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XQuery Java API JSR 225 Available for Public Review

Posted by Abhay Bakshi on Jul 03, 2007

Sections: Architecture & Design,; Development,; Operations & Infrastructure
Topics: Java ,; Data Access
Tags: XQuery

The first public review draft of JSR 225: XQuery API for Java has been posted for review. The spec (being led by Oracle) aims to provide standard programmatic access for XQuery implementations in Java. XQJ is a generic XQuery data access framework, which provides a uniform interface on top of a variety of different XQuery implementations.

The W3C XQuery is a query language for XML. Wikipedia defines it's function as providing the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents.XQuery uses XPath expression syntax to address specific parts of an XML document. One of the language syntax requirements for XQuery is that the Query language MUST be convenient for humans to read and write. The XQuery language now has both a human-readable syntax and an XML-based syntax. The XML-based syntax for XQuery is available at XQueryX 1.0.

A basic XQJ expression in Java looks like (from the spec):

...
// establish a connection to the XQuery engine
XQConnection conn = xqds.getConnection();

// create an expression object that is later used
// to execute an XQuery expression
XQExpression expr = conn.createExpression();

// the XQuery expression to be executed
String es = "for $n in fn:doc('catalog.xml')//item " +
"return fn:data($n/name)";

// execute the XQuery expression
XQResultSequence result = expr.executeQuery(es);

// process the result (sequence) iteratively
while (result.next()) {
  // retrieve the current item of the sequence as a String
  String str = result.getAtomicValue();
  System.out.println("Product name: " + str);
}

// free all resources allocated for the result
result.close();

// free all resources allocated for the expression
expr.close();

// free all resources allocated for the connection
conn.close();
...

While the XQJ spec has been led by Jim Melton (Oracle), its JCP expert group involves members including Jason Hunter, DataDirect , BEA Systems et al. Industry leaders outside of JCP have also been following the XQJ specification, contributing ideas, and improving their own products for conformity. Michael Kay - creator of Saxon and author of XPath 2.0 Programmer's Reference - writes:

It's the first new version for about a year, and the spec is developed under conditions of absolute secrecy, so I was interested to see what was going to be in it. I wasn't expecting too much, because most of my comments on the previous draft had been politely rejected (about 8 months after I submitted them, with no open discussion). Sure enough, they've tidied up quite a lot of little things, but the overall design hasn't changed much. (It's sufficiently incompatible, however, that most applications will have to be tweaked: not rewritten, but amended here and there.)

It's still uncompromisingly based on a client-server, connection-oriented model where the application lives on a different machine from the database (so 1980s!). ... prepared expressions are not thread-safe - you can't compile an expression and then run it in multiple threads simultaneously, because the XQPreparedExpression holds its own dynamic execution context.

Are there any good points? Yes, quite a few. The mapping of XPath values (the 19 primitive data types) to equivalent objects in Java is done reasonably well - much better than JAXP ... This version also has a full representation in Java terms of all XPath types ...
Still, I think one can do better. ...

The W3C XQuery specification and 8 related specifications (including XSLT 2.0 and XPath 2.0) were submitted to the W3C on June 8 2006. Many of them were updated on 23 January 2007. The industry has already begun embracing XQuery in their products, even JSR 170/Java Content Repositories use it as their standard query mechanism. InfoQ has been tracking development of XQuery specification, XQJ specification, and the implementations.