EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/Embeddable

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Key API

• @Embeddable
• @Embedded
• @ElementCollection
• @AttributeOverride
• @AttributeOverrides
• @AssociationOverride
• @AssociationOverrides

Native API

• AggregateObjectMapping

@Embeddable

You can use the @Embeddable annotation or <embeddable> XML element to map an embedded class. An embeddable is a special type of class that is not directly persistent, but persisted only with its parent entity. An embeddable has no persistent identity, and no Id mapping. An embeddable can contain any type of attribute mapping including @Basic, @OneToOne, @ManyToOne, @OneToMany, @ManyToMany, @ElementCollection or another @Embedded. Embeddables cannot contain an @Id mapping.

An embeddable can be used to define an entities Id, normally in the case of a composite id. The id attributes in the embeddable will be mapped as @Basic not as @Id, as all attributes are required to be part of the @Id. The parent entity will use the @EmbeddedId to map the embeddeable instead of the @Embedded mapping.

An embeddable can be referenced from an entity or another embeddable using the @Embedded annotation for a single reference, or the @ElementCollection annotation for a Collection or Map reference. An embeddable can also be used in any Map key using the @MapKeyClass annotation.

Embeddables do not currently support the @Inheritance annotation, but inheritance is supported in EclipseLink through usage of a DescriptorCustomizer.

An embeddable cannot define a @Table annotation, and the columns used in its mappings are by default the names of the column is its parent's table. If the embeddable class is shared with multiple parents, then each parent can override the column names using the @AttributeOverride and @AssociationOverride annotations.

The @Embeddable annotation does not have any attributes.

For more information, see Section 2.5 "Embeddable Classes" in the JPA Specification.

For more information, see Section 11.1.13 "Embeddable Annotation" in the JPA Specification.

The following example shows usage of an embbeddable.

Example: Using @Embeddable annotation

@Embeddable 
public class EmployeementStatus implements Serializable {
    @Basic
    private String status;
    @Basic
    private String level;
    @Basic
    private java.sql.Date startDate;
    @Basic
    private java.sql.Date endDate;
    ...
}

@Entity
@Table(name="EMP")
public class Employee implements Serializable {
    @Id
    private Long id;
    @Basic
    private String name;
    @Embedded
    private EmployeementStatus status;
    ...
}

Example: Using <embeddable> XML

<entity class="Employee">
    <table name="EMP">
    <attributes>
        <id name="id"/>
        <basic name="name"/>
        <embeddable name="status"/>
        ...
    </attributes>
</entity>
<embeddable class="EmployeementStatus">
    <attributes>
        <basic name="status"/>
        <basic name="level"/>
        <basic name="startDate"/>
        <basic name="endDate"/>
    </attributes>
</embeddable >

Advanced Embeddable Configuration

Sharing

A embeddable class can be shared by more than one class, or even referenced twice in the same class. Since an embeddable does not define a table, this means the class will be stored in multiple different tables. As long as all of the tables that stored the embeddable have its columns this is fine. If any of the table have different columns names, this must be handle through using @AttributeOverride and @AssociationOverride.

Note that the same embeddable object instance should never be shared, each parent should have its own object instance as the objects are always stored in their parents table. Only the class can be shared.

@AttributeOverride

You can use the @AttributeOverride and @AttributeOverrides annotations, or <attribute-override> XML element to override the column for a basic attribute in an embedded relationship mapping. This allows for the column name to be different if the embeddable class is used by more than entity or if an entity defines multiple embedded relationships.

@AttributeOverride Attributes

Attribute	Description	Default	Required?
name	The name of the attribute.		Yes
column	The column in the source table for the embedded relationship.	column defined in embeddable	No

For more information, see Section 11.1.4 "AttributeOverride Annotation" in the JPA Specification.

@AssociationOverride

You can use the @AssociationOverride and @AssociationOverrides annotations, or <association-override> XML element to override the join column or join table for a relationship attribute in an embedded relationship mapping. This allows for the join column name to be different if the embeddable class is used by more than entity or if an entity defines multiple embedded relationships.

@AssociationOverrideAttributes

Attribute	Description	Default	Required?
name	The name of the attribute.		Yes
joinColumn	The join column in the source table for the embedded relationship.	join column defined in embeddable	No
joinTable	The join table for the subclass.	join table defined in embeddable	No

For more information, see Section 11.1.2 "AssociationOverride Annotation" in the JPA Specification.

Example: Shared embeddable

@Embeddable 
public class Status implements Serializable {
    @Basic
    private String status;
    @Basic
    private String level;
    @Basic
    private java.sql.Date startDate;
    @Basic
    private java.sql.Date endDate;
    ...
}

@Entity
@Table(name="EMP")
public class Employee implements Serializable {
    @Id
    private Long id;
    @Basic
    private String name;
    @Embedded
    @AttributeOverrides({
      @AttributeOverride(name="level", column=@Column(name="EMPLOYMENT_LEVEL"))
      @AttributeOverride(name="status", column=@Column(name="EMPLOYMENT_STATUS"))})
    private Status status;
    ...
}

@Entity
@Table(name="PROJ")
public class Project implements Serializable {
    @Id
    private Long id;
    @Basic
    private String name;
    @Embedded
    @AttributeOverrides({
      @AttributeOverride(name="level", column=@Column(name="PROJECT_LEVEL"))
      @AttributeOverride(name="status", column=@Column(name="PROJECT_STATUS"))})
    private Status status;
    ...
}

Relationships

Embeddable objects can define relationships. Any relationship type can be used including a nested embedded relationship.

For bi-directional relationships, the target object's inverse relationship must be back to the parent of the embeddable. The mappedBy can either reference the embeddable attribute directly, or use the dot notation from the parent class. Relationships to another object's embeddable are not allowed.

There is no standard way for an embeddable to reference its parent entity. The easiest way to implement this is to use property access to set the embedded object on the entity and in the set method set the back reference from the embeddable to its parent. The parent would be marked as @Transient as it is not a persistent relationship.

Example: Embeddable with relationships

@Embeddable 
public class ProjectInfo implements Serializable {
    @Basic
    private boolean available;
    @ManyToMany
    private List<Project> projects;
    ...
}

@Entity
@Table(name="EMP")
public class Employee implements Serializable {
    @Id
    private Long id;
    @Basic
    private String name;
    @Embedded
    private ProjectInfo projectInfo;
    ...
}

@Entity
@Table(name="PROJ")
public class Project implements Serializable {
    @Id
    private Long id;
    @Basic
    private String name;
    @ManyToMany(mappedBy="projectInfo.projects")
    private List<Employee> employees;
    ...
}

Nesting

An embeddable object can be embedded inside another embeddable object. All of the columns will be aggregated into the parent entity's table.

If an entity needs to override the column names for a nested embeddable the dot notation can be used inside the @AttributeOverride.

Nested embeddables cannot contain embedded references to the same embeddable class or cycles, as there would be no way to store this in a relational table.

Example: Nested embeddable

@Embeddable 
public class ProjectInfo implements Serializable {
    @Basic
    private String name;
    @Basic
    private BigDecimal budget;
    @Embedded
    private ProjectStatus status;
    ...
}

@Embeddable 
public class ProjectStatus implements Serializable {
    @Basic
    private String status;
    @Basic
    private boolean isBehindSchedule;
    ...
}

@Entity
public class Project implements Serializable {
    @Id
    private long id;
    @Embedded
    @AttributeOverride(name="budget", column=@Column(name="PROJECT_BUDGET"))
    @AttributeOverride(name="status.isBehindSchedule", column=@Column(name="BEHIND_SCHEDULE"))
    private ProjectInfo info;
    ...
}

Inheritance

The JPA specification does not define inheritance for embeddables. EclipseLink does not currently support using annotations or XML to define inheritance with embeddables, but EclipseLink does support inheritance with embeddables through the native API.

To define inheritance with a hierarchy of embedded classes a DescriptorCustomizer and SessionCustomizer can be used. The customizers can be used to define a discriminator column (class indicator field) in the root embeddable class' descriptor. The child descriptors can either be defined using a SessionCustomizer, or use a DescriptorCustomizer to set their parent class.

Example: Using a DescriptorCustomizer to enable inheritance in an embeddable

public class StatusCustomizer implements DescriptorCustomizer {
    public void customize(ClassDescriptor descriptor) {
        descriptor.getInheritancePolicy().setClassIndicatorFieldName("STATUS_TYPE");
        descriptor.getInheritancePolicy().addClassIndicator(RetiredStatus.class, "R");
        descriptor.getInheritancePolicy().addClassIndicator(TerminatedStatus.class, "T");
        descriptor.getInheritancePolicy().addClassIndicator(ActiveStatus.class, "A");
        descriptor.getInheritancePolicy().addClassIndicator(UnActiveStatus.class, "U");
    }
}

public class ActiveStatusCustomizer implements DescriptorCustomizer {
    public void customize(ClassDescriptor descriptor) {
        descriptor.getInheritancePolicy().setParentClass(Status.class);
    }
}

Embedded Ids

See, @EmbeddedId.

Nullable embedded values

In Java the value of an embedded reference could be null, but since only the attributes of the embeddable are stored into columns in the database table, there is no simple way to represent null in the database. One way of representing null is to assume that if every column value in the database is null, then this means that the reference to the embeddable is null. However, this means that an embeddable object with all nulls cannot be represented.

EclipseLink allows you to configure each embedded mapping to either not allow a null reference, or to map null to having each column value be null. The default is to map null to null column values, unless the embeddable has relationships, then null is not allowed.

The allowNull option cannot currently be configure through annotations or XML, but a DescriptorCustomizer can be used.

Example: Using a DescriptorCustomizer to disallow a null embedded value

public class EmployeeCustomizer implements DescriptorCustomizer {
    public void customize(ClassDescriptor descriptor) {
        ((AggregateObjectMapping)descriptor.getMappingForAttributeName("status")).setIsNullAllowed(false);
    }
}

Querying

Embeddable objects cannot be queried directly, but they can be queried in the context of their parent. Typically it is best to select the parent, and access the embeddable from the parent. This will ensure the embeddable is registered with the persistence context. If the embeddable is selected in a query, the resulting objects will be detached, and changes will not be tracked.

Example: Querying an embeddable

SELECT employee.period FROM Employee employee WHERE employee.period.endDate = :param

Version: 2.2.0 DRAFT
Other versions...

@MappedSuperclass	Configuring Entities	Configuring Mappings
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