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    <p><strong>Note:</strong> I'm the <a href="http://www.eclipse.org/eclipselink/moxy.php"><strong>EclipseLink JAXB (MOXy)</strong></a> lead and a member of the <a href="http://jcp.org/en/jsr/detail?id=222"><strong>JAXB (JSR-222)</strong></a> expert group.</p> <p>What you have done is right, the error you are seeing is due to what I believe is a bug in the <a href="http://jaxb.java.net/"><strong>JAXB reference implementation</strong></a>. The JAXB RI should be able to handle a null value being returned from an <code>XmlAdapter</code>. This use case works with EclipseLink JAXB (MOXy), I'll demonstrate below with an example.</p> <p><strong>StringAdapter</strong></p> <p>Below is an implmentation that does approximately what the one that you will get after you generate your Java model from the XML schema (see <a href="http://blog.bdoughan.com/2011/08/xml-schema-to-java-generating.html">http://blog.bdoughan.com/2011/08/xml-schema-to-java-generating.html</a>).</p> <pre><code>package forum11894193; import javax.xml.bind.annotation.adapters.XmlAdapter; public class StringAdapter extends XmlAdapter&lt;String, String&gt; { @Override public String marshal(String string) throws Exception { if("".equals(string)) { return null; } return string; } @Override public String unmarshal(String string) throws Exception { return string; } } </code></pre> <p><strong>package-info</strong></p> <p>Since you are registering a global adapter, it will be referenced from a <code>package-info</code> class like the one below (see: <a href="http://blog.bdoughan.com/2012/02/jaxb-and-package-level-xmladapters.html">http://blog.bdoughan.com/2012/02/jaxb-and-package-level-xmladapters.html</a>).</p> <pre><code>@XmlJavaTypeAdapters({ @XmlJavaTypeAdapter(value=StringAdapter.class, type=String.class) }) package forum11894193; import javax.xml.bind.annotation.adapters.*; </code></pre> <p><strong>Root</strong></p> <p>Below is a sample domain class with a few <code>String</code> fields. Since the <code>XmlAdapter</code> was registered at the package level it will apply to all mapped String fields/properties in that package.</p> <pre><code>package forum11894193; import javax.xml.bind.annotation.*; @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Root { String a; String b; String c; } </code></pre> <p><strong>Demo</strong></p> <p>In the demo code below we'll create an instance of <code>Root</code> set a couple of the fields to <code>""</code> and then marshal it to XML.</p> <pre><code>package forum11894193; import javax.xml.bind.*; public class Demo { public static void main(String[] args) throws Exception { JAXBContext jc = JAXBContext.newInstance(Root.class); Root root = new Root(); root.a = ""; root.b = "b"; root.c = ""; Marshaller marshaller = jc.createMarshaller(); marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true); marshaller.marshal(root, System.out); } } </code></pre> <p><strong>Output Using JAXB RI</strong></p> <p>Using the JAXB RI with this example results in a NPE. The stack trace is different, but most likely to us using different marshal methods. I am also using the version of the JAXB RI included in the JDK which is repackaged to <code>com.sun.xml.internal.bind.v2</code>.</p> <pre><code>Exception in thread "main" java.lang.NullPointerException at com.sun.xml.internal.bind.v2.runtime.output.Encoded.setEscape(Encoded.java:96) at com.sun.xml.internal.bind.v2.runtime.output.UTF8XmlOutput.doText(UTF8XmlOutput.java:294) at com.sun.xml.internal.bind.v2.runtime.output.UTF8XmlOutput.text(UTF8XmlOutput.java:283) at com.sun.xml.internal.bind.v2.runtime.output.IndentingUTF8XmlOutput.text(IndentingUTF8XmlOutput.java:141) at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.leafElement(XMLSerializer.java:293) at com.sun.xml.internal.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$1.writeLeafElement(RuntimeBuiltinLeafInfoImpl.java:179) at com.sun.xml.internal.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$1.writeLeafElement(RuntimeBuiltinLeafInfoImpl.java:166) at com.sun.xml.internal.bind.v2.runtime.reflect.TransducedAccessor$CompositeTransducedAccessorImpl.writeLeafElement(TransducedAccessor.java:239) at com.sun.xml.internal.bind.v2.runtime.property.SingleElementLeafProperty.serializeBody(SingleElementLeafProperty.java:87) at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:306) at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsSoleContent(XMLSerializer.java:561) at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:290) at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:462) at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:314) at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:243) at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:75) at forum11894193.Demo.main(Demo.java:17) </code></pre> <p><strong>Output Using EclipseLink JAXB (MOXy)</strong></p> <p>When MOXy is used as the JAXB provider you get the desired output. For information on specifying MOXy as your JAXB provider see: <a href="http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html">http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html</a>.</p> <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt; &lt;root&gt; &lt;b&gt;b&lt;/b&gt; &lt;/root&gt; </code></pre>
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    1. COI'd seen your post a few times, but didn't realise the 'bug' was the same because the stack trace is different. I agree that it is a bug in the JAXB implementation. I'll give switching to MOXy a try. Would you expect the resulting XML to be identical? If not, it might not be an option.
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    2. CO@JBarclay - Yes the resulting XML should be identical. EclipseLink JAXB (MOXy) is now the default JAXB provider in WebLogic to give you some sense of the compatibility that you can expect.
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    3. COI've switched to MOXy, which fixed the problem as you said. Unfortunately, MOXy isn't doing so well with nillable elements. I've updated the question, any ideas?
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