How to marshall a string using JAXB that sometimes contains XML content and sometimes does not?

Note: I’m the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

This use case is mapped using the @XmlAnyElement annotation and specifying a DOMHandler. There appears to be bug when doing this with the JAXB RI, but the following use case works with EclipseLink JAXB (MOXy).

BodyDomHandler

By default a JAXB impleemntation will represent unmapped content as a DOM node. You can leverage a DomHandler to an alternate representation of the DOM, In this case we will represent the DOM as a String.

import java.io.*;
import javax.xml.bind.ValidationEventHandler;
import javax.xml.bind.annotation.DomHandler;
import javax.xml.transform.Source;
import javax.xml.transform.stream.*;

public class BodyDomHandler implements DomHandler<String, StreamResult> {

    private static final String BODY_START_TAG = "<body>";
    private static final String BODY_END_TAG = "</body>";

    private StringWriter xmlWriter = new StringWriter();

    public StreamResult createUnmarshaller(ValidationEventHandler errorHandler) {
        return new StreamResult(xmlWriter);
    }

    public String getElement(StreamResult rt) {
        String xml = rt.getWriter().toString();
        int beginIndex = xml.indexOf(BODY_START_TAG) + BODY_START_TAG.length();
        int endIndex = xml.indexOf(BODY_END_TAG);
        return xml.substring(beginIndex, endIndex);
    }

    public Source marshal(String n, ValidationEventHandler errorHandler) {
        try {
            String xml = BODY_START_TAG + n.trim() + BODY_END_TAG;
            StringReader xmlReader = new StringReader(xml);
            return new StreamSource(xmlReader);
        } catch(Exception e) {
            throw new RuntimeException(e);
        }
    }

}

Message

Below is how you would specify the @XmlAnyElement annotation on your Message class.

import javax.xml.bind.annotation.XmlAnyElement;
import javax.xml.bind.annotation.XmlType;

@XmlType(propOrder = { "body" })
public class Message
{
    private String body;
    public String getBody() { return body; }
    @XmlAnyElement(BodyDomHandler.class)
    public void setBody(String body) { this.body = body; }
}

Output

Below is the output from running your SerialziationTest:

<?xml version="1.0" encoding="UTF-8"?>
<Report>
   <message>
      <body>Sample report message.</body>
   </message>
</Report>
<?xml version="1.0" encoding="UTF-8"?>
<Report>
   <message>
      <body>
         <rootTag>
            <body>All systems online.</body>
         </rootTag>
      </body>
   </message>
</Report>

For More Information

NOTE – Bug in JAXB RI

There appears to be a bug in the JAXB reference implementation, and the example code will result in a stack trace like the following:

Exception in thread "main" javax.xml.bind.MarshalException
 - with linked exception:
[com.sun.istack.internal.SAXException2: unable to marshal type "java.lang.String" as an element because it is missing an @XmlRootElement annotation]
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:317)
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:243)
    at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:75)
    at forum12428727.SerializationTest.main(SerializationTest.java:20)
Caused by: com.sun.istack.internal.SAXException2: unable to marshal type "java.lang.String" as an element because it is missing an @XmlRootElement annotation
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.reportError(XMLSerializer.java:216)
    at com.sun.xml.internal.bind.v2.runtime.LeafBeanInfoImpl.serializeRoot(LeafBeanInfoImpl.java:126)
    at com.sun.xml.internal.bind.v2.runtime.property.SingleReferenceNodeProperty.serializeBody(SingleReferenceNodeProperty.java:100)
    at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:306)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:664)
    at com.sun.xml.internal.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:141)
    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)
    ... 3 more

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