input.xml

The following is the XML document I will use for this example. The 2nd "other-phone-number" element refers to the same data as the "primary-phone-number" element, and the 3rd "other-phone-number" element refers to the same data as the 1st "other-phone-number" element.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <primary-phone-number id="B">555-BBBB</primary-phone-number>
    <other-phone-number id="A">555-AAAA</other-phone-number>
    <other-phone-number reference="B"/>
    <other-phone-number reference="A"/>
</customer>

Customer

The Customer class maintains references to PhoneNumber objects. The same instance of PhoneNumber may be referenced multiple times.

package package blog.xmladapter.stateful;
 
import java.util.List;
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlType(propOrder={"primaryPhoneNumber", "otherPhoneNumbers"})
public class Customer {
 
    private PhoneNumber primaryPhoneNumber;
    private List<PhoneNumber> otherPhoneNumbers;
 
    @XmlElement(name="primary-phone-number")
    public PhoneNumber getPrimaryPhoneNumber() {
        return primaryPhoneNumber;
    }
 
    public void setPrimaryPhoneNumber(PhoneNumber primaryPhoneNumber) {
        this.primaryPhoneNumber = primaryPhoneNumber;
    }
 
    @XmlElement(name="other-phone-number")
    public List<PhoneNumber> getOtherPhoneNumbers() {
        return otherPhoneNumbers;
    }
 
    public void setOtherPhoneNumbers(List<PhoneNumber> phoneNumbers) {
        this.otherPhoneNumbers = 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 blog.xmladapter.stateful;
 
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlTransient;
import javax.xml.bind.annotation.XmlValue;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
 
@XmlJavaTypeAdapter(PhoneNumberAdapter.class)
@XmlTransient
public class PhoneNumber {
 
    private String id;
    private String number;
 
    @XmlAttribute
    public String getId() {
        return id;
    }
 
    public void setId(String id) {
        this.id = id;
    }
 
    @XmlValue
    public String getNumber() {
        return number;
    }
 
    public void setNumber(String number) {
        this.number = number;
    }
 
    @Override
    public boolean equals(Object object) {
        if(null == object || object.getClass() != this.getClass()) {
            return false;
        }
        PhoneNumber test = (PhoneNumber) object;
        if(!equals(id, test.getId())) {
            return false;
        }
        return equals(number, test.getNumber());
    }
 
    private boolean equals(String control, String test) {
        if(null == control) {
            return null == test;
        } else {
            return control.equals(test);
        }
    }
 
    @Override
    public int hashCode() {
        return id.hashCode();
    }
 
}

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 blog.xmladapter.stateful;
 
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.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>();
 
    public static class AdaptedPhoneNumber extends PhoneNumber {
        @XmlAttribute
        public String reference;
    }
 
    @Override
    public AdaptedPhoneNumber marshal(PhoneNumber phoneNumber) throws Exception {
        AdaptedPhoneNumber adaptedPhoneNumber = new AdaptedPhoneNumber();
        if(phoneNumberList.contains(phoneNumber)) {
            adaptedPhoneNumber.reference = phoneNumber.getId();
        } else {
            adaptedPhoneNumber.setId(phoneNumber.getId());
            adaptedPhoneNumber.setNumber(phoneNumber.getNumber());
            phoneNumberList.add(phoneNumber);
        }
        return adaptedPhoneNumber;
    }
 
    @Override
    public PhoneNumber unmarshal(AdaptedPhoneNumber adaptedPhoneNumber) throws Exception {
        PhoneNumber phoneNumber = phoneNumberMap.get(adaptedPhoneNumber.reference);
        if(null == phoneNumber) {
            phoneNumber = new PhoneNumber();
            phoneNumber.setId(adaptedPhoneNumber.getId());
            phoneNumber.setNumber(adaptedPhoneNumber.getNumber());
            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 blog.xmladapter.stateful;
 
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/blog/xmladapter/stateful/input.xml");
        Customer customer = (Customer) unmarshaller.unmarshal(xml);
 
        PhoneNumber primaryPN = customer.getPrimaryPhoneNumber();
        PhoneNumber otherPN1 = customer.getOtherPhoneNumbers().get(0);
        PhoneNumber otherPN2 = customer.getOtherPhoneNumbers().get(1);
        PhoneNumber otherPN3 = customer.getOtherPhoneNumbers().get(2);
        System.out.println(primaryPN == otherPN2);
        System.out.println(otherPN1 == otherPN3);
 
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.setAdapter(new PhoneNumberAdapter());
        marshaller.marshal(customer, System.out);
    }
 
}

Output