Any value generator plugin will need to implement org.datanucleus.store.valuegenerator.ValueGenerator So you need to implement the following interface

public interface ValueGenerator
{
    String getName ();

    void allocate (int additional);

    Object next ();
    Object current ();

    long nextValue();
    long currentValue();
}

DataNucleus provides an abstract base class org.datanucleus.store.valuegenerator.AbstractValueGenerator to extend if you don't require datastore access. If you do require (RDBMS) datastore access for your ValueGenerator then you can extend org.datanucleus.store.rdbms.valuegenerator.AbstractRDBMSValueGenerator Let's give an example, here we want a generator that provides a form of UUID identity. We define our class as

package mydomain;

import org.datanucleus.store.valuegenerator.ValueGenerationBlock;
import org.datanucleus.store.valuegenerator.AbstractValueGenerator;

public class MyUUIDValueGenerator extends AbstractValueGenerator
{
    public MyUUIDValueGenerator(String name, Properties props)
    {
        super(name, props);
    }

    /**
     * Method to reserve "size" ValueGenerations to the ValueGenerationBlock.
     * @param size The block size
     * @return The reserved block
     */
    public ValueGenerationBlock reserveBlock(long size)
    {
        Object[] ids = new Object[(int) size];
        for (int i = 0; i < size; i++)
        {
            ids[i] = getIdentifier();
        }
        return new ValueGenerationBlock(ids);
    }

    /**
     * Create a UUID identifier.
     * @return The identifier
     */
    private String getIdentifier()
    {
        ... Write this method to generate the identifier
    }
}

As show you need a constructor taking 2 arguments String and java.util.Properties . The first being the name of the generator, and the second containing properties for use in the generator.

• class-name Name of the class that the value is being added to
• root-class-name Name of the root class in this inheritance tree
• field-name Name of the field whose value is being set (not provided if this is datastore identity field)
• catalog-name Catalog that objects of the class are stored in
• schema-name Schema that objects of the class are stored in
• table-name Name of the (root) table storing this field
• column-name Name of the column storing this field
• sequence-name Name of the sequence (if specified in the MetaData)

So we now have our custom "value generator" and we just need to make this into a DataNucleus plugin. To do this you simply add a file plugin.xml to your JAR at the root. The file plugin.xml should look like this

<?xml version="1.0"?>
<plugin id="mydomain" name="DataNucleus plug-ins" provider-name="My Company">
    <extension point="org.datanucleus.store_valuegenerator">
        <valuegenerator name="myuuid" class-name="mydomain.MyUUIDValueGenerator" unique="true"/>
    </extension>
</plugin>

Note that you also require a MANIFEST.MF file as per the Plugins Guide.

The name "myuuid" is what you will use as the "strategy" when specifying to use it in MetaData. The flag "unique" is only needed if your generator is to be unique across all requests. For example if your generator was only unique for a particular class then you should omit that part. Thats all. You now have a DataNucleus "value generator" plugin.

To use your value generator you would reference it in your JDO MetaData like this

<class name="MyClass">
    <datastore-identity strategy="myuuid"/>
    ...
</class>

Don't forget that if you write a value generator that could be of value to others you could easily donate it to DataNucleus for inclusion in the next release.