Question

I’m wondering if it’s possible to annotate my classes so that the first time the marshaller encounters an object, it generates an XML element of the appropriate type, but any subsequent reference to this object by anything else will have an XML IDREF entry created?

Answer 1

You can leverage the concept of JAXB’s XmlAdapter to do something like the following:

input.xml

The following is the XML document I will use for this example. The 3rd phone-number entry is a reference to the 1st phone-number entry, and the 5th phone-number entry is a reference to the 4th.:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <phone-number id="A">
        <number>555-AAAA</number>
    </phone-number>
    <phone-number id="B">
        <number>555-BBBB</number>
    </phone-number>
    <phone-number id="A"/>
    <phone-number xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="work-phone-number" id="W">
        <number>555-WORK</number>
        <extension>1234</extension>
    </phone-number>
    <phone-number xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="work-phone-number" id="W"/>
</customer>

Customer

The customer class maintains a collection of PhoneNumber objects. The same instance of PhoneNumber may appear multiple times in the collection.

package forum7587095;

import java.util.List;

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

@XmlRootElement
public class Customer {

    private List<PhoneNumber> phoneNumbers;

    @XmlElement(name="phone-number")
    public List<PhoneNumber> getPhoneNumbers() {
        return phoneNumbers;
    }

    public void setPhoneNumbers(List<PhoneNumber> phoneNumbers) {
        this.phoneNumbers = phoneNumbers;
    }

}

PhoneNumber

This is a class that can either appear in the document itself or as a reference. This will be handled using an XmlAdapter. An XmlAdapter is configured using the @XmlJavaTypeAdapter annotation. Since we have specified this adapter at the type/class level it will apply to all properties referencing the PhoneNumber class:

package forum7587095;

import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlJavaTypeAdapter(PhoneNumberAdapter.class)
public class PhoneNumber {

    private String id;
    private String number;

    public String getId() {
        return id;
    }

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

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    @Override
    public boolean equals(Object arg0) {
        if(null == arg0 || arg0.getClass() != this.getClass()) {
            return false;
        }
        PhoneNumber test = (PhoneNumber) arg0;
        if(!equals(id, test.getId())) {
            return false;
        }
        return equals(number, test.getNumber());
    }

    protected boolean equals(String control, String test) {
        if(null == control) {
            return null == test;
        } else {
            return control.equals(test);
        }
    }

    @Override
    public int hashCode() {
        return id.hashCode();
    }

}

WorkPhoneNumber

Based on your comment I have added a subclass of PhoneNumber.

package forum7587095;

public class WorkPhoneNumber extends PhoneNumber {

    private String extension;

    public String getExtension() {
        return extension;
    }

    public void setExtension(String extension) {
        this.extension = extension;
    }

    @Override
    public boolean equals(Object arg0) {
        if(!super.equals(arg0)) {
            return false;
        }
        return equals(extension, ((WorkPhoneNumber) arg0).getExtension());
    }

}

PhoneNumberAdapter

Below is the implementation of the XmlAdapter. Note that we must maintain if the PhoneNumber object has been seen before. If it has we only populate the id portion of the AdaptedPhoneNumber object.

package forum7587095;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlSeeAlso;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class PhoneNumberAdapter extends XmlAdapter<PhoneNumberAdapter.AdaptedPhoneNumber, PhoneNumber>{

    private List<PhoneNumber> phoneNumberList = new ArrayList<PhoneNumber>();
    private Map<String, PhoneNumber> phoneNumberMap = new HashMap<String, PhoneNumber>();

    @XmlSeeAlso(AdaptedWorkPhoneNumber.class)
    @XmlType(name="phone-number")
    public static class AdaptedPhoneNumber {
        @XmlAttribute public String id;
        public String number;

        public AdaptedPhoneNumber() {
        }

        public AdaptedPhoneNumber(PhoneNumber phoneNumber) {
            id = phoneNumber.getId();
            number = phoneNumber.getNumber();
        }

        public PhoneNumber getPhoneNumber() {
            PhoneNumber phoneNumber = new PhoneNumber();
            phoneNumber.setId(id);
            phoneNumber.setNumber(number);
            return phoneNumber;
        }

    }

    @XmlType(name="work-phone-number")
    public static class AdaptedWorkPhoneNumber extends AdaptedPhoneNumber {

        public String extension;

        public AdaptedWorkPhoneNumber() {
        }

        public AdaptedWorkPhoneNumber(WorkPhoneNumber workPhoneNumber) {
            super(workPhoneNumber);
            extension = workPhoneNumber.getExtension();
        }

        @Override
        public WorkPhoneNumber getPhoneNumber() {
            WorkPhoneNumber phoneNumber = new WorkPhoneNumber();
            phoneNumber.setId(id);
            phoneNumber.setNumber(number);
            phoneNumber.setExtension(extension);
            return phoneNumber;
        }
}

    @Override
    public AdaptedPhoneNumber marshal(PhoneNumber phoneNumber) throws Exception {
        AdaptedPhoneNumber adaptedPhoneNumber;
        if(phoneNumberList.contains(phoneNumber)) {
            if(phoneNumber instanceof WorkPhoneNumber) {
                adaptedPhoneNumber = new AdaptedWorkPhoneNumber();
            } else {
                adaptedPhoneNumber = new AdaptedPhoneNumber();
            }
            adaptedPhoneNumber.id = phoneNumber.getId();
        } else {
            if(phoneNumber instanceof WorkPhoneNumber) {
                adaptedPhoneNumber = new AdaptedWorkPhoneNumber((WorkPhoneNumber)phoneNumber);
            } else {
                adaptedPhoneNumber = new AdaptedPhoneNumber(phoneNumber);
            }
            phoneNumberList.add(phoneNumber);
        }
        return adaptedPhoneNumber;
    }

    @Override
    public PhoneNumber unmarshal(AdaptedPhoneNumber adaptedPhoneNumber) throws Exception {
        PhoneNumber phoneNumber = phoneNumberMap.get(adaptedPhoneNumber.id);
        if(null != phoneNumber) {
            return phoneNumber;
        }
        phoneNumber = adaptedPhoneNumber.getPhoneNumber();
        phoneNumberMap.put(phoneNumber.getId(), phoneNumber);
        return phoneNumber;
    }

}

Demo

To ensure the same instance of the XmlAdapter is used for the entire marshal and unmarshal operations we must specifically set an instance of the XmlAdapter on both the Marshaller and Unmarshaller:

package forum7587095;

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Customer.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        unmarshaller.setAdapter(new PhoneNumberAdapter());
        File xml = new File("src/forum7587095/input.xml");
        Customer customer = (Customer) unmarshaller.unmarshal(xml);

        System.out.println(customer.getPhoneNumbers().get(0) == customer.getPhoneNumbers().get(2));
        System.out.println(customer.getPhoneNumbers().get(3) == customer.getPhoneNumbers().get(4));

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.setAdapter(new PhoneNumberAdapter());
        marshaller.marshal(customer, System.out);
    }

}

Output

true
true
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <phone-number id="A">
        <number>555-AAAA</number>
    </phone-number>
    <phone-number id="B">
        <number>555-BBBB</number>
    </phone-number>
    <phone-number id="A"/>
    <phone-number xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="work-phone-number" id="W">
        <number>555-WORK</number>
        <extension>1234</extension>
    </phone-number>
    <phone-number xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="work-phone-number" id="W"/>
</customer>

For More Information

• http://blog.bdoughan.com/2011/09/mixing-nesting-and-references-with.html
• http://blog.bdoughan.com/2010/10/jaxb-and-shared-references-xmlid-and.html
• http://blog.bdoughan.com/search/label/XmlAdapter
• http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-xsitype.html
• http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-substitution.html