Interface

Any value generator plugin will need to implement org.datanucleus.store.types.converters.TypeConverter So you need to implement the following interface

public interface TypeConverter<X, Y> extends Serializable
{
    /**
     * Method to convert the passed member value to the datastore type.
     * @param memberValue Value from the member
     * @return Value for the datastore
     */
    Y toDatastoreType(X memberValue);

    /**
     * Method to convert the passed datastore value to the member type.
     * @param datastoreValue Value from the datastore
     * @return Value for the member
     */
    X toMemberType(Y datastoreValue);
}

Implementation

Let's take an example. If we look at the Java type URI we want to persist it as a String since a native URI type isn't present in datastores. We define our class as

public class URIStringConverter implements TypeConverter<URI, String>
{
    public URI toMemberType(String str)
    {
        if (str == null)
        {
            return null;
        }

        return java.net.URI.create(str.trim());
    }

    public String toDatastoreType(URI uri)
    {
        return uri != null ? uri.toString() : null;
    }
}

So when converting it for the datastore it will use the toString() form of the URI, and will be converted back to a URI (on retrieval from the datastore) using the URI.create method. Obviously this particular TypeConverter is included in DataNucleus, but hopefully it gives an idea of what to do to provide your own.

Plugin Specification

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.type_converter">
        <type-converter name="dn.uri-string" member-type="java.net.URI" datastore-type="java.lang.String"
            converter-class="mydomain.converters.URIStringConverter"/>
    </extension>
</plugin>

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

The name "dn.uri-string" can be used to refer to this converter from within a java_types extension point definition for the default converter to use for a Java type.