Unable to marshal type as XML element because @XmlRootElement annotation is missing
<p>I want to marshal object to XML. </p>

<p>However, it fails with exception:</p>

<pre><code>javax.xml.bind.MarshalException
 - with linked exception:
[com.sun.istack.SAXException2: unable to marshal type "FreightOfferDetail" as an element because it is missing an @XmlRootElement annotation]
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:331)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:257)
    at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:96)
    at com.wktransportservices.fx.test.util.jaxb.xmltransformer.ObjectTransformer.toXML(ObjectTransformer.java:27)
    at com.wktransportservices.fx.test.sampler.webservice.connect.FreightOfferToConnectFreight.runTest(FreightOfferToConnectFreight.java:59)
    at org.apache.jmeter.protocol.java.sampler.JavaSampler.sample(JavaSampler.java:191)
    at org.apache.jmeter.threads.JMeterThread.process_sampler(JMeterThread.java:429)
    at org.apache.jmeter.threads.JMeterThread.run(JMeterThread.java:257)
    at java.lang.Thread.run(Thread.java:662)
Caused by: com.sun.istack.SAXException2: unable to marshal type "FreightOfferDetail" as an element because it is missing an @XmlRootElement annotation
    at com.sun.xml.bind.v2.runtime.XMLSerializer.reportError(XMLSerializer.java:244)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:303)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:490)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:328)
</code></pre>

<p>In fact, this annotation is present (for parent and delivered class):</p>

<pre><code>@XmlRootElement(name = "Freight_Offer")
@XmlAccessorType(XmlAccessType.FIELD)
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_EMPTY)
public class FreightOffer {
    @JsonIgnore
    @XmlTransient
    private String freightId;
    private String id;

    private String externalSystemId;

    private AddressLocation pickUp;

    private AddressLocation delivery;

    private FreightDescription freightDescription;

    private ListContacts contacts;

    private Customer customer;

    private ListSla slas;

    private String pushId;

    private CompanyProfile company;

    private Route route;

    private String href;

    private Lifecycle lifecycle;

    private Visibility visibility;

    private Boolean unfoldedVXMatching;
    // getters / setters
</code></pre>

<p>Child class:</p>

<pre><code>@XmlAccessorType(XmlAccessType.PROPERTY)
public class FreightOfferDetail extends FreightOffer {

    private List&lt;Contact&gt; contact;

    @XmlElement(name = "contacts")
    @JsonProperty("contacts")
    public List&lt;Contact&gt; getContact() {
        return contact;
    }

    public void setContact(List&lt;Contact&gt; contact) {
        this.contact = contact;
    }
</code></pre>

<p>It fails exactly at this method <code>toXML()</code>:</p>

<pre><code>public class ObjectTransformer&lt;T&gt; implements Transformer&lt;T&gt; {

    protected final JAXBContext context;
    protected final Marshaller marshaller;

    protected final int okStatusCode = 200;
    protected final String okSubErrorCode = "OK";

    public ObjectTransformer(JAXBContext context) throws JAXBException {
        this.context = context;
        marshaller = context.createMarshaller();
        marshaller.setProperty("jaxb.encoding", "UTF-8");
        marshaller.setProperty("jaxb.formatted.output", Boolean.TRUE);
    }

    public String toXML(T object) throws JAXBException {
        StringWriter writer = new StringWriter();
        marshaller.marshal(object, writer);
        String xmlOffer = writer.toString();
        return xmlOffer;
    }
</code></pre>

<p>It should work, but it shouldn't. </p>

<p>I couldn't find what is missed or wrong here.</p>

<p><strong><em>UPDATE:</em></strong></p>

<p>Here is snippet from test:</p>

<pre><code>public SampleResult runTest(JavaSamplerContext context) {
    AbstractSamplerResults results = new XMLSamplerResults(new SampleResult());
    results.startAndPauseSampler();

    if (failureCause != null) {
        results.setExceptionFailure("FAILED TO INSTANTIATE connectTransformer", failureCause);
    } else {
        FreightOfferDTO offer = null;
        FreightOffer freightOffer = null;
        try {
            results.resumeSampler();            

            RouteInfo routeDTO = SamplerUtils.getRandomRouteFromRepo(context.getIntParameter(ROUTES_TOUSE_KEY));

            offer = FreightProvider.createRandomFreight(routeDTO, createUserWithLoginOnly(context));

            freightOffer = connectTransformer.fromDTO(offer);
            String xmlOfferString = connectTransformer.toXML(freightOffer); // &lt;- it fails here.
</code></pre>

<p>I take the date from <strong><em>CSV</em></strong> file and converting to <strong><em>DTO</em></strong> object. This method returns to me 
<code>FreightOfferDetail.</code></p>

<p>Here is snippet from this method:</p>

<pre><code>public FreightOfferDetail freightFromDTO(FreightOfferDTO freightDTO, boolean fullFormat){
    FreightOfferDetail freight = new FreightOfferDetail();

    freight.setFreightId(freightDTO.getIds().getAtosId());
    freight.setId(freightDTO.getIds().getFxId());
    // ...
</code></pre>

<p><strong><em>How to marshal object to XML file, at this case?</em></strong></p>