Extensions : Value Generators


DataNucleus is developed as a plugin-driven framework and one of the components that is pluggable is the generation of identity or field values. DataNucleus provides a large selection of generators but is structured so that you can easily add your own variant and have it usable within your DataNucleus usage. Below are listed some of those available, but each store plugin typically will define its own. The JDO/JPA specs define various that are required. You can extend DataNucleuss capabilities using the plugin extension org.datanucleus.store_valuegenerator.

Plugin extension-point Key Datastore Description Location
org.datanucleus.store_valuegenerator auid all datastores Value Generator using AUIDs datanucleus-core
org.datanucleus.store_valuegenerator uuid-hex all datastores Value Generator using uuid-hex datanucleus-core
org.datanucleus.store_valuegenerator uuid-string all datastores Value Generator using uuid-string datanucleus-core
org.datanucleus.store_valuegenerator timestamp all datastores Value Generator using Timestamp datanucleus-core
org.datanucleus.store_valuegenerator timestamp-value all datastores Value Generator using Timestamp millisecs value datanucleus-core
org.datanucleus.store_valuegenerator increment rdbms Value Generator using increment strategy datanucleus-rdbms
org.datanucleus.store_valuegenerator sequence rdbms Value Generator using datastore sequences datanucleus-rdbms
org.datanucleus.store_valuegenerator table-sequence rdbms Value Generator using a database table to generate sequences (same as increment) datanucleus-rdbms
org.datanucleus.store_valuegenerator max rdbms Value Generator using max(COL)+1 strategy datanucleus-rdbms
org.datanucleus.store_valuegenerator datastore-uuid-hex rdbms Value Generator using uuid-hex attributed by the datastore datanucleus-rdbms
org.datanucleus.store_valuegenerator identity mongodb Value Generator for MongoDB using identity strategy datanucleus-mongodb
org.datanucleus.store_valuegenerator identity neo4j Value Generator for Neo4j using identity strategy datanucleus-neo4j

The following sections describe how to create your own value generator plugin for DataNucleus.


Any value generator plugin will need to implement org.datanucleus.store.valuegenerator.ValueGenerator Javadoc. 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 dont require datastore access. If you do require (RDBMS) datastore access for your ValueGenerator then you can extend org.datanucleus.store.rdbms.valuegenerator.AbstractRDBMSValueGenerator Lets 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)

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.store_valuegenerator">
        <valuegenerator name="myuuid" class-name="mydomain.MyUUIDValueGenerator" unique="true"/>

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

Plugin Usage

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

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

Dont 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.