Marshalling XML by Using Object-XML Mappers

Introduction

This chapter, describes Spring’s Object-XML Mapping support. Object-XML Mapping (O-X mapping for short) is the act of converting an XML document to and from an object. This conversion process is also known as XML Marshalling, or XML Serialization. This chapter uses these terms interchangeably.

Within the field of O-X mapping, a marshaller is responsible for serializing an object (graph) to XML. In similar fashion, an unmarshaller deserializes the XML to an object graph. This XML can take the form of a DOM document, an input or output stream, or a SAX handler.

Some of the benefits of using Spring for your O/X mapping needs are:

Ease of configuration

Spring’s bean factory makes it easy to configure marshallers, without needing to construct JAXB context, JiBX binding factories, and so on. You can configure the marshallers as you would any other bean in your application context. Additionally, XML namespace-based configuration is available for a number of marshallers, making the configuration even simpler.

Consistent Interfaces

Spring’s O-X mapping operates through two global interfaces: Marshaller and Unmarshaller. These abstractions let you switch O-X mapping frameworks with relative ease, with little or no change required on the classes that do the marshalling. This approach has the additional benefit of making it possible to do XML marshalling with a mix-and-match approach (for example, some marshalling performed using JAXB and some by XStream) in a non-intrusive fashion, letting you use the strength of each technology.

Consistent Exception Hierarchy

Spring provides a conversion from exceptions from the underlying O-X mapping tool to its own exception hierarchy with the XmlMappingException as the root exception. These runtime exceptions wrap the original exception so that no information is lost.

Marshaller and Unmarshaller

As stated in the introduction, a marshaller serializes an object to XML, and an unmarshaller deserializes XML stream to an object. This section describes the two Spring interfaces used for this purpose.

Understanding Marshaller

Spring abstracts all marshalling operations behind the org.springframework.oxm.Marshaller interface, the main method of which follows:

public interface Marshaller {

	/**
	 * Marshal the object graph with the given root into the provided Result.
	 */
	void marshal(Object graph, Result result) throws XmlMappingException, IOException;
}

The Marshaller interface has one main method, which marshals the given object to a given javax.xml.transform.Result. The result is a tagging interface that basically represents an XML output abstraction. Concrete implementations wrap various XML representations, as the following table indicates:

Result implementation Wraps XML representation

DOMResult

org.w3c.dom.Node

SAXResult

org.xml.sax.ContentHandler

StreamResult

java.io.File, java.io.OutputStream, or java.io.Writer
Although the marshal() method accepts a plain object as its first parameter, most Marshaller implementations cannot handle arbitrary objects. Instead, an object class must be mapped in a mapping file, be marked with an annotation, be registered with the marshaller, or have a common base class. Refer to the later sections in this chapter to determine how your O-X technology manages this.

Understanding Unmarshaller

Similar to the Marshaller, we have the org.springframework.oxm.Unmarshaller interface, which the following listing shows:

public interface Unmarshaller {

	/**
	 * Unmarshal the given provided Source into an object graph.
	 */
	Object unmarshal(Source source) throws XmlMappingException, IOException;
}

This interface also has one method, which reads from the given javax.xml.transform.Source (an XML input abstraction) and returns the object read. As with Result, Source is a tagging interface that has three concrete implementations. Each wraps a different XML representation, as the following table indicates:

Source implementation Wraps XML representation

DOMSource

org.w3c.dom.Node

SAXSource

org.xml.sax.InputSource, and org.xml.sax.XMLReader

StreamSource

java.io.File, java.io.InputStream, or java.io.Reader

Even though there are two separate marshalling interfaces (Marshaller and Unmarshaller), all implementations in Spring-WS implement both in one class. This means that you can wire up one marshaller class and refer to it both as a marshaller and as an unmarshaller in your applicationContext.xml.

Understanding XmlMappingException

Spring converts exceptions from the underlying O-X mapping tool to its own exception hierarchy with the XmlMappingException as the root exception. These runtime exceptions wrap the original exception so that no information will be lost.

Additionally, the MarshallingFailureException and UnmarshallingFailureException provide a distinction between marshalling and unmarshalling operations, even though the underlying O-X mapping tool does not do so.

The O-X Mapping exception hierarchy is shown in the following figure:

oxm exceptions

Using Marshaller and Unmarshaller

You can use Spring’s OXM for a wide variety of situations. In the following example, we use it to marshal the settings of a Spring-managed application as an XML file. In the following example, we use a simple JavaBean to represent the settings:

  • Java

  • Kotlin

public class Settings {

	private boolean fooEnabled;

	public boolean isFooEnabled() {
		return fooEnabled;
	}

	public void setFooEnabled(boolean fooEnabled) {
		this.fooEnabled = fooEnabled;
	}
}
class Settings {
	var isFooEnabled: Boolean = false
}

The application class uses this bean to store its settings. Besides a main method, the class has two methods: saveSettings() saves the settings bean to a file named settings.xml, and loadSettings() loads these settings again. The following main() method constructs a Spring application context and calls these two methods:

  • Java

  • Kotlin

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.stream.StreamSource;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.oxm.Marshaller;
import org.springframework.oxm.Unmarshaller;

public class Application {

	private static final String FILE_NAME = "settings.xml";
	private Settings settings = new Settings();
	private Marshaller marshaller;
	private Unmarshaller unmarshaller;

	public void setMarshaller(Marshaller marshaller) {
		this.marshaller = marshaller;
	}

	public void setUnmarshaller(Unmarshaller unmarshaller) {
		this.unmarshaller = unmarshaller;
	}

	public void saveSettings() throws IOException {
		try (FileOutputStream os = new FileOutputStream(FILE_NAME)) {
			this.marshaller.marshal(settings, new StreamResult(os));
		}
	}

	public void loadSettings() throws IOException {
		try (FileInputStream is = new FileInputStream(FILE_NAME)) {
			this.settings = (Settings) this.unmarshaller.unmarshal(new StreamSource(is));
		}
	}

	public static void main(String[] args) throws IOException {
		ApplicationContext appContext =
				new ClassPathXmlApplicationContext("applicationContext.xml");
		Application application = (Application) appContext.getBean("application");
		application.saveSettings();
		application.loadSettings();
	}
}
class Application {

	lateinit var marshaller: Marshaller

	lateinit var unmarshaller: Unmarshaller

	fun saveSettings() {
		FileOutputStream(FILE_NAME).use { outputStream -> marshaller.marshal(settings, StreamResult(outputStream)) }
	}

	fun loadSettings() {
		FileInputStream(FILE_NAME).use { inputStream -> settings = unmarshaller.unmarshal(StreamSource(inputStream)) as Settings }
	}
}

private const val FILE_NAME = "settings.xml"

fun main(args: Array<String>) {
	val appContext = ClassPathXmlApplicationContext("applicationContext.xml")
	val application = appContext.getBean("application") as Application
	application.saveSettings()
	application.loadSettings()
}

The Application requires both a marshaller and an unmarshaller property to be set. We can do so by using the following applicationContext.xml:

<beans>
	<bean id="application" class="Application">
		<property name="marshaller" ref="xstreamMarshaller" />
		<property name="unmarshaller" ref="xstreamMarshaller" />
	</bean>
	<bean id="xstreamMarshaller" class="org.springframework.oxm.xstream.XStreamMarshaller"/>
</beans>

This application context uses XStream, but we could have used any of the other marshaller instances described later in this chapter. Note that, by default, XStream does not require any further configuration, so the bean definition is rather simple. Also note that the XStreamMarshaller implements both Marshaller and Unmarshaller, so we can refer to the xstreamMarshaller bean in both the marshaller and unmarshaller property of the application.

This sample application produces the following settings.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<settings foo-enabled="false"/>

XML Configuration Namespace

You can configure marshallers more concisely by using tags from the OXM namespace. To make these tags available, you must first reference the appropriate schema in the preamble of the XML configuration file. The following example shows how to do so:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:oxm="http://www.springframework.org/schema/oxm" (1)
	xsi:schemaLocation="http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/oxm
		https://www.springframework.org/schema/oxm/spring-oxm.xsd"> (2)
1 Reference the oxm schema.
2 Specify the oxm schema location.

The schema makes the following elements available:

Each tag is explained in its respective marshaller’s section. As an example, though, the configuration of a JAXB2 marshaller might resemble the following:

<oxm:jaxb2-marshaller id="marshaller" contextPath="org.springframework.ws.samples.airline.schema"/>

JAXB

The JAXB binding compiler translates a W3C XML Schema into one or more Java classes, a jaxb.properties file, and possibly some resource files. JAXB also offers a way to generate a schema from annotated Java classes.

Spring supports the JAXB 2.0 API as XML marshalling strategies, following the Marshaller and Unmarshaller interfaces described in Marshaller and Unmarshaller. The corresponding integration classes reside in the org.springframework.oxm.jaxb package.

Using Jaxb2Marshaller

The Jaxb2Marshaller class implements both of Spring’s Marshaller and Unmarshaller interfaces. It requires a context path to operate. You can set the context path by setting the contextPath property. The context path is a list of colon-separated Java package names that contain schema derived classes. It also offers a classesToBeBound property, which allows you to set an array of classes to be supported by the marshaller. Schema validation is performed by specifying one or more schema resources to the bean, as the following example shows:

<beans>
	<bean id="jaxb2Marshaller" class="org.springframework.oxm.jaxb.Jaxb2Marshaller">
		<property name="classesToBeBound">
			<list>
				<value>org.springframework.oxm.jaxb.Flight</value>
				<value>org.springframework.oxm.jaxb.Flights</value>
			</list>
		</property>
		<property name="schema" value="classpath:org/springframework/oxm/schema.xsd"/>
	</bean>

	...

</beans>

XML Configuration Namespace

The jaxb2-marshaller element configures a org.springframework.oxm.jaxb.Jaxb2Marshaller, as the following example shows:

<oxm:jaxb2-marshaller id="marshaller" contextPath="org.springframework.ws.samples.airline.schema"/>

Alternatively, you can provide the list of classes to bind to the marshaller by using the class-to-be-bound child element:

<oxm:jaxb2-marshaller id="marshaller">
	<oxm:class-to-be-bound name="org.springframework.ws.samples.airline.schema.Airport"/>
	<oxm:class-to-be-bound name="org.springframework.ws.samples.airline.schema.Flight"/>
	...
</oxm:jaxb2-marshaller>

The following table describes the available attributes:

Attribute Description Required

id

The ID of the marshaller

No

contextPath

The JAXB Context path

No

JiBX

The JiBX framework offers a solution similar to that which Hibernate provides for ORM: A binding definition defines the rules for how your Java objects are converted to or from XML. After preparing the binding and compiling the classes, a JiBX binding compiler enhances the class files and adds code to handle converting instances of the classes from or to XML.

For more information on JiBX, see the JiBX web site. The Spring integration classes reside in the org.springframework.oxm.jibx package.

Using JibxMarshaller

The JibxMarshaller class implements both the Marshaller and Unmarshaller interface. To operate, it requires the name of the class to marshal in, which you can set using the targetClass property. Optionally, you can set the binding name by setting the bindingName property. In the following example, we bind the Flights class:

<beans>
	<bean id="jibxFlightsMarshaller" class="org.springframework.oxm.jibx.JibxMarshaller">
		<property name="targetClass">org.springframework.oxm.jibx.Flights</property>
	</bean>
	...
</beans>

A JibxMarshaller is configured for a single class. If you want to marshal multiple classes, you have to configure multiple JibxMarshaller instances with different targetClass property values.

XML Configuration Namespace

The jibx-marshaller tag configures a org.springframework.oxm.jibx.JibxMarshaller, as the following example shows:

<oxm:jibx-marshaller id="marshaller" target-class="org.springframework.ws.samples.airline.schema.Flight"/>

The following table describes the available attributes:

Attribute Description Required

id

The ID of the marshaller

No

target-class

The target class for this marshaller

Yes

bindingName

The binding name used by this marshaller

No

XStream

XStream is a simple library to serialize objects to XML and back again. It does not require any mapping and generates clean XML.

For more information on XStream, see the XStream web site. The Spring integration classes reside in the org.springframework.oxm.xstream package.

Using XStreamMarshaller

The XStreamMarshaller does not require any configuration and can be configured in an application context directly. To further customize the XML, you can set an alias map, which consists of string aliases mapped to classes, as the following example shows:

<beans>
	<bean id="xstreamMarshaller" class="org.springframework.oxm.xstream.XStreamMarshaller">
		<property name="aliases">
			<props>
				<prop key="Flight">org.springframework.oxm.xstream.Flight</prop>
			</props>
		</property>
	</bean>
	...
</beans>

By default, XStream lets arbitrary classes be unmarshalled, which can lead to unsafe Java serialization effects. As such, we do not recommend using the XStreamMarshaller to unmarshal XML from external sources (that is, the Web), as this can result in security vulnerabilities.

If you choose to use the XStreamMarshaller to unmarshal XML from an external source, set the supportedClasses property on the XStreamMarshaller, as the following example shows:

<bean id="xstreamMarshaller" class="org.springframework.oxm.xstream.XStreamMarshaller">
	<property name="supportedClasses" value="org.springframework.oxm.xstream.Flight"/>
	...
</bean>

Doing so ensures that only the registered classes are eligible for unmarshalling.

Additionally, you can register custom converters to make sure that only your supported classes can be unmarshalled. You might want to add a CatchAllConverter as the last converter in the list, in addition to converters that explicitly support the domain classes that should be supported. As a result, default XStream converters with lower priorities and possible security vulnerabilities do not get invoked.
Note that XStream is an XML serialization library, not a data binding library. Therefore, it has limited namespace support. As a result, it is rather unsuitable for usage within Web Services.