Resolver APIshttp://norman.walsh.name/2007/02/09/resolvers10102007-02-09T15:52:52-05:00$Date: 2007-04-25 07:53:59 -0400 (Wed, 25 Apr 2007) $NormanWalsh2007Norman WalshHow can I resolve thee? Let me count the ways.Working on my XMLResolver
project has made me look again at the range of entity and URI resolution APIs
in the Java platform. How many are there?
I got to six without breaking a sweat.You can resolve with SAX,
org.xml.sax.EntityResolver:InputSource resolveEntity(String publicId, String systemId);Here we can't look at the entity name nor can we look at the
system identifier before it's been made absolute. There's also no way
for this interface to indicate failure to resolve, so we can't
effectively chain resolvers.Also with SAX,
org.xml.sax.ext.EntityResolver2:InputSource getExternalSubset(String name, String baseURI);
InputSource resolveEntity(String name,
String publicId,
String baseURI,
String systemId);Getting an external subset for a document that doesn't have one is kind of
cool, though I've never actually tried it.This version of
resolveEntity fixes the bugs in the earlier
API: we can examine the system identifier before and after making it
absolute and we can return null to indicate that we
weren't able to resolve it.With StAX,
javax.xml.stream.XMLResolver:Object resolveEntity(String publicID,
String systemID,
String baseURI,
String namespace)Oh, joy! It returns Object! And notice how its signature
conflicts with the EntityResolver2 interface so you
can't even implement both APIs in a single class.I can't imagine how a namespace URI comes into play for
resolving an entity that includes either
a system or public identifier. And vice versa.
And where's the name of the entity? That I could actually use!
We're just warming up,
DOM
gets into the act too, with
org.w3c.dom.ls.LSResourceResolver:LSInput resolveResource(String type,
String namespaceURI,
String publicId,
String systemId,
String baseURI);Another bizarre conflation of
external identifiers
and
namespace
names.The string valued “type” makes this a sort of chameleon method.
If you specify a type of “http://www.w3.org/TR/REC-xml” then it's
supposed to resolve “XML resources (i.e. entities)” (sic). But without the
entity name, apparently.If you specify a type of “http://www.w3.org/2001/XMLSchema”
then it's supposed to resolve “XML Schema [resources]”. But when does
anything have both a namespace and
an external identifier?(If you specify any other type, you're on your own. Fair enough,
I guess.)The Transformer API has one too,
javax.xml.transform.URIResolver:Source resolve(String href, String base);This is just for URIs, so it's at least a little less complicated.Finally, contributing to the problem myself, I've proposed one more in
org.xmlresolver.NamespaceResolver:Source resolveNamespace(String uri, String nature, String purpose);This one resolves namespaces using
RDDL natures and purposes. (You
aren't supposed
to use relative URIs as namespace names, so my decision not to support them in
this API was intentional.)On the whole: Bleh!I wonder how many of these are actually used with any
regularity? I wonder if there's any opportunity for unification
and simplification? (And I wonder how many more there are out there.)