Section : Documentation > Extensions

Extensions : Annotations

Plugin

DataNucleus is developed as a plugin-driven framework and one of the components that is pluggable is the reading of annotations. 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.

DataNucleus supports Java annotations. More than this, it actually provides a pluggable framework whereby you can plug in your own annotations support. The JDO API plugin provides support for JDO annotations, and the JPA API plugin provides support for JPA annotations. You can extend DataNucleuss capabilities using the plugin extension org.datanucleus.annotations.

Plugin extension-point Key Description Location
org.datanucleus.annotations @PersistenceCapable JDO annotation reader datanucleus-api-jdo
org.datanucleus.annotations @PersistenceAware JDO annotation reader datanucleus-api-jdo
org.datanucleus.annotations @Entity JPA annotation reader datanucleus-api-jpa
org.datanucleus.annotations @MappedSuperclass JPA annotation reader datanucleus-api-jpa
org.datanucleus.annotations @Embeddable JPA annotation reader datanucleus-api-jpa

Interface

Any annotation reader plugin will need to implement org.datanucleus.metadata.annotations.AnnotationReader. Javadoc So you need to implement the following interface

package org.datanucleus.metadata.annotations;

import org.datanucleus.metadata.PackageMetaData;
import org.datanucleus.metadata.ClassMetaData;

public interface AnnotationReader
{
    /**
     * Accessor for the annotations packages supported by this reader.
     * @return The annotations packages that will be processed.
     */
    String[] getSupportedAnnotationPackages();

    /**
     * Method to get the ClassMetaData for a class from its annotations.
     * @param cls The class
     * @param pmd MetaData for the owning package (that this will be a child of)
     * @return The ClassMetaData (unpopulated and unitialised)
     */
    public ClassMetaData getMetaDataForClass(Class cls, PackageMetaData pmd);
}

Plugin Specification

So we now have our custom annotation reader 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.annotations">
        <annotations annotation-class="mydomain.annotations.MyAnnotationType" 
            reader="mydomain.annotations.MyAnnotationReader"/>
    </extension>
</plugin>

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

So here we have our annotations reader class MyAnnotationReader which will process any classes annotated with the annotation MyAnnotationType.