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Difference between revisions of "EclipseLink/Examples/MOXy/JPA"

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=== Shared Reference Relationships ===
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=== Relationships - Shared Reference ===
  
 
These relationships apply when the target object(s) may be referenced by multiple source objects.  This type of relationship can not be safely represented as nesting in XML.
 
These relationships apply when the target object(s) may be referenced by multiple source objects.  This type of relationship can not be safely represented as nesting in XML.

Revision as of 10:18, 1 October 2009

Overview

This example demonstrates how to derive an XML representation from a set of JPA entities using JAXB. In simplest terms JAXB will be used to apply a tree structure to the object graph of JPA entities. Multiple tree representations of an object graph are possible and will depend on the root object chosen and the direction the relationships are traversed.

JPA Entities

For this example the following JPA entities will be used. In order to save space package names, import statements, and the get /set methods have been omitted from the following code samples. All annotations shown below are standard JPA annotations.

@Entity
public class Department {
 
    @Id
    @Column(name="D_ID")
    private BigDecimal dId;
 
    private String name;
 
    @ManyToMany
    @JoinTable(name="DEPT_EMP", joinColumns = 
        @JoinColumn(name="D_ID", referencedColumnName = "D_ID"), 
            inverseJoinColumns = @JoinColumn(name="E_ID", 
                referencedColumnName = "E_ID"))
    private List<Employee> member;
 
}
@Entity
public class Employee {
 
    @Id
    @Column(name="E_ID")
    private BigDecimal eId;
 
    private String name;
 
    @OneToOne(mappedBy="resident")
    private Address residence;
 
    @OneToMany(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
    @ManyToMany(mappedBy="member")
    private List<Department> team;
 
}
@Entity
public class Address {
 
    @Id
    @Column(name="E_ID", insertable=false, updatable=false)
    private BigDecimal eId;
 
    private String city;
 
    private String street;
 
    @OneToOne
    @JoinColumn(name="E_ID")
    private Employee resident;
 
}
@Entity
@Table(name="PHONE_NUMBER")
public class PhoneNumber {
 
    @Id
    @Column(name="P_ID")
    private BigDecimal pId;
 
    @ManyToOne
    @JoinColumn(name="E_ID", referencedColumnName = "E_ID")
    private Employee contact;
 
    private String num;
 
}

JAXB Binding

For this example the XML acessor type will be set to FIELD for all the model classes. This can be set as a package level JAXB annotation.

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

Relationships - Privately Owned

These relationships apply when the target object(s) is only referenced by a single source object. This type of relationship can be safely represented as nesting in XML (the default in JAXB).

One To One

In JPA the OneToOne annotation indicates that only one instance of the source entity is able to refer to the same target entity instance. Since the relationship between Employee and Address is bi-directional, we need to mark one side transient in JAXB to prevent a cycle.

@Entity
public class Employee {
 
    @OneToOne(mappedBy="resident")
    private Address residence;
 
}
@Entity
public class Address {
 
    @OneToOne
    @JoinColumn(name="E_ID")
    @XmlTransient
    private Employee resident;
 
}

One To Many

In JPA the OneToMany annotation indicates that only one instance of the source entity is able to refer to the same target entity instance. Since the relationship between Employee and Address is bi-directional, we need to mark one side transient in JAXB to prevent a cycle.

@Entity
public class Employee {
 
    @OneToMany(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
}
@Entity
public class PhoneNumber {
 
    @ManyToOne
    @JoinColumn(name="E_ID", referencedColumnName = "E_ID")
    @XmlTransient
    private Employee contact;
 
}

Relationships - Shared Reference

These relationships apply when the target object(s) may be referenced by multiple source objects. This type of relationship can not be safely represented as nesting in XML.

Many To One

In JPA the ManyToOne annotation indicates that one or more instances of the source entity are able to refer to the same target entity instance.

@Entity
public class PhoneNumber {
 
    @ManyToOne
    @JoinColumn(name="E_ID", referencedColumnName = "E_ID")
    private Employee contact;
 
}
@Entity
public class Employee {
 
    @OneToMany(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
}

Many To Many

In JPA the ManyToMany annotation indicates that one or more instances of the source entity are able to refer to the same target entity instance.

@Entity
public class Department {
 
    @Id
    @Column(name="D_ID")
    private BigDecimal dId;
 
    private String name;
 
    @ManyToMany
    @JoinTable(name="DEPT_EMP", joinColumns = 
        @JoinColumn(name="D_ID", referencedColumnName = "D_ID"), 
            inverseJoinColumns = @JoinColumn(name="E_ID", 
                referencedColumnName = "E_ID"))
    private List<Employee> member;
 
}
@Entity
public class Employee {
 
    @ManyToMany(mappedBy="member")
    private List<Department> team;
 
}