XQuery API for Java

XQuery API for Java (XQJ) refers to the common Java API for the W3C XQuery 1.0 specification.

XQJ
Developer(s)Java Community Process
Stable release
1.0 / 24 June 2009 (2009-06-24)
TypeData Access API
WebsiteJSR 225: XQuery API for Java
General architecture of how an XQJ driver is used to communicate with an XML Database from Java Applications.

The XQJ API enables Java programmers to execute XQuery against an XML data source (e.g. an XML database) while reducing or eliminating vendor lock in.

The XQJ API provides Java developers with an interface to the XQuery Data Model.[1] Its design is similar to the JDBC API which has a client/server feel and as such lends itself well to Server-based XML Databases and less well to client-side XQuery processors, although the "connection" part is a very minor part of the entire API. Users of the XQJ API can bind Java values to XQuery expressions, preventing code injection attacks.[2] Also, multiple XQuery expressions can be executed as part of an atomic transaction.

History and implementation

The XQuery API for Java was developed at the Java Community Process as JSR 225. It had some big technology backers such as Oracle,[3][4][5][6] IBM,[5][6] BEA Systems,[7] Software AG,[8] Intel, Nokia and DataDirect.[4]

Version 1.0 of the XQuery API for Java Specification was released on June 24, 2009,[9] along with JavaDocs, a reference implementation and a TCK (Technology Compatibility Kit) which implementing vendors must conform to.

The XQJ classes are contained in the Java package javax.xml.xquery

There is no (visible) activity to create a version of XQJ that provides support for XQuery 3.0 or 3.1, for example by providing Java bindings for additions to the data model such as functions, arrays, or maps.

Functionality

XQJ allows multiple implementations to exist and be used by the same application.

XQJ connections support creating and executing XQuery expressions. Expressions may be updating[10] and may include full text searches.[11] XQJ represents XQuery expressions using one of the following classes:

  • XQExpression – the expression is sent to the XQuery processor every time.
  • XQPreparedExpression – the expression is cached and the execution path is pre-determined allowing it to be executed multiple times in an efficient manner.

XQuery expressions return a result sequence of XDM[1] items which in XQJ are represented through the XQResultSequence interface. The programmer can use an XQResultSequence to walk over individual XDM[1] items in the result sequence. Each item in the sequence has XDM[1] type information associated with it, such as its node type e.g. element(), document-node() or an XDM atomic type such as xs:string, xs:integer or xs:dateTime. XDM type information in XQJ can be retrieved via the XQItemType interface.

Atomic XQuery items can be easily cast to Java primitives via XQItemAccessor methods such as getByte() and getFloat(). Also XQuery items and sequences can be serialized to DOM Node, SAX ContentHandler, StAX XMLStreamReader and the generic IO Reader and InputStream classes.

Examples

Basic example

The following example illustrates creating a connection to an XML Database, submitting an XQuery expression, then processing the results in Java. Once all of the results have been processed, the connection is closed to free up all resources associated with it.

// Create a new connection to an XML database
XQConnection conn = vendorDataSource.getConnection("myUser", "myPassword");

XQExpression expr = conn.createExpression(); // Create a reusable XQuery Expression object

XQResultSequence result = expr.executeQuery(
  "for $n in fn:collection('catalog')//item " +
  "return fn:data($n/name)"); // execute an XQuery expression

// Process the result sequence iteratively
while (result.next()) {
    // Print the current item in the sequence
    System.out.println("Product name: " + result.getItemAsString(null));
}

// Free all resources created by the connection
conn.close();

Binding a value to an external variable

The following example illustrates how a Java value can be bound to an external variable in an XQuery expression. Assume that the connection conn already exists:

XQExpression expr = conn.createExpression();

// The XQuery expression to be executed
String es = "declare variable $x as xs:integer external;" +
            " for $n in fn:collection('catalog')//item" +
            " where $n/price <= $x" +
            " return fn:data($n/name)";

// Bind a value (21) to an external variable with the QName x
expr.bindInt(new QName("x"), 21, null);

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

// Process the result (sequence) iteratively
while (result.next()) {
    // Process the result ...
}

Default data type mapping

Mapping between Java and XQuery data types is largely flexible, however the XQJ 1.0 specification does have default mapping rules mapping data types when they are not specified by the user. These mapping rules bear great similarities to the mapping rules found in JAXB.

The following table illustrates the default mapping rules for when binding Java values to external variables in XQuery expressions.

Default conversion rules when mapping from Java data types to XQuery data types
Java Datatype Default XQuery Data Type(s)
boolean xs:boolean
byte xs:byte
byte[] xs:hexBinary
double xs:double
float xs:float
int xs:int
long xs:long
short xs:short
Boolean xs:boolean
Byte xs:byte
Float xs:float
Double xs:double
Integer xs:int
Long xs:long
Short xs:short
String xs:string
BigDecimal xs:decimal
BigInteger xs:integer
Duration xs:dayTimeDuration if the Duration Object's state is xs:dayTimeDuration
xs:yearMonthDuration if the Duration Object's state is xs:yearMonthDuration
xs:duration if the Duration Object's state is xs:duration
XMLGregorianCalendar xs:date if the XMLGregorianCalendar Object's state is xs:date
xs:dateTime if the XMLGregorianCalendar Object's state is xs:dateTime
xs:gDay if the XMLGregorianCalendar Object's state is xs:gDay
xs:gMonth if the XMLGregorianCalendar Object's state is xs:gMonth
xs:gMonthDay if the XMLGregorianCalendar Object's state is xs:gMonthDay
xs:gYear if the XMLGregorianCalendar Object's state is xs:gYear
xs:gYearMonth if the XMLGregorianCalendar Object's state is xs:gYearMonth
xs:time if the XMLGregorianCalendar Object's state is xs:time
QName xs:QName
Document document-node(element(*, xs:untyped))
DocumentFragment document-node(element(*, xs:untyped))
Element element(*, xs:untyped)
Attr attribute(*, xs:untypedAtomic)
Comment comment()
ProcessingInstruction processing-instruction()
Text text()

Known implementations

Native XML databases

The following is a list of Native XML Databases which are known to have XQuery API for Java implementations.

Relational databases

DataDirect provide XQJ adapters for relational databases, by translating XQuery code into SQL on the fly, then converting SQL result sets into a format suitable for XQJ to process further. The following is a couple of known implementations.

Non-database implementations

The following is a list of non-database XQuery processors which provide an XQuery API for Java interface (typically allowing query against documents parsed from XML in filestore, and held in memory as DOM or similar trees).

License

The specification is marked as "Copyright © 2003, 2006 - 2009 Oracle. All rights reserved."

The specification contains two separate licenses: a "specification license" and a "reference implementation license".

The specification license allows free copying of the specification provided that copyright notices are retained; it also grants a license to create and distribute an implementation of the specification provided that it fully implements the entire specification, that it does not modify or extend any interfaces, and that it passes the compatibility tests.

This provision has caused some controversy. Firstly, it is not universally accepted that implementing a published specification is something that requires a license (that is, that copyright law would disallow this in the absence of a license).[20][21] Secondly, the license does not meet the criteria to qualify as an open source license (see Open Source Definition), because of the ban on making extensions and modifications. This has led some open source enthusiasts to challenge whether XQJ implementations can ever be considered truly open source.

The license for the reference implementation is a fairly conventional BSD-style open source license.

References
  1. XQuery 1.0 and XPath 2.0 Data Model (XDM)
  2. Binding Java Variables
  3. Querying XML: XQuery, XPath, and SQL/XML in context - Jim Melton and Stephen Buxton. ISBN 978-1558607118
  4. XQJ - XQuery Java API is Completed, Marc Van Cappellen, Zhen Hua Liu, Jim Melton and Maxim Orgiyan Archived 28 July 2012 at the Wayback Machine
  5. IBM and Oracle Submit XQuery API for Java (XQJ) Java Specification Request.
  6. An Early Look at XQuery API for Java (XQJ) - Andrew Eisenberg, IBM and Jim Melton, Oracle Archived 28 July 2012 at the Wayback Machine
  7. The BEA Streaming XQuery Processor
  8. XQJ Interface for Tamino Native XML Database Archived 30 May 2013 at the Wayback Machine
  9. JSR-000225 XQuery API for Java (Final Release)
  10. XQuery Update Facility
  11. XQuery Full Text
  12. MarkLogic XQJ API
  13. eXist XQJ API
  14. BaseX XQJ API
  15. Sedna XQJ API
  16. Oracle XML DB Support for XQJ
  17. Software AG - Working with the CentraSite XQJ Interface
  18. Zorba 2.5 ships with a long awaited XQJ binding, 14 June 2012
  19. Oracle XML Developer's Kit (XDK) provides a standalone XQuery 1.0 processor for use by Java applications.
  20. http://rosenlaw.com/wp-content/uploads/Open-Standards.pdf
  21. http://www.groklaw.net/articlebasic.php?story=20120221094600287
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