@XmlRootElement is an annotation that people are used to using with JAXB (JSR-222). It's purpose is to uniquely associate a root element with a class. Since JAXB classes map to complex types, it is possible for a class to correspond to multiple root elements. In this case @XmlRootElement can not be used and people start getting a bit confused. In this post I'll demonstrate how @XmlElementDecl can be used to map this use case. XML Schema The XML schema below contains three root elements: customer, billing-address, and shipping-address. The customer element has an anonymous complex type, while billing-address and shipping-address are of the same named type (address-type). createBillingAddress(AddressType value) { return new JAXBElement(_BillingAddress_QNAME, AddressType.class, null, value); } @XmlElementDecl(namespace = "http://www.example.org/customer", name = "shipping-address") public JAXBElement createShippingAddress(AddressType value) { return new JAXBElement(_ShippingAddress_QNAME, AddressType.class, null, value); } } package-info The package-info class is used to specify the namespace mapping (see JAXB & Namespaces). @XmlSchema(namespace = "http://www.example.org/customer", elementFormDefault = XmlNsForm.QUALIFIED) package org.example.customer; import javax.xml.bind.annotation.*; Unmarshal Operation Now we look at the impact of the type of root element when unmarshalling XML. customer.xml Below is a sample XML document with customer as the root element. Remember the customer element had an anonymous complex type. 1 Any Street 2 Another Road shipping.xml Here is a sample XML document with shipping-address as the root element. The shipping-address element had a named complex type. 2 Another Road Unmarshal Demo When unmarshalling XML that corresponds to a class annotated with @XmlRootElement you get an instance of the domain object. But when unmarshalling XML that corresponds to a class annotated with @XmlElementDecl you get the domain object wrapped in an instance of JAXBElement. In this example you may need to use the QName from the JAXBElement to determine if you unmarshalled a billing or shipping address. package org.example.customer; import java.io.File; import javax.xml.bind.*; public class UnmarshalDemo { public static void main(String[] args) throws Exception { JAXBContext jc = JAXBContext.newInstance("org.example.customer"); Unmarshaller unmarshaller = jc.createUnmarshaller(); // Unmarshal Customer File customerXML = new File("src/org/example/customer/customer.xml"); Customer customer = (Customer) unmarshaller.unmarshal(customerXML); // Unmarshal Shipping Address File shippingXML = new File("src/org/example/customer/shipping.xml"); JAXBElement je = (JAXBElement) unmarshaller.unmarshal(shippingXML); AddressType shipping = je.getValue(); } } Unmarshal Demo - JAXBIntrospector If you don't want to deal with remembering whether the result of the unmarshal operation will be a domain object or JAXBElement, then you can use the JAXBIntrospector.getValue(Object) method to always get the domain object. package org.example.customer; import java.io.File; import javax.xml.bind.*; public class JAXBIntrospectorDemo { public static void main(String[] args) throws Exception { JAXBContext jc = JAXBContext.newInstance("org.example.customer"); Unmarshaller unmarshaller = jc.createUnmarshaller(); // Unmarshal Customer File customerXML = new File("src/org/example/customer/customer.xml"); Customer customer = (Customer) JAXBIntrospector.getValue(unmarshaller .unmarshal(customerXML)); // Unmarshal Shipping Address File shippingXML = new File("src/org/example/customer/shipping.xml"); AddressType shipping = (AddressType) JAXBIntrospector .getValue(unmarshaller.unmarshal(shippingXML)); } } Marshal Operation You can directly marshal an object annotated with @XmlRootElement to XML. Classes corresponding to @XmlElementDecl annotations must first be wrapped in an instance of JAXBElement. The factory method you you annotated with @XmlElementDecl is the easiest way to do this. The factory method is in the ObjectFactory class if you generated your model from an XML schema. package org.example.customer; import javax.xml.bind.*; public class MarshalDemo { public static void main(String[] args) throws Exception { JAXBContext jc = JAXBContext.newInstance("org.example.customer"); Marshaller marshaller = jc.createMarshaller(); marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true); // Create Domain Objects AddressType billingAddress = new AddressType(); billingAddress.setStreet("1 Any Street"); Customer customer = new Customer(); customer.setBillingAddress(billingAddress); // Marshal Customer marshaller.marshal(customer, System.out); // Marshal Billing Address ObjectFactory objectFactory = new ObjectFactory(); JAXBElement je = objectFactory.createBillingAddress(billingAddress); marshaller.marshal(je, System.out); } } Output Below is the output from running the demo code. 1 Any Street 1 Any Street
August 12, 2012
by Blaise Doughan
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