Final Draft of XQJ

By Michael Kay on November 21, 2007 at 02:18p.m.

A "Final Draft" of XQJ has been published ( There aren't many changes, but this time the specification is accompanied by actual interface definitions, and by a test suite.

Swapping in the published interface definitions for my previous handmade versions proved no trouble - one or two discrepancies as regards which methods throw exceptions, and one or two methods that have had an extra argument added.

Running the test suite is likely to be a bit more effort. Problems so far:

(a) The spec generally requires an XQException to be raised when arguments to methods are null. I was generally ignoring that requirement, often throwing a NullPointerException instead (which is more usual).

(b) The test suite takes the rules on closed connections very seriously. I had previously treated this as a bit of a nonsense - there's no real connection in Saxon, so closing it doesn't mean very much, and if things still work after you've closed it, then no-one is going to complain. Except the test suite, that is... A particular problem is that closing a connection requires invalidating all expressions created using that connection. That means Saxon is going to have to retain a reference to all such expressions, which is likely to result in the expressions and quite possibly their results being locked in memory for the duration of the session unless I can find a way of doing it using weak references. That's a lot of work to satisfy a pointless requirement, but unfortunately the spec and the test suite leave no latitude here.

(c) More seriously, there are rules on the manipulation of the static context that the Saxon implementation wasn't following (it's possible these rules have been tightened up in the latest draft, I'm not sure). For example when you set the static context on the connection, the connection is supposed to take a snapshot of the state of the static context, and subsequent changes to the static context object are supposed to have no effect. Also, options like ordering mode have to be respected in the API even though the actual XQuery implementation is allowed to ignore them.

So far the test suite has found perhaps three or four bugs that real live users would notice and complain about, and perhaps 150 errors (like throwing the wrong exception on null arguments, or delivering a successful result when a closed connection is used) that can be treated as trivial - though of course they will all need to be fixed.

Postscript (2007-11-23)

I've now run all the tests - with just a couple of failed tests left where there was clearly a discrepancy between the test result and the spec. Apart from the issues highlighted above, there were one or two problems to be sorted out with StAX (XMLStreamReader) interfaces (caused largely by the woeful inadequacy of the StAX specification itself), and quite a few details especially with the XQItemType interface where it only became clear from reading the tests what the specification authors had really had in mind (baseType() in XQJ, for example, means something that you couldn't possibly guess from the XQuery and XML Schema uses of the term).

The change that will probably have most impact on users is the rules on a forwards-only result sequence that say (with one or two exceptions) that you're only allowed to access each item once. There's a workaround - you call getItem() to grab a copy of the item and that counts as your one access - but it's not very natural.

It's painful having to add lots of code to prevent users doing useful things that the implementation previously allowed, but one just has to breathe deeply and remember that conformance comes first.