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copied!<h1>TL;DR</h1> <p>For</p> <pre><code>@XmlElementRef(name="foo", required=false) protected JAXBElement<String> foo; </code></pre> <p>An absent node in the document will correspond to this field being null. An XML element present in the document with <code>xsi:nil="true"</code> will correspond to the value being an instance of <code>JAXBElement</code> with a value of <code>null</code>.</p> <p>You can also provide an XML schema instead of having JAXB generate one using the <code>location</code> property on the package level <code>@XmlSchema</code> annotation.</p> <pre><code>@XmlSchema( ... location="http://www.example.com/schema/root.xsd") package forum19665550; import javax.xml.bind.annotation.XmlSchema; </code></pre> <hr> <h1>Marshal/Unmarshal</h1> <h2>Java Model</h2> <p><strong>Root</strong></p> <p>This is an object with two fields that can represent optional and nillable data.</p> <pre><code>import javax.xml.bind.JAXBElement; import javax.xml.bind.annotation.*; @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Root { @XmlElementRef(name="foo", required=false) protected JAXBElement<String> foo; @XmlElementRef(name="bar", required=false) protected JAXBElement<String> bar; } </code></pre> <p><strong>ObjectFactory</strong></p> <pre><code>import javax.xml.bind.JAXBElement; import javax.xml.bind.annotation.*; import javax.xml.namespace.QName; @XmlRegistry public class ObjectFactory { @XmlElementDecl(name="foo") public JAXBElement<String> createFoo(String foo) { return new JAXBElement<String>(new QName("foo"), String.class, foo); } @XmlElementDecl(name="bar") public JAXBElement<String> createBar(String bar) { return new JAXBElement<String>(new QName("bar"), String.class, bar); } } </code></pre> <h2>Demo Code</h2> <p><strong>Demo</strong></p> <p>The demo code below will investigate the differences in the values for <code>foo</code> and <code>bar</code>. You can use the <code>JAXBIntrospector</code> class to get the real value for an instance of <code>JAXBElement</code>. There is a bug in EclipseLink JAXB (MOXy) related to unmarshalling an instance of <code>JAXBElement</code> wrapping a null value (see: <a href="http://bugs.eclipse.org/420746" rel="noreferrer">http://bugs.eclipse.org/420746</a>).</p> <pre><code>import java.io.File; import javax.xml.bind.*; public class Demo { public static void main(String[] args) throws Exception { JAXBContext jc = JAXBContext.newInstance(Root.class, ObjectFactory.class); Unmarshaller unmarshaller = jc.createUnmarshaller(); File xml = new File("src/forum19665550/input.xml"); Root root = (Root) unmarshaller.unmarshal(xml); System.out.println("foo was set: " + (root.foo != null)); System.out.println("bar was set: " + (root.bar != null)); System.out.println("foo value: " + root.foo); System.out.println("bar value: " + root.bar); System.out.println("foo unwrapped value: " + JAXBIntrospector.getValue(root.foo)); System.out.println("bar unwrapped value: " + JAXBIntrospector.getValue(root.bar)); Marshaller marshaller = jc.createMarshaller(); marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true); marshaller.marshal(root, System.out); } } </code></pre> <p><strong>input.xml/Output</strong></p> <p>In the resulting output we see that we can differentiate between an element being absent from the document and an element with `xsi:nil="true" and still have the resulting value be null.</p> <pre><code>foo was set: false bar was set: true foo value: null bar value: javax.xml.bind.JAXBElement@4af42ea0 foo unwrapped value: null bar unwrapped value: null <?xml version="1.0" encoding="UTF-8" standalone="yes"?> <root> <bar xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/> </code></pre> <h2> </h2> <h1>Generating an XML Schema</h1> <h2>Demo Code</h2> <p><strong>GenerateSchema</strong></p> <p>Below is some JAXB code that will generate an XML Schema from the annotated model.</p> <pre><code>import java.io.IOException; import javax.xml.bind.*; import javax.xml.transform.Result; import javax.xml.transform.stream.StreamResult; public class GenerateSchema { public static void main(String[] args) throws Exception { JAXBContext jc = JAXBContext.newInstance(Root.class); jc.generateSchema(new SchemaOutputResolver() { @Override public Result createOutput(String namespaceUri, String suggestedFileName) throws IOException { StreamResult result = new StreamResult(System.out); result.setSystemId(suggestedFileName); return result; } }); } } </code></pre> <p><strong>Output</strong></p> <p>Here is the resulting XML Schema. You are correct that it doesn't indicate that the <code>foo</code> and <code>bar</code> elements are nillable.</p> <pre><code><?xml version="1.0" encoding="UTF-8" standalone="yes"?> <xs:schema version="1.0" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="bar" type="xs:string"/> <xs:element name="foo" type="xs:string"/> <xs:element name="root" type="root"/> <xs:complexType name="root"> <xs:sequence> <xs:element ref="foo" minOccurs="0"/> <xs:element ref="bar" minOccurs="0"/> </xs:sequence> </xs:complexType> </xs:schema> </code></pre> <hr> <h2>Providing an XML Schema</h2> <p>Instead of having JAXB derive an XML Schema from your model, you can point to your existing one that will contain more information.</p> <p><strong>package-info</strong></p> <p>This is done by specifying the <code>location</code> property on the package level <code>@XmlSchema</code> annotation.</p> <pre><code>@XmlSchema( ... location="http://www.example.com/schema/root.xsd") package forum19665550; import javax.xml.bind.annotation.XmlSchema; </code></pre>

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