title: How do you configure an EclipseLink HistoryPolicy to track changes to an Element Collection Property of a JPA Entity in GlassFish 4?
body: <p>I successfully configured an EclipseLink HistoryPolicy on a Person entity and verified that EclipseLink running on GlassFish 4 tracked changes to the PERSON table in the PERSON_VERSION table. I then extended my Person entity with a Map property to hold the person's telephone numbers as shown in the following class:</p>
  
  <pre><code>@Customizer(PersonCustomizer.class)
  public class Person {
      @ID
      private Long id;
      @Version
      private Integer version;
      ....
      @ElementCollection
      @MapKeyEnumerated(EnumType.STRING)
      private Map&lt;TelephoneNumberType, String&gt; telephoneNumber;
      ...
  }
  </code></pre>
  
  <p>I then tried to extend my DescriptorCustomizer class to track the changes to the PERSON_TELEPHONENUMBER table as show in the following class:</p>
  
  <pre><code>public class PersonCustomizer implements DescriptorCustomizer {
      public void customize(ClassDescriptor descriptor) throws Exception {
          HistoryPolicy policy = new HistoryPolicy();
          policy.addStartFieldName("VALIDFROM");
          policy.addEndFieldName("VALIDTO");
          policy.setShouldHandleWrites(true);
          policy.addHistoryTableName("PERSON", "PERSON_VERSION");
          descriptor.setHistoryPolicy(policy);
  
          DirectMapMapping mapping = (DirectMapMapping) descriptor.getMappingForAttributeName("telephoneNumber");
          HistoryPolicy policy2 = new HistoryPolicy();
          policy2.setShouldHandleWrites(true);
          policy2.addHistoryTableName("PERSON_TELEPHONENUMBER", "PERSON_VERSION_TELEPHONENUMBER");
          mapping.setHistoryPolicy(policy2);
      }
  }
  </code></pre>
  
  <p>Unfortunately, GlassFish refuses to deploy my application.  If I comment out the mapping.setHistoryPolicy(policy2) line from the above PersonCustomizer class, the application deploys but changes to the PERSON_TELEPHONENUMBER table are not tracked in the PERSON_VERSION_TELEPHONENUMBER table. </p>
  
  <p>The following is a sample of the error message that GlassFish reports during application deployment:</p>
  
  <pre><code>Local Exception Stack: 
  Exception [EclipseLink-0] (Eclipse Persistence Services - 2.5.0.v20130507-3faac2b): org.eclipse.persistence.exceptions.IntegrityException
  Descriptor Exceptions: 
  ---------------------------------------------------------
  
  Exception [EclipseLink-41] (Eclipse Persistence Services - 2.5.0.v20130507-3faac2b): org.eclipse.persistence.exceptions.DescriptorException
  Exception Description: A non-read-only mapping must be defined for the sequence number field.
  Descriptor: RelationalDescriptor(Person --&gt; [DatabaseTable(PERSON)])
  
  Runtime Exceptions: 
  ---------------------------------------------------------
  
  java.lang.NullPointerException
  
      at org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.initializeDescriptors(DatabaseSessionImpl.java:689)
      at org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.initializeDescriptors(DatabaseSessionImpl.java:625)
      at org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.initializeDescriptors(DatabaseSessionImpl.java:565)
      at org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.postConnectDatasource(DatabaseSessionImpl.java:792)
  
  ...
  
  Descriptor Exceptions: 
  ---------------------------------------------------------
  
  Local Exception Stack: 
  Exception [EclipseLink-41] (Eclipse Persistence Services - 2.5.0.v20130507-3faac2b): org.eclipse.persistence.exceptions.DescriptorException
  Exception Description: A non-read-only mapping must be defined for the sequence number field.
  Descriptor: RelationalDescriptor(Person --&gt; [DatabaseTable(PERSON)])
      at org.eclipse.persistence.exceptions.DescriptorException.mappingForSequenceNumberField(DescriptorException.java:929)
      at org.eclipse.persistence.internal.descriptors.ObjectBuilder.validate(ObjectBuilder.java:4090)
      at org.eclipse.persistence.descriptors.ClassDescriptor.selfValidationAfterInitialization(ClassDescriptor.java:4061)
      at org.eclipse.persistence.descriptors.ClassDescriptor.validateAfterInitialization(ClassDescriptor.java:5910)
  
  ...
  
  Runtime Exceptions: 
  ---------------------------------------------------------
  
  
  java.lang.NullPointerException
      at org.eclipse.persistence.history.HistoryPolicy.initialize(HistoryPolicy.java:320)
      at org.eclipse.persistence.mappings.DirectCollectionMapping.initialize(DirectCollectionMapping.java:1457)
      at org.eclipse.persistence.mappings.DirectMapMapping.initialize(DirectMapMapping.java:367)
      at org.eclipse.persistence.descriptors.ClassDescriptor.initialize(ClassDescriptor.java:2980)
  
  ...
  </code></pre>
  
  <p>Thank you for you help.</p>