JPA/Hibernate - Embedding an Attribute

Question

I am having a trouble mapping an embedded attribute of a class. I have created some classes that are similar to what I am trying to do to illustrate. Basically, I have an @Embeddable class hierarchy that uses Inheritance. The top level class "Part Number" has only one attribute, and the extending classes add no attributes to the "Part Number" class, they only add some validation/logic.

Here is what I mean:

PART

@Entity
@Table(name="PART")
public class Part {
    private Integer id;
    private String name;
    private PartNumber partNumber;

    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE)
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name="PART_NAME")
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    @Embedded
    public PartNumber getPartNumber() {
        return partNumber;
    }
    public void setPartNumber(PartNumber partNumber) {
        this.partNumber = partNumber;
    }

}

PARTNUMBER

@Embeddable
public abstract class PartNumber {

    protected String partNumber;
    private String generalPartNumber;
    private String specificPartNumber;

    private PartNumber() {

    }

    public PartNumber(String partNumber) {
        this.partNumber = partNumber;

    }

    @Column(name = "PART_NUMBER")
    public String getPartNumber() {
        return partNumber;
    }

    public void setPartNumber(String partNumber) {
        this.partNumber = partNumber;
    }

    /**
     * @param partNumber
     * @return
     */
    public boolean validate(String partNumber) {
        // do some validation
        return true;
    }

    /**
     * Returns the first half of the Part Number
     * 
     * @return generalPartNumber
     */
    @Transient
    public String getGeneralPartNumber() {
        return generalPartNumber;

    }

    /**
     * Returns the last half of the Part Number 
     * which is specific to each Car Brand
     * 
     * @return specificPartNumber
     */
    @Transient
    public String getSpecificPartNumber() {
        return specificPartNumber;

    }

}

FORD PARTNUMBER

public class FordPartNumber extends PartNumber {

    /**
     * Ford Part Number is formatted as 1234-#1234
     * 
     * @param partNumber
     */
    public FordPartNumber(String partNumber) {
        super(partNumber);
        validate(partNumber);
    }

    /*
     * (non-Javadoc)
     * 
     * @see com.test.PartNumber#validate(java.lang.String)
     */
    @Override
    public boolean validate(String partNumber) {
        // do some validation
        return true;
    }

    /*
     * (non-Javadoc)
     * 
     * @see com.test.PartNumber#getGeneralPartNumber()
     */
    @Override
    public String getGeneralPartNumber() {
        return partNumber;

    }

    /*
     * (non-Javadoc)
     * 
     * @see com.test.PartNumber#getSpecificPartNumber()
     */
    @Override
    public String getSpecificPartNumber() {
        return partNumber;

    }

}

CHEVY PARTNUMBER

public class ChevyPartNumber extends PartNumber {

    /**
     * Chevy Part Number is formatted as 1234-$1234
     * 
     * @param partNumber
     */
    public ChevyPartNumber(String partNumber) {
        super(partNumber);
        validate(partNumber);
    }

    /*
     * (non-Javadoc)
     * 
     * @see com.test.PartNumber#validate(java.lang.String)
     */
    @Override
    public boolean validate(String partNumber) {
        // do some validation
        return true;
    }

    /*
     * (non-Javadoc)
     * 
     * @see com.test.PartNumber#getGeneralPartNumber()
     */
    @Override
    public String getGeneralPartNumber() {
        return partNumber;

    }

    /*
     * (non-Javadoc)
     * 
     * @see com.test.PartNumber#getSpecificPartNumber()
     */
    @Override
    public String getSpecificPartNumber() {
        return partNumber;

    }
}

Of course this does not work, because Hibernate ignores the Inheritance Hierarchy and doesn't like the fact that PartNumber is abstract. Is there some way to do this using JPA or Hibernate Annotations? I have tried using the @Inheritance JPA annotation.

I am not able to refactor the "PartNumber" part of the hierarchy because the original Developer wants to be able to extend PartNumber with N many XXXXPartNumber classes.

Does anyone know if anything like this will be a part of the JPA 2.0 or a new version of Hibernate?

Answer 1

Component (e.g. @Embeddable) inheritance is not supported and most likely never will be. There is a good reason for that - entity identifier plays a critical role in all inheritance strategies supported by Hibernate and components don't have (mapped) identifiers.

You have three choices:

A) Map PartNumber (and all its descendants) as entities. PartNumber may remain abstract:

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="part_type", discriminatorType=DiscriminatorType.STRING)
public abstract class PartNumber {
...
}

@Entity
@DiscriminatorValue("Ford")
public class FordPartNumber extends PartNumber {
...
}

B) Based on your example it seems that all PartNumber descendants differ in behavior only (they don't introduce any new properties to be stored). If that's indeed the case, you can map PartNumber properties plus your own discriminator value (so you know which class to instantiate) as @Embedded private property and have get/setPartNumber() accessors in Part class marshall / unmarshall appropriate subclasses. You can even write your own Hibernate custom type to do that for you (it's pretty straightforward).

C) If PartNumber descendants DO differ in properties that have to be stored and mapping them as entities is unacceptable for whatever reason, you can use marshall / unmarshall them to string (as XML or anything else that fits the bill) and store that. I'm using XStream for this exact purpose and I wrote a simple Hibernate type to go with it. Your Part mapping would look something like

@Type(type="xmlBean")
public PartNumber getPartNumber() {
    return partNumber;
}
public void setPartNumber(PartNumber partNumber) {
    this.partNumber = partNumber;
}

and PartNumber descendants won't have to be mapped at all. The downside, of course, is that dealing with XML in the database is a bit more of a hassle so that may not be the ideal approach for something you would potentially need to report on. OTOH, I'm using this for storing plugin settings and it saved me a lot of trouble with mappings / DB maintenance.