Retrieves a resource. This resource can be of the following three return types: (1) java.io.InputStream (2) javax.xml.stream.XMLStreamReader (3) java.xml.stream.XMLEventReader. If this method returns null the processor will attempt to resolve the entity using its default mechanism.

Parameters:

publicID		The public identifier of the external entity being referenced, or null if none was supplied.
systemID		The system identifier of the external entity being referenced.
baseURI		Absolute base URI associated with systemId.
namespace		The namespace of the entity to resolve.

Returns:  The resource requested or null.

Exceptions:

XMLStreamException

if there was a failure attempting to resolve the resource.