public InputSource getExternalSubset(String name,
    String baseURI) throws SAXException, IOException
Allows applications to provide an external subset for documents
that don't explicitly define one.  Documents with DOCTYPE declarations
that omit an external subset can thus augment the declarations
available for validation, entity processing, and attribute processing
(normalization, defaulting, and reporting types including ID).
This augmentation is reported
through the startDTD()  method as if
the document text had originally included the external subset;
this callback is made before any internal subset data or errors
are reported.


This method can also be used with documents that have no DOCTYPE
declaration.  When the root element is encountered,
but no DOCTYPE declaration has been seen, this method is
invoked.  If it returns a value for the external subset, that root
element is declared to be the root element, giving the effect of
splicing a DOCTYPE declaration at the end the prolog of a document
that could not otherwise be valid.  The sequence of parser callbacks
in that case logically resembles this:

... comments and PIs from the prolog (as usual)
startDTD ("rootName", source.getPublicId (), source.getSystemId ());
startEntity ("[dtd]");
... declarations, comments, and PIs from the external subset
endEntity ("[dtd]");
endDTD ();
... then the rest of the document (as usual)
startElement (..., "rootName", ...);


Note that the InputSource gets no further resolution.
Implementations of this method may wish to invoke
resolveEntity()  to gain benefits such as use
of local caches of DTD entities.  Also, this method will never be
used by a (non-validating) processor that is not including external
parameter entities. 

Uses for this method include facilitating data validation when
interoperating with XML processors that would always require
undesirable network accesses for external entities, or which for
other reasons adopt a "no DTDs" policy.
Non-validation motives include forcing documents to include DTDs so
that attributes are handled consistently.
For example, an XPath processor needs to know which attibutes have
type "ID" before it can process a widely used type of reference.

Warning: Returning an external subset modifies
the input document.  By providing definitions for general entities,
it can make a malformed document appear to be well formed.

 public InputSource resolveEntity(String name,
    String publicId,
    String baseURI,
    String systemId) throws SAXException, IOException
Allows applications to map references to external entities into input
sources, or tell the parser it should use conventional URI resolution.
This method is only called for external entities which have been
properly declared.
This method provides more flexibility than the EntityResolver 
interface, supporting implementations of more complex catalogue
schemes such as the one defined by the OASIS XML Catalogs specification.


Parsers configured to use this resolver method will call it
to determine the input source to use for any external entity
being included because of a reference in the XML text.
That excludes the document entity, and any external entity returned
by getExternalSubset() .
When a (non-validating) processor is configured not to include
a class of entities (parameter or general) through use of feature
flags, this method is not invoked for such entities.  

Note that the entity naming scheme used here is the same one
used in the LexicalHandler , or in the ContentHandler.skippedEntity() 
method.