(dateOfBirth); person.setFirstName(new StrBuilder(firstName).appendPadding(50, '0').toString()); person.setHeight(height); person.setLastName(lastName); person.setWeight(weight); try { marshaller.marshal(person); Assert.fail("First name length restriction not applied."); } catch (MarshallingFailureException marshallingFailureException) { Throwable rootCause = marshallingFailureException.getRootCause(); Assert.assertFalse(rootCause.getMessage().indexOf("is not facet-valid with respect to maxLength '50'") == -1); } } @Test public void marshallPersonInvalidHeight() { Person person = new Person(); person.setDateOfBirth(dateOfBirth); person.setFirstName(firstName); person.setHeight(height.add(new BigDecimal("0.1111"))); person.setLastName(lastName); person.setWeight(weight); try { marshaller.marshal(person); Assert.fail("Height precision restriction not applied."); } catch (MarshallingFailureException marshallingFailureException) { Throwable rootCause = marshallingFailureException.getRootCause(); Assert.assertFalse(rootCause.getMessage().indexOf("the number of fraction digits has been limited to 2") == -1); } } @Test public void marshallPersonInvalidLastName() { Person person = new Person(); person.setDateOfBirth(dateOfBirth); person.setFirstName(firstName); person.setHeight(height); person.setLastName(new StrBuilder(lastName).appendPadding(50, '0').toString()); person.setWeight(weight); try { marshaller.marshal(person); Assert.fail("First name length restriction not applied."); } catch (MarshallingFailureException marshallingFailureException) { Throwable rootCause = marshallingFailureException.getRootCause(); Assert.assertFalse(rootCause.getMessage().indexOf("is not facet-valid with respect to maxLength '50'") == -1); } } @Test public void marshallPersonInvalidWeight() { Person person = new Person(); person.setDateOfBirth(dateOfBirth); person.setFirstName(firstName); person.setHeight(height); person.setLastName(lastName); person.setWeight(weight.add(new BigDecimal("0.1111"))); try { marshaller.marshal(person); Assert.fail("Weight precision restriction not applied."); } catch (MarshallingFailureException marshallingFailureException) { Throwable rootCause = marshallingFailureException.getRootCause(); Assert.assertFalse(rootCause.getMessage().indexOf("the number of fraction digits has been limited to 2") == -1); } } @Test public void unmarshallPerson() { Person person = (Person) marshaller.unmarshal(MARSHALLED_PERSON); Assert.assertNotNull(person); Assert.assertTrue(dateOfBirth.compareTo(person.getDateOfBirth()) == 0); Assert.assertEquals(firstName, person.getFirstName()); Assert.assertEquals(height, person.getHeight()); Assert.assertEquals(lastName, person.getLastName()); Assert.assertEquals(weight, person.getWeight()); }} |
Removing JAXB Annotations
If you are using the reference implementation of JAXB you are required
to use annotations to map objects to XML. As I mentioned in a previous
post, I’m not a fan of using annotations for the purpose of mapping
objects to some other format (relational database or XML or A.N.
Other). I find it creates code clutter especially in instances were
entity model classes are mapped to both a relational database and XML
(using JAXB). In modular projects, it also creates unnecessary
dependencies on the model module.
Luckily, some bright spark came along and created JAXB Introductions
which allows you to define the mapping of objects to XML in an XML file.
I’m not going to regurgitate the information on the JAXB
Introductions website but I will outline the modifications I had to
make to allow me to use JAXB Introductions with the example above.
First, I added the maven dependency to the project.
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| <dependency> <groupId>jboss.jaxbintros</groupId> <artifactId>jboss-jaxb-intros</artifactId> <version>1.0.1.GA</version></dependency> |
Second, I created a JAXB Introductions mapping file.
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| <?xml version = "1.0" encoding = "UTF-8"?> <Class name="net.thoughtforge.model.Person"> <XmlType/> <XmlAccessorType value="FIELD"/> <Field name="dateOfBirth"> </Field> <Field name="firstName"> </Field> <Field name="height"> </Field> <Field name="lastName"> </Field> <Field name="weight"> </Field> </Class></jaxb-intros> |
Thirdly, I modified the Spring configuration to inject the JAXB Introductions annotation reader into the JAXB marshaller.
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| <?xml version="1.0" encoding="UTF-8"?> xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:component-scan base-package="net.thoughtforge.marshaller"/> <bean id="jaxb2Marshaller" class="org.springframework.oxm.jaxb.Jaxb2Marshaller"> <property name="classesToBeBound"> <list> <value>net.thoughtforge.model.Person</value> </list> </property> <property name="jaxbContextProperties"> <map> <entry> <key> <util:constant static-field="com.sun.xml.bind.api.JAXBRIContext.ANNOTATION_READER"/> </key> <bean class="org.jboss.jaxb.intros.IntroductionsAnnotationReader"> <constructor-arg ref="jaxbIntroductions"/> </bean> </entry> </map> </property> <property name="schema" value="classpath:schema/person.xsd"/> </bean> <bean id="jaxbIntroductions" class="org.jboss.jaxb.intros.IntroductionsConfigParser" factory-method="parseConfig"> <constructor-arg><value>classpath:marshaller-mapping.xml</value></constructor-arg> </bean></beans> |
Finally, I removed the JAXB annotations from the model class (Person.java).
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| package net.thoughtforge.model;import java.io.Serializable;import java.math.BigDecimal;import java.util.Calendar;public class Person implements Serializable { private static final long serialVersionUID = 8465162879793776395L; private Calendar dateOfBirth; private String firstName; private BigDecimal height; private String lastName; private BigDecimal weight; public Calendar getDateOfBirth() { return dateOfBirth; } public void setDateOfBirth(Calendar dateOfBirth) { this.dateOfBirth = dateOfBirth; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public BigDecimal getHeight() { return height; } public void setHeight(BigDecimal height) { this.height = height; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } public BigDecimal getWeight() { return weight; } public void setWeight(BigDecimal weight) { this.weight = weight; }} |
Hopefully, this short example will prove useful for those starting out
with JAXB and OXM. For those already familiar with JAXB and OXM,
perhaps it has shown you how you can use JAXB without annotations.
