Whether or not XMLReader extends Parser, it looks as tho' this utility class is going to have to be replaced for SAX2. Seeing as we've got this opportunity, I'd like to propose we use an alternative (or at least supplementary) mechanism for specifying an XMLReader implementation class. The current mechanism is to have the org.xml.sax.parser System property specify the name of a Parser implementing class for ParserFactory to instantiate. This works fine, but has a couple of drawbacks. First, it's only possible for an application to bind to one parser implementation using this technique ... applications that want to use more than one have to specify at least one of the parser classes explicitly. Second, it requires some external configuration: it isn't just enough to put the parser implementation on the CLASSPATH for the parser to be made available. There is another mechanism that avoids both of these (admittedly minor) problems. The general idea can be found in the description of the new Service Provider mechanism found in the JDK 1.3 beta release, http://java.sun.com/products/jdk/1.3/docs/guide/jar/jar.html We can't use that directly (two reasons: we want to support pre 1.3, and in any case in 1.3 we'd have to reference a class in sun.misc.*) but we can do the same sort of thing. Here's how it works (sorry if this is a bit cryptic). Rather than using a System property we use one or more provider files (these are just text files somewhere on the CLASSPATH which can be accessed via ClassLoader.getSystemResources()) which contain the names of the concrete classes which implement XMLReader. An XMLReaderFactory could return an Iterator or Enumeration over the available implementation classes (more likely over some meta-information providing access to the features of the various installed parsers so an application can choose if there's more than one) and allow an app to select and instantiate one. The neat thing about this is that if we choose a well known location on the CLASSPATH to put the provider files it'll be possible to bundle a parser implementation along with it's own provider file, making it possible to configure an app to use it merely by making sure that the parser is on the CLASSPATH. For example, suppose we put the provider files at org/xml/sax2/XMLReader.providers, then I could distribute a jar file of my parser that looked like this, com cromwellmedia markup xml NonValidatingXMLReaderImpl.class ValidatingXMLReaderImpl.class org xml sax2 XMLReader.providers where XMLReader.providers contains the lines, com.cromwellmedia.markup.xml.NonValidatingXMLReaderImpl com.cromwellmedia.markup.xml.ValidatingXMLReaderImpl An XMLReaderFactory would be able to reference my providers file via ClassLoader.getSystemResources() and hence instantiate (or provide meta-information about) my two parsers. Note that this mechanism is compatible with there also being someone else's parser(s) being on the CLASSPATH too. So long as that parser is in a separate jar file (or in a separate class tree) ClassLoader.getSystemResources() will be able to locate both provider files, so an XMLReaderFactory would be able to amalgamate the files and offer access to all the parsers on the CLASSPATH. One complication tho'. ClassLoader.getSystemResources() (nb. plural) is new in JDK 1.2. With a bit of fiddling about we can get this mechanism to work with 1.1 as well, but with the limitation that multiple provider files can't be automatically amalgamated (ie. if you had more than one parser you'd have to stitch the two provider files together and put the manually joined file at the front of the CLASSPATH). Oh, and there's no reason why we couldn't also support an org.xml.sax2.parser System property as well. If there's any interest I can probably find the time to put together a sample implementation. Incidentally, I'd really like to see a mechanism of this sort used for bootstrapping into a DOM implementation. Cheers, Miles -- Miles Sabin Cromwell Media Internet Systems Architect 5/6 Glenthorne Mews +44 (0)20 8817 4030 London, W6 0LJ, England msabin@c... http://www.cromwellmedia.com/ xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ or CD-ROM/ISBN 981-02-3594-1 Please note: New list subscriptions now closed in preparation for transfer to OASIS.
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