The maven plugin example here is incomplete: https://www.objectdb.com/java/jpa/tool/enhancer#Maven_and_ANT_Enhancement_
Taken literally it can't work because it's missing this:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>1.2</version>
<executions>
May I suggest you also link from there to the tutorial example: https://www.objectdb.com/tutorial/jpa/web/maven
Needs also an explanation about:
<phase>process-classes</phase> Or: <phase>compile</phase>
Related: https://www.objectdb.com/forum/240