Extensions : Class Annotation Handler


DataNucleus is developed as a plugin-driven framework and one of the components that is pluggable is the handling of annotations at class-level. DataNucleus provides a support for JDO and JPA annotations, but is structured so that you can easily add your own annotations and have them usable within your DataNucleus usage.


Any class annotation handler plugin will need to implement org.datanucleus.metadata.annotations.ClassAnnotationHandler. Javadoc. So you need to implement the following interface

package org.datanucleus.metadata.annotations;

import org.datanucleus.ClassLoaderResolver;
import org.datanucleus.metadata.AbstractClassMetaData;

public interface ClassAnnotationHandler
     * Method to process a class level annotation.
     * @param annotation The annotation
     * @param cmd Metadata for the class to update with any necessary information.
     * @param clr ClassLoader resolver
    void processClassAnnotation(AnnotationObject annotation, AbstractClassMetaData cmd, ClassLoaderResolver clr);

Plugin Specification

So we now have our custom annotation handler 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.annotations" name="DataNucleus plug-ins" provider-name="My Company">
    <extension point="org.datanucleus.class_annotation_handler">
        <class-annotation-handler annotation-class="mydomain.annotations.MyAnnotation" handler="mydomain.annotations.MyAnnotationHandler"/>

Note that you also require a MANIFEST.MF file as per the Extensions Guide. So here, when the metadata for our class is processed, if it finds the @MyAnnotation annotation it will call this handler after generating the basic metadata for the class, allowing us to update it.