Sharing attributes across Entities

One way to share common attributes across entities is to use inheritance, and put the common attributes in a super class. Another way is to abstract the common attributes into a separate Embeddable class and then embed it within the regular Entity classes. The latter approach has been used for some of the entities in the EJB3Demo project.

The entities Customer, District and Warehouse all contain some very similar columns for storing address information. I created a new class called Address to hold this common data. The Address class is marked as @Embeddable and contains the getters/setters for the attributes. Here is an extract:

@Embeddable public class Address implements Serializable {
    String street1;
    String city;
    @Column(length=20)
    public String getCity() {
        return city;
    }
    @Column(length=20)
    public String getStreet1() {
        return street1;
    }
}

Note that the getters have been annotated using @Column - this is useful for capturing the bits that are shared across the entities.

When you embed the Address object within an Entity, you can fine-tune the column mapping. Example:

@Entity
public class Customer {
    Address address;
    @Embedded
    @AttributeOverrides({
        @AttributeOverride(name="street1", column=@Column(name="C_STREET_1")),
        @AttributeOverride(name="city", column=@Column(name="C_CITY")),
    })
    public Address getAddress() {
        return address;
    }
}

The getter for the Address object is decorated with the @Embedded annotation. The address object can be shared amongst multiple entities, and each entity can define its own column names, etc.

When you access embedded objects in EJBQL, you use the property name in the Query statement. For example, to access the city field in Customer, you would write:

SELECT c.address.city FROM Customer c

This syntax is also used when you have relationship mappings. For example, a District belongs to a Warehouse, and hence the District object contains an instance of the Warehouse object. To access the fields within Warehouse, you would write:

SELECT d.warehouse.name FROM District d

Although one could use inheritance to model above, I prefer to use the Embeddable class option as it more accurately reflects what is happening here. A Customer is not an instance of Address - a Customer has an Address. In this situation, Has-A relationship is more natural than Is-A relationship.