It is possible to declare an embedded component inside an entity and even override its column mapping. Component classes have to be annotated at the class level with the @Embeddable annotation. It is possible to override the column mapping of an embedded object for a particular entity using the @Embedded and @AttributeOverride annotation in the associated property:

@Entity
public class Person implements Serializable {

    // Persistent component using defaults
    Address homeAddress;

    @Embedded
    @AttributeOverrides( {
            @AttributeOverride(name="iso2", column = @Column(name="bornIso2") ),
            @AttributeOverride(name="name", column = @Column(name="bornCountryName") )
    } )
    Country bornIn;
    ...
}
            

@Embeddable
public class Address implements Serializable {
    String city;
    Country nationality; //no overriding here
}
            

@Embeddable
public class Country implements Serializable {
    private String iso2;
    @Column(name="countryName") private String name;

    public String getIso2() { return iso2; }
    public void setIso2(String iso2) { this.iso2 = iso2; }

    
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    ...
}
            

A embeddable object inherit the access type of its owning entity (note that you can override that using the Hibernate specific @AccessType annotations (see Hibernate Annotation Extensions).

The Person entity bean has two component properties, homeAddress and bornIn. homeAddress property has not been annotated, but Hibernate will guess that it is a persistent component by looking for the @Embeddable annotation in the Address class. We also override the mapping of a column name (to bornCountryName) with the @Embedded and @AttributeOverride annotations for each mapped attribute of Country. As you can see, Country is also a nested component of Address, again using auto-detection by Hibernate and EJB3 defaults. Overriding columns of embedded objects of embedded objects is currently not supported in the EJB3 spec, however, Hibernate Annotations supports it through dotted expressions.

    @Embedded
    @AttributeOverrides( {
            @AttributeOverride(name="city", column = @Column(name="fld_city") )
            @AttributeOverride(name="nationality.iso2", column = @Column(name="nat_Iso2") ),
            @AttributeOverride(name="nationality.name", 
	         column = @Column(name="nat_CountryName") )
            //nationality columns in homeAddress are overridden
    } )
    Address homeAddress;

Hibernate Annotations supports one more feature that is not explicitly supported by the EJB3 specification. You can annotate a embedded object with the @MappedSuperclass annotation to make the superclass properties persistent (see @MappedSuperclass for more informations).

While not supported by the EJB3 specification, Hibernate Annotations allows you to use association annotations in an embeddable object (ie @*ToOne nor @*ToMany). To override the association columns you can use @AssociationOverride.

If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work: at least one of the columns will have to be explicit. Hibernate goes beyond the EJB3 spec and allows you to enhance the defaulting mechanism through the NamingStrategy. DefaultComponentSafeNamingStrategy is a small improvement over the default EJB3NamingStrategy that allows embedded objects to be defaulted even if used twice in the same entity.