@XmlRegistry - how does it work?

java, jaxb, jaxb2, xml

Solution

@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<Address> createAddress(Address value) {
        return new JAXBElement<Address>(new QName("address"), Address.class, value);
    }

    @XmlElementDecl(name = "address")
    JAXBElement<String> createStringAddress(String value) {
        return new JAXBElement<String>(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<JAXBElement<Address>> 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<JAXBElement<Address>> getAddresses() {
        return addresses;
    }

    public void setAddresses(List<JAXBElement<Address>> 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<Address> 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<Address> getAddresses() {
        return addresses;
    }

    public void setAddresses(List<Address> 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

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<employee>
    <id>1</id>
    <name>Vaishali</name>
    <email>Vaishali@example.com</email>
    <addresses>
        <address>
            <addressLine1>300</addressLine1>
            <addressLine2>Mumbai</addressLine2>
            <addressLine3>India</addressLine3>
        </address>
        <address>
            <addressLine1>301</addressLine1>
            <addressLine2>Pune</addressLine2>
            <addressLine3>India</addressLine3>
        </address>
    </addresses>
</employee>

Problem

Original source