Tapestry5, JPA, Netbeans and a FilerException

Tapestry5.4 supports JPA out of the box which is nice. Also Netbeans supports JPA, doubly nice. Now if you're using 2 or more Persistence Units, you have probably encountered a FilerException when you try to compile or run your project. This shouldn't be a problem if you're using only single Persistence Unit. I suspect the fix would be the same if you encounter the same exception.

java.lang.RuntimeException: javax.annotation.processing.FilerException: Attempt to recreate a file for type {myclass}

The {myclass} here is always an entity class.

I think, a secondary symptom of this is if all your entity classes are all in a single package. This should be the case if you're doing a standard Tapestry5 web application.

The fix to this is to edit the Persistence Unit and add a  property and explicitly declare the entity classes on the Persistence Unit.

<persistence-unit name="PayrollPU" transaction-type="RESOURCE_LOCAL">  
   <provider>org.hibernate.ejb.HibernatePersistence</provider>  
   <class>cu.ictso.miscapps.entities.Departments</class>  
   <class>cu.ictso.miscapps.entities.Employee</class>  
   <class>cu.ictso.miscapps.entities.Groups</class>  
   <class>cu.ictso.miscapps.entities.ViewAttendance</class>  
   <exclude-unlisted-classes>true</exclude-unlisted-classes>  
   <properties>  
    <property name="javax.persistence.jdbc.url" value="jdbc:sqlserver://localhost;databaseName=DTR-Payroll2;Integrated Security=false"/>  
    <property name="javax.persistence.jdbc.user" value="PayMaster"/>  
    <property name="javax.persistence.jdbc.driver" value="com.microsoft.sqlserver.jdbc.SQLServerDriver"/>  
    <property name="javax.persistence.jdbc.password" value="123456"/>  
    <property name="javax.persistence.schema-generation.database.action" value="create"/>  
   </properties>  
 </persistence-unit>  

Here's an example Persistence Unit.