Revision as of 12:59, 25 November 2009

Dynamic Persistence with EclipseLink OXM

This page provides an overview for the work done to support Dynamic Persistence in EclipseLink OXM.

Document History

Overview

The first step in creating a JPA-JAXB meet-in-the-middle solution is to enable EclipseLink OXM to use Dynamic Persistence, a.k.a. processing Projects, Mappings & Descriptors without having concrete Java classes available. This work builds off of the initial Dynamic Persistence functionality (org.eclipse.persistence.dynamic) current used by the JPA component. By supplying a DynamicClassLoader and loading the EclipseLink project by way of DynamicTypeBuilder.loadDynamicProject(), we can inject Dynamic Persistence functionality into EclipseLink OXM in a fairly transparent way.

Prototype

We currently have a prototype available that can read an EclipseLink project, unmarshal an XML document, modify the Java domain objects (using reflection), and marshal the modified objects, all without having actual Java class files available on the classpath. To get this prototype working the following changes were required:

Instantiating a "Dynamic" Project

Dynamic classes come into play when we use the DynamicTypeBuilder.loadDynamicProject() to load a Project. In this example, even though we have a concrete Project object (DynamicTestProject) which only contains Java class names and no references to concrete Java classes, we will pass this project through to DynamicTypeBuilder to cause the Dynamic classes to be created.

We also pass in an instance of DynamicClassLoader that we create using a system classloader as its parent.

ClassLoader sysClassLoader = Thread.currentThread().getContextClassLoader();
DynamicClassLoader dynamicClassLoader = new DynamicClassLoader(sysClassLoader);
 
Project dynamicTestProject = DynamicTypeBuilder.loadDynamicProject((Project) new DynamicTestProject(), null, dynamicClassLoader);
 
setProject(dynamicTestProject);

We can also load a project.xml file and use it to create our Dynamic project.

DynamicTypeBuilder (org.eclipse.persistence.dynamic)

• A new method to load a dynamic Project by passing in a "dry" EclipseLink project (one with only class names defined) was added for convenience:

public static Project loadDynamicProject(Project project, DatabaseLogin login, DynamicClassLoader dynamicClassLoader)

• A change was needed to avoid a ClassCastException (because we are now potentially dealing with XML mappings):

private boolean requiresInitialization(DatabaseMapping mapping) {
   ...
   if (mapping.isAggregateMapping() && !mapping.isXMLMapping()) {
      return !((AggregateObjectMapping) mapping).isNullAllowed();
   }
   ...
}

OXM Back-Pointer Support

Some changes were needed in the way that back-pointers are configured in OXM. Previously, a back-pointer would be configured on the source descriptor's mapping, e.g.:

...
XMLCompositeCollectionMapping phone = new XMLCompositeCollectionMapping();
phone.setAttributeName("phoneNumbers");
phone.setXPath("phone-numbers/number");
phone.setReferenceClassName("rick.dynamic.XPhoneNumber");
...
phone.getBiDirectionalPolicy().setBidirectionalTargetAttributeName("owningEmployee");
phone.getBiDirectionalPolicy().setBidirectionalTargetGetMethodName("getOwningEmployee");
phone.getBiDirectionalPolicy().setBidirectionalTargetSetMethodName("setOwningEmployee");
...

The problem is that when Dynamic Types are being created, class attributes are created for each of the descriptor's mappings; and since the owningEmployee attribute is not mapped on the Phone descriptor, one would not be generated on the dynamic Phone class.

The solution was to create a new type of mapping (tentatively XMLInverseReferenceMapping) that can be set on the target descriptor, and delegate it's functionality back to the original back-pointer code in XMLCompositeObjectMapping:

public class XMLInverseReferenceMapping extends AggregateMapping implements ContainerMapping {
...
@Override
public void postInitialize(AbstractSession session) throws DescriptorException {
   // Get the corresponding mapping from the reference descriptor and set up the 
   // inverse mapping.
   DatabaseMapping mapping = getReferenceDescriptor().getMappingForAttributeName(this.mappedBy);
 
   if (mapping instanceof XMLCompositeCollectionMapping) {
      XMLCompositeCollectionMapping oppositeMapping = (XMLCompositeCollectionMapping) mapping;
      oppositeMapping.setInverseReferenceMapping(this);
   }
   ...
}
...

NOTE: In moving the bi-directional aspect from a property on the source descriptor to its own XML Mapping, it no longer seemed appropriate to use "bi-directional" terminology, which is why we chose XMLInverseReferenceMapping. On 091124 in a conference call with Shaun we changed this from the original XMLBackReferenceMapping to XMLInverseReferenceMapping.

With these changes, back-pointers are now configured as follows:

// EMPLOYEE Descriptor
XMLCompositeCollectionMapping phone = new XMLCompositeCollectionMapping();
phone.setAttributeName("phoneNumbers");
phone.setXPath("phone-numbers/number");
phone.setReferenceClassName("org.eclipse.persistence.testing.oxm.dynamic.XPhoneNumber");
phone.useCollectionClass(ArrayList.class);
phone.setContainerPolicy(ContainerPolicy.buildPolicyFor(ArrayList.class));
descriptor.addMapping(phone);
 
// PHONE Descriptor
XMLInverseReferenceMapping owningEmployee = new XMLInverseReferenceMapping();
owningEmployee.setReferenceClassName("org.eclipse.persistence.testing.oxm.dynamic.XEmployee");
owningEmployee.setMappedBy("phoneNumbers");
owningEmployee.setAttributeName("owningEmployee");
owningEmployee.setSetMethodName("setOwningEmployee");
owningEmployee.setGetMethodName("getOwningEmployee");
descriptor.addMapping(owningEmployee);

What was previously containerAttributeName/containerGetMethodName/containerSetMethodName on XMLCompositeCollectionMapping is now accessed through that mapping's new inverseReferenceMapping field. The "opposite property" is specified with the setMappedBy() API.

Several changes were required throughout OXM to support this change, e.g.:

...
if (xmlCompositeCollectionMapping.getBidirectionalPolicy().getBidirectionalTargetContainerPolicy() == null) {
...

becomes

...
if (xmlCompositeCollectionMapping.getInverseReferenceMapping().getContainerPolicy() == null) {
...