import org.datanucleus.identity;

public interface DatastoreId
{
    /**
     * Provides the datastore id in a form that can be used by the database as a key.
     * @return The key value
     */
    public Object getKeyAsObject();

    /**
     * Accessor for the PC class name 
     * @return the PC Class
     */
    public String getTargetClassName();

    /**
     * Equality operator.
     * @param obj Object to compare against
     * @return Whether they are equal
     */
    public abstract boolean equals(Object obj);

    /**
     * Accessor for the hashcode
     * @return Hashcode for this object
     */
    public abstract int hashCode();

    /**
     * Returns the string representation of the datastore id.
     * The string representation should contain enough information to be usable as input to a String constructor
     * to create the datastore id.
     * @return the string representation of the datastore id.
     */
    public abstract String toString();
}

package org.datanucleus.identity;

public class DatastoreIdImpl implements java.io.Serializable, DatastoreId
{
    /** Separator to use between fields. */
    private transient static final String STRING_DELIMITER = "[OID]";

    // JDO spec 5.4.3 - serializable fields required to be public.

    public final Object keyAsObject;
    public final String targetClassName;

    /** pre-created toString to improve performance **/ 
    public final String toString;

    /** pre-created hasCode to improve performance **/ 
    public final int hashCode;

    public DatastoreIdImpl()
    {
        keyAsObject = null;
        targetClassName = null; 
        toString = null;
        hashCode = -1;
    }

    public DatastoreIdImpl(String pcClass, Object object)
    {
        this.targetClassName = pcClass;
        this.keyAsObject = object;

        StringBuilder s = new StringBuilder();
        s.append(this.keyAsObject.toString());
        s.append(STRING_DELIMITER);
        s.append(this.targetClassName);
        toString = s.toString();
        hashCode = toString.hashCode();        
    }

    /**
     * Constructs an identity from its string representation that is consistent with the output of toString().
     * @param str the string representation of a datastore id
     * @exception IllegalArgumentException if the given string representation is not valid.
     * @see #toString
     */
    public DatastoreIdImpl(String str)
    throws IllegalArgumentException
    {
        if (str.length() < 2)
        {
            throw new IllegalArgumentException(Localiser.msg("038000", str));
        }
        else if (str.indexOf(STRING_DELIMITER) < 0)
        {
            throw new IllegalArgumentException(Localiser.msg("038000", str));
        }

        int start = 0;
        int end = str.indexOf(STRING_DELIMITER, start);
        String oidStr = str.substring(start, end);
        Object oidValue = null;
        try
        {
            // Use Long if possible, else String
            oidValue = Long.valueOf(oidStr);
        }
        catch (NumberFormatException nfe)
        {
            oidValue = oidStr;
        }
        keyAsObject = oidValue;

        start = end + STRING_DELIMITER.length();
        this.targetClassName = str.substring(start, str.length());

        toString = str;
        hashCode = toString.hashCode();
    }

    public Object getKeyAsObject()
    {
        return keyAsObject;
    }

    public String getTargetClassName()
    {
        return targetClassName;
    }

    public boolean equals(Object obj)
    {
        if (obj == null)
        {
            return false;
        }
        if (obj == this)
        {
            return true;
        }
        if (!(obj.getClass().getName().equals(ClassNameConstants.IDENTITY_OID_IMPL)))
        {
            return false;
        }
        if (hashCode() != obj.hashCode())
        {
            return false;
        }
        if (!((DatastoreId)obj).toString().equals(toString))
        {
            // Hashcodes are the same but the values aren't
            return false;
        }
        return true;
    }

    public int compareTo(Object o)
    {
        if (o instanceof DatastoreIdImpl)
        {
            DatastoreIdImpl c = (DatastoreIdImpl)o;
            return this.toString.compareTo(c.toString);
        }
        else if (o == null)
        {
            throw new ClassCastException("object is null");
        }
        throw new ClassCastException(this.getClass().getName() + " != " + o.getClass().getName());
    }

    public int hashCode()
    {
        return hashCode;
    }

    /**
     * Creates a String representation of the datastore identity, formed from the target class name and the key value. This will be something like
     * <pre>3254[OID]mydomain.MyClass</pre>
     * @return The String form of the identity
     */
    public String toString()
    {
        return toString;
    }
}

<?xml version="1.0"?>
<plugin id="mydomain" name="DataNucleus plug-ins" provider-name="My Company">
    <extension point="org.datanucleus.store_datastoreidentity">
        <datastoreidentity name="myoid" class-name="mydomain.MyOIDImpl" unique="true"/>
    </extension>
</plugin>