JAXB - Marshal Element Missing @XmlRootElement Annotation
Table of Contents
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. Let’s 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]
This concludes our code samples on how to marshal and unmarshal with missing @XmlRootElement annotation. If you found this post helpful or have any questions or remarks, please leave a comment below.