Primary Key Classes

As has been described in the application identity guide, when you choose application identity you are defining which fields of the class are part of the primary key, and you are taking control of the specification of id's to DataNucleus. Application identity requires a primary key (PK) class, and each persistent capable class may define a different class for its primary key, and different persistent capable classes can use the same primary key class, as appropriate. You specify the primary key class like this 

<entity class="MyClass">
    <id-class class="MyIdClass"/>
    ...
</entity>

or using annotations 

@Entity
@IdClass(class=MyIdClass.class)
public class MyClass
{
    ...
}

You now need to define the PK class to use. This is simplified for you because if you have only one PK field then you dont need to define a PK class and you only define it when you have a composite PK.

An important thing to note is that the PK can only be made up of fields of the following Java types

  • Primitives : boolean , byte , char , int , long , short
  • java.lang : Boolean , Byte , Character , Integer , Long , Short , String , Enum , StringBuffer
  • java.math : BigInteger
  • java.sql : Date , Time , Timestamp
  • java.util : Date , Currency, Locale, TimeZone, UUID
  • java.net : URI, URL
  • javax.jdo.spi : PersistenceCapable
Note that the types in bold are JPA standard types. Any others are DataNucleus extensions and, as always, check the specific datastore docs to see what is supported for your datastore.

Single primary-key field

The simplest way of using application identity is where you have a single PK field, and in this case you use an inbuilt primary key class that DataNucleus provides, so you don't need to specify the objectid-class . Let's take an example 

public class MyClass
{
    long id;
    String name;
    String description;
    ...
}

<entity class="MyClass">
    <attributes>
        <id name="id"/>
        <basic name="name"/>
        <basic name="description"/>
    </attributes>
</entity>

So we didnt specify the JPA "id-class". You will, of course, have to give the field a value before persisting the object, either by setting it yourself, or by using a value-strategy on that field.



Rules for User-Defined Primary Key classes

If you wish to use application identity and don't want to use the "SingleFieldIdentity" builtin PK classes then you must define a Primary Key class of your own. You can't use classes like java.lang.String, or java.lang.Long directly. You must follow these rules when defining your primary key class.

  • the Primary Key class must be public
  • the Primary Key class must implement Serializable
  • the Primary Key class must have a public no-arg constructor, which might be the default constructor
  • the field types of all non-static fields in the Primary Key class must be serializable, and are recommended to be primitive, String, Date, or Number types
  • all serializable non-static fields in the Primary Key class must be public
  • the names of the non-static fields in the Primary Key class must include the names of the primary key fields in the JDO class, and the types of the common fields must be identical
  • the equals() and hashCode() methods of the Primary Key class must use the value(s) of all the fields corresponding to the primary key fields in the JDO class
  • if the Primary Key class is an inner class, it must be static
  • the Primary Key class must override the toString() method defined in Object, and return a String that can be used as the parameter of a constructor
  • the Primary Key class must provide a String constructor that returns an instance that compares equal to an instance that returned that String by the toString() method.
  • the Primary Key class must be only used within a single inheritence tree.
Primary Key Example - Multiple Field

Here's an example of a composite (multiple field) primary key class 

public class ComposedIdKey implements Serializable
{
    public String field1;
    public String field2;

    /**
     *  Default constructor.
     */
    public ComposedIdKey ()
    {
    }

    /**
     * String constructor.
     */
    public ComposedIdKey(String value) 
    {
        StringTokenizer token = new StringTokenizer (value, "::");
        //className
        token.nextToken ();
        //field1
        this.field1 = token.nextToken ();
        //field2
        this.field2 = token.nextToken ();
    }

    /**
     * Implementation of equals method.
     */
    public boolean equals(Object obj)
    {
        if (obj == this)
        {
            return true;
        }
        if (!(obj instanceof ComposedIdKey))
        {
            return false;
        }
        ComposedIdKey c = (ComposedIdKey)obj;

        return field1.equals(c.field1) && field2.equals(c.field2);
    }

    /**
     *  Implementation of hashCode method that supports the
     *  equals-hashCode contract.
     */
    public int hashCode ()
    {
        return this.field1.hashCode() ^ this.field2.hashCode();
    }

    /**
     *  Implementation of toString that outputs this object id's PK values.
     */
    public String toString ()
    {
        return this.getClass().getName() + "::"  + this.field1 + "::" + this.field2;
    }
}