@XmlRegistry - how does it work?

Question

I have found some examples of JAXB2 @XmlRegistry over the internet but no good in-depth tutorials that talk about the concept of using @XmlRegistry wit

Answer 1

@XmlRegistry - how does it work?

@XmlRegistry is used to mark a class that has @XmlElementDecl annotations. To have your JAXB implementation process this class you need to ensure that it is included in the list of classes used to bootstrap the JAXBContext. It is not enough for it to be a static inner class of one of your domain model classes:

JAXBContext context = JAXBContext.newInstance(Employee.class, Employee.XMLObjectFactory.class);

@XmlElementDecl - how does it work?

If the value of the field/property is going to be a JAXBElement then you need to leverage @XmlElementDecl. A JAXBElement captures information that is can be useful:

• Element name, this is necessary if you are mapping to a choice structure where multiple elements are of the same type. If the element name does not correspond to a unique type then you would not be able to round-trip the document.
• JAXBElement can be used to represent an element with xsi:nil="true".

XmlObjectFactory

@XmlElementDecl also allows you to specify a scope. I have modified the model from you post a bit. I have introduced an XmlObjectFactory class that has two @XmlElementDecl. Both specify a name of address. I have leveraged the scope property so that for properties within the Employee class the @XmlElementDecl corresponding to the Address class with be used.

package forum11078850;

import javax.xml.bind.JAXBElement;
import javax.xml.bind.annotation.XmlElementDecl;
import javax.xml.bind.annotation.XmlRegistry;
import javax.xml.namespace.QName;

@XmlRegistry
public class XmlObjectFactory {

    @XmlElementDecl(scope = Employee.class, name = "address")
    JAXBElement
 createAddress(Address value) {
        return new JAXBElement
(new QName("address"), Address.class, value);
    }

    @XmlElementDecl(name = "address")
    JAXBElement createStringAddress(String value) {
        return new JAXBElement(new QName("address"), String.class, value);
    }

}

Employee

The @XmlElementRef annotation will cause the value of the property to be matched on its root element name. Possible matches will include classes mapped with @XmlRootElement or @XmlElementDecl.

package forum11078850;

import java.util.List;

import javax.xml.bind.JAXBElement;
import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlType(propOrder = { "id", "name", "email", "addresses" })
public class Employee {
    private int id;
    private String name;
    private String email;

    private List> addresses;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @XmlElementWrapper
     @XmlElementRef(name="address")
    public List> getAddresses() {
        return addresses;
    }

    public void setAddresses(List> addresses) {
        this.addresses = addresses;
    }

}

ObjectFactoryTest

package forum11078850;

import java.io.FileReader;
import javax.xml.bind.*;

public class ObjectFactoryTest {
    public static void main(String[] args) throws Exception {
        FileReader reader = new FileReader("src/forum11078850/input.xml");
        JAXBContext context = JAXBContext.newInstance(Employee.class, XmlObjectFactory.class);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        Object obj = unmarshaller.unmarshal(reader);

        Marshaller marshaller = context.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(obj, System.out);
    }
}

The Address class and input.xml from my original answer can be used to run this example.

---

ORIGINAL ANSWER

I'm not sure how you are attempting to use @XmlRegistry, so I will focus on the following part of your post:

When I unmarshal the employee xml using above code, the address list does not get populated. The resulting employee object only has a blank list of adresses. Is there anything wrong with my mappings?

Your list of Address objects is wrapped in a grouping element (addresses), so you need to use the @XmlElementWrapper annotation to map this use case. Below is a complete example:

Employee

package forum11078850;

import java.util.List;
import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlType(propOrder = { "id", "name", "email", "addresses" })
public class Employee {
    private int id;
    private String name;
    private String email;

    private List
 addresses;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @XmlElementWrapper
    @XmlElement(name = "address")
    public List
 getAddresses() {
        return addresses;
    }

    public void setAddresses(List
 addresses) {
        this.addresses = addresses;
    }

}

Address

package forum11078850;

public class Address {
    private String addressLine1;
    private String addressLine2;
    private String addressLine3;

    public String getAddressLine1() {
        return addressLine1;
    }

    public void setAddressLine1(String addressLine1) {
        this.addressLine1 = addressLine1;
    }

    public String getAddressLine2() {
        return addressLine2;
    }

    public void setAddressLine2(String addressLine2) {
        this.addressLine2 = addressLine2;
    }

    public String getAddressLine3() {
        return addressLine3;
    }

    public void setAddressLine3(String addressLine3) {
        this.addressLine3 = addressLine3;
    }

}

ObjectFactoryTest

package forum11078850;

import java.io.FileReader;
import javax.xml.bind.*;

public class ObjectFactoryTest {
    public static void main(String[] args) throws Exception {
        FileReader reader = new FileReader("src/forum11078850/input.xml");
        JAXBContext context = JAXBContext.newInstance(Employee.class);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        Object obj = unmarshaller.unmarshal(reader);

        Marshaller marshaller = context.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(obj, System.out);
    }
}

input.xml/Output

    1
    Vaishali
    Vaishali@example.com
    
        

            300
            Mumbai
            India
        
        

            301
            Pune
            India