Using the Embedded Key Class

With JAXB, you can derive an XML representation from a set of JPA entities, when a JPA entity has an embedded ID class.

In Example 7-4, the EmployeeId is the embedded ID of the Employee class:

@Entity
public class PhoneNumber {
 
    @ManyToOne
    @JoinColumns({
        @JoinColumn(name="E_ID", referencedColumnName = "E_ID"),
        @JoinColumn(name="E_COUNTRY", referencedColumnName = "COUNTRY")
    })
    private Employee contact;
 
}
 
@Entity
@IdClass(EmployeeId.class)
public class Employee {
 
    @EmbeddedId
    private EmployeeId id;
 
    @OneToMany(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
}
 
@Embeddable
public class EmployeeId {
 
    @Column(name="E_ID")
    private BigDecimal eId;
 
    private String country;
 
}
 

For the JAXB bindings, the XML accessor type will be set to FIELD for all the model classes. This can be set as a package level JAXB annotation, as shown here:

@XmlAccessorType(XmlAccessType.FIELD)
package com.example.model;
 
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
 

Example 7-5 uses the EclipseLink extension @XmlCustomizer which extends the JAXB specification. Because the contact attribute is a bidirectional relationship, it includes the EclipseLink extension @XmlInverseReference.

@Entity
@IdClass(EmployeeId.class)
@XmlCustomizer(EmployeeCustomizer.class)
public class Employee {
 
    @EmbeddedId
    private EmployeeId id;
 
    @OneToMany(mappedBy="contact")
    @XmlInverseReference(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
}
 

To embed the content of the EmployeeId class in the complex type corresponding to the Employee class, change the XPath on the mapping for the id property to be self or . . Then specify the XPath to the XML nodes which represent the ID.

import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.oxm.mappings.XMLCompositeObjectMapping;
 
public class EmployeeCustomizer implements DescriptorCustomizer {
 
    public void customize(ClassDescriptor descriptor) throws Exception {
        XMLCompositeObjectMapping idMapping = 
            (XMLCompositeObjectMapping) descriptor.getMappingForAttributeName("id");
        idMapping.setXPath(".");
 
        descriptor.addPrimaryKeyFieldName("eId/text()");
        descriptor.addPrimaryKeyFieldName("country/text()");
    }
 
}
 

If the target object had a single ID then we would use @XmlIDREF. Since the target object has a compound key, we will mark the field @XmlTransient, and use the EclipseLink extension @XmlCustomizer to set up the mapping.

@Entity
@XmlCustomizer(PhoneNumberCustomizer.class)
public class PhoneNumber {
 
    @ManyToOne
    @JoinColumns({
        @JoinColumn(name="E_ID", referencedColumnName = "E_ID"),
        @JoinColumn(name="E_COUNTRY", referencedColumnName = "COUNTRY")
    })
    @XmlTransient
    private Employee contact;
 
}
 

An XMLObjectReferenceMapping will be created. The mapping will include multiple key mappings.

import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.oxm.mappings.XMLObjectReferenceMapping;
 
public class PhoneNumberCustomizer implements DescriptorCustomizer {
 
    public void customize(ClassDescriptor descriptor) throws Exception {
        XMLObjectReferenceMapping contactMapping = new XMLObjectReferenceMapping();
        contactMapping.setAttributeName("contact");
        contactMapping.setReferenceClass(Employee.class);
        contactMapping.addSourceToTargetKeyFieldAssociation("contact/@eID", "eId/text()");
        contactMapping.addSourceToTargetKeyFieldAssociation("contact/@country", "country/text()");
        descriptor.addMapping(contactMapping);
    }
 
}