PersistentResourceXMLBuilder.build() requires a PersistentResourceDefinition as an input. This is a problem because the PersistentResourcXmlDefinition is needed to initialize parsers for Extension.initializeParsers(), which is called before Extension.initialize(). And Extension.initialize() is when the PersistentResourceDefinition would normally be constructed.

An Extension implementation could overcome this by maintaining internal state. Construct the PersistentResourceDefinition in initializeParsers() and store it in an instance field for use in initialize(). Or vice versa. That gets messy though as now the Extension impl is needing to worry about the order in which the two methods are called and tracking whether both have been called so it can drop the cached object.

A possible thing to do is the have the ExtensionContext and ExtensionParsingContext offer an attachment API, with the lifecycle of attachments documented as being scoped to a single overall extension initialization. That could work but isn't very elegant.