Information on the root XML element is required when marshalling to or unmarshalling from a Java object. JAXB provides this information via the @XmlRootElement annotation which contains the name and namespace of the root XML element.

When trying to marshal a class which does not have a @XMLRootElement annotation defined, following error will be thrown: "unable to marshal as an element because it is missing an @XmlRootElement annotation". Alternatively when trying to unmarshal, the Java runtime will report that an "unsuspected element" is found.

For this example let’s use following class representing a car with a basic structure. Note that a XmlRootElement is not defined!

package com.codenotfound.jaxb.model;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;

public class Car {

  private String make;
  private String manufacturer;
  private String id;

  public String getMake() {
    return make;
  }

  @XmlElement
  public void setMake(String make) {
    this.make = make;
  }

  public String getManufacturer() {
    return manufacturer;
  }

  @XmlElement
  public void setManufacturer(String manufacturer) {
    this.manufacturer = manufacturer;
  }

  public String getId() {
    return id;
  }

  @XmlAttribute
  public void setId(String id) {
    this.id = id;
  }

  @Override
  public String toString() {
    return "Car [" + "make=" + make + ", manufacturer=" + manufacturer + ", id=" + id + "]";
  }
}

Marshal when @XMLRootElement is missing

Marshalling is the process of transforming the memory representation of an object to a data format suitable for storage or transmission. In the case of JAXB it means converting a Java object into XML. The below code snippet shows the creation of a new Car instance.

  car = new Car();
  car.setMake("Passat");
  car.setManufacturer("Volkswagen");
  car.setId("ABC-123");

The method below takes as input the above car object and tries to marshal it using JAXB.

public static String marshalError(Car car) throws JAXBException {
  StringWriter stringWriter = new StringWriter();

  JAXBContext jaxbContext = JAXBContext.newInstance(Car.class);
  Marshaller jaxbMarshaller = jaxbContext.createMarshaller();

  // format the XML output
  jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

  jaxbMarshaller.marshal(car, stringWriter);

  String result = stringWriter.toString();
  LOGGER.info(result);
  return result;
}

When running the above method, the runtime returns an error as the Car class is missing the required @XMLRootElement annotation.

unable to marshal type "com.codenotfound.jaxb.model.Car" as an
element because it is missing an @XmlRootElement annotation

In order to be able to marshal the car object we need to provide a root XML element. This is done as shown below by first creating a qualified name which contains the name and namespace of the root XML element. In a next step we create a new JAXBElement and pass the qualified name, class and object. Using the created JAXBElement we call the marshal() method.

public static String marshal(Car car) throws JAXBException {
  StringWriter stringWriter = new StringWriter();

  JAXBContext jaxbContext = JAXBContext.newInstance(Car.class);
  Marshaller jaxbMarshaller = jaxbContext.createMarshaller();

  // format the XML output
  jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

  QName qName = new QName("com.codenotfound.jaxb.model", "car");
  JAXBElement<Car> root = new JAXBElement<>(qName, Car.class, car);

  jaxbMarshaller.marshal(root, stringWriter);

  String result = stringWriter.toString();
  LOGGER.info(result);
  return result;
}

This time JAXB is able to successfully marshal the object and the result is the following:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:car id="ABC-123" xmlns:ns2="com.codenotfound.jaxb.model">
  <make>Passat</make>
  <manufacturer>Volkswagen</manufacturer>
</ns2:car>

Unmarshal when @XMLRootElement is missing

Unmarshalling in JAXB is the process of converting XML content into a Java object. Lets reuse the XML representation of a car that we generated in the previous section.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:car id="DEF-456" xmlns:ns2="com.codenotfound.jaxb.model">
  <make>Golf</make>
  <manufacturer>Volkswagen</manufacturer>
</ns2:car>

In the method below we pass the above XML file and try to unmarshal it to an instance of the Car class. Note that it is also possible to use JAXB to create an object from an XML String instead of using a file.

public static Car unmarshalError(File file) throws JAXBException {
  JAXBContext jaxbContext = JAXBContext.newInstance(Car.class);
  Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();

  Car car = (Car) jaxbUnmarshaller.unmarshal(file);

  LOGGER.info(car.toString());
  return car;
}

When running the above code, the runtime returns an error as a root XML element is found (the XML has a root element) but the Car class does not define a @XMLRootElement and as such it is not expected.

unexpected element (uri:"com.codenotfound.jaxb.model", local:"car").
Expected elements are (none)

In order to be able to unmarshal the object we need to define a root XML element. This is done as shown below by first manually creating the root JAXBElement of type Car by using the XML file and the class we are trying to unmarshal to. Then we create a Car object and assign the value of the previous created root JAXBElement.

public static Car unmarshal(File file) throws JAXBException {
  JAXBContext jaxbContext = JAXBContext.newInstance(Car.class);
  Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();

  JAXBElement<Car> root = jaxbUnmarshaller.unmarshal(new StreamSource(
            file), Car.class);
  Car car = root.getValue();

  LOGGER.info(car.toString());
  return car;
}

This time JAXB is able to successfully unmarshal the object and the result is the following:

Car [make=Golf, manufacturer=Volkswagen, id=DEF-456]

github mark If you would like to run the above code sample you can get the full source code here.

This concludes the marshal & unmarshal with missing @XmlRootElement annotation code samples. If you found this post helpful or have any questions or remarks, please leave a comment below.

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