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Difference between revisions of "EclipseLink/UserGuide/MOXy/Relationships/Bidirectional Relationships"

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= Bidirectional Relationships  =
 
= Bidirectional Relationships  =
  
In order to map bidirectional relationships in EclipseLink MOXy, one of the properties must be annotated as an '''@XmlInverseReference'''.  Without this annotation, the cyclic relationship will result in an infinite loop during marshalling.
+
In order to map bidirectional relationships in EclipseLink MOXy, the back-pointer must be annotated as an '''@XmlInverseReference'''.  Without this annotation, the cyclic relationship will result in an infinite loop during marshalling.
  
 
'''@XmlInverseReferences''' must specify the '''mappedBy''' attribute, which indicates the property on the opposite side of the relationship.
 
'''@XmlInverseReferences''' must specify the '''mappedBy''' attribute, which indicates the property on the opposite side of the relationship.
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</source>
 
</source>
  
 +
In addition, when using '''@XmlInverseReference''', it is not necessary to explicitly set the back-pointer in your Java code; EclipseLink will do this for you automatically:
  
 +
<source lang="java">
 +
Employee emp = new Employee();
 +
emp.setName("Bob Smith");
  
 +
PhoneNumber p = new PhoneNumber();
 +
p.setNumber("555-1212");
  
 +
emp.getPhones().add(p);
 +
 +
// Not Necessary
 +
// p.setEmployee(emp);
 +
</source>
  
  

Revision as of 14:12, 19 April 2011


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Bidirectional Relationships

In order to map bidirectional relationships in EclipseLink MOXy, the back-pointer must be annotated as an @XmlInverseReference. Without this annotation, the cyclic relationship will result in an infinite loop during marshalling.

@XmlInverseReferences must specify the mappedBy attribute, which indicates the property on the opposite side of the relationship.

In this example, an Employee has a collection of PhoneNumbers, and each PhoneNumber has a back-pointer back to its Employee:

@XmlAccessorType(XmlAccessType.FIELD)
public class Employee {
   private String name;
   private List<PhoneNumber> phones = new ArrayList<PhoneNumber>();
   ...
}
 
@XmlAccessorType(XmlAccessType.FIELD)
public class PhoneNumber {
   private String number;
   @XmlInverseReference(mappedBy="phones")
   private Employee employee;
   ...
}

In addition, when using @XmlInverseReference, it is not necessary to explicitly set the back-pointer in your Java code; EclipseLink will do this for you automatically:

Employee emp = new Employee();
emp.setName("Bob Smith");
 
PhoneNumber p = new PhoneNumber();
p.setNumber("555-1212");
 
emp.getPhones().add(p);
 
// Not Necessary
// p.setEmployee(emp);










When binding JPA entities to XML, there are MOXy extensions will help you map a bidirectional relationship.

In this sample model, notice that Customer has a relationship to Address, and Address has a relationship back to Customer. In JPA, one direction of the relationship is mapped (the customer property on Address); the other direction specifies a mapping to leverage.

Sample Model

 
import javax.persistence.*;
 
@Entity
public class Customer {
 
    @Id
    private long id;
 
    @OneToOne(mappedBy="customer", cascade={CascadeType.ALL})
    private Address address;
 
}
 
 
import javax.persistence.*;
 
@Entity
public class Address implements Serializable {
 
    @Id
    private long id;
 
    @MapsId
    @OneToOne
    @JoinColumn(name="ID")
    private Customer customer;
 
}

To marshal these objects to XML, mark one direction @XmlTransient' to prevent a JAXB infinite loop during marshalling. Normally, during unmarshalling (from XML-to-object), you are responsible for populating the back pointer

 
import javax.persistence.*;
import javax.xml.bind.annotation.*;
 
@Entity
public class Address implements Serializable {
 
    @Id
    private long id;
 
    @OneToOne
    @JoinColumn(name="ID")
    @MapsId
    @XmlTransient
    private Customer customer;
 
}


@XMLInverseReference

With the @XmlInverseReference annotation, MOXy will populate the back pointer automatically. In this sample, notice that the @XmlInverseReference annotation leverages the same "mappedBy" concept.

 
import javax.persistence.*;
import org.eclipse.persistence.oxm.annotations.*;
 
@Entity
public class Address implements Serializable {
 
    @Id
    private long id;
 
    @OneToOne
    @JoinColumn(name="ID")
    @MapsId
    @XmlInverseReference(mappedBy="address")
    private Customer customer;
 
}


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