Dr. Dobb's Journal - December 2007 - (Page 30)

d12trem_p4ma.qxp 10/11/07 12:46 PM Page 30 Core Technology by Sam Edwards and Paul Tremblett XQuery The same power for XML that SQL has with relational data Widespread use of XML has established it as a lingua franca. The choice of XML to represent ever-increasing amounts of data led to the need for a query language that uses the structure of Sam is a developer at Harmonia. He can be contacted at ssaammee@gmail.com. Paul is a lead developer at Harmonia. He can be contacted at paul@tremblett.ca. XML intelligently to express queries across the diverse kinds of data XML can be used to represent. W3C has developed a formal specification of such a query language; it is called XQuery. The best approach to learning XQuery is one that comes naturally to developers—examine a few examples and start writing queries. The eXist open-source native XML database (http://exist.sourceforge.net) comes with a sandbox that makes such an approach easy; see Figure 1. In the upper window, the query doc("musicians.xml")/musicians/musician searches the document contained in the file “musicians.xml” (Listing One) for all musician nodes that are children of the node named musicians. The results of the query are displayed in the lower window. suitable driver has been written. If you examine the Java code, you see that it is roughly analogous to using JDBC. You start by using the following code to load the driver: String driver = "org.exist.xmldb.DatabaseImpl"; Class cl = Class.forName(driver); Database database = (Database)cl.newInstance(); DatabaseManager.registerDatabase (database); Using XQuery From Java The eXist sandbox is an excellent learning tool and can be useful for composing and executing ad hoc queries; however, as you surely discovered when you used SQL, the real power of XQuery comes when it is used in an application. QueryExample.java (available online; see “Resource Center,” page 5) is a Java program from the eXist Developer’s Guide that can be used to execute XQueries. There are many implementations of XQuery. Because we had already installed eXist to use the sandbox, we chose the XML:DB API that eXist uses. This API can work with any native or XML-enabled database for which a 30 Dr. Dobb’s Journal l www.ddj.com l December 2007 The only difference between this code and its JDBC counterpart is the last line, which registers the Database object with the DatabaseManager. Each vendor delivers its own implementation of Database, which is the encapsulation of the database driver functionality that is necessary to access an XML database. The DatabaseManager class provides access to a Collection. The Collection interface represents a collection of Resources stored in an XML database. The Resource interface represents a container for data stored within the database. Next, you get a Collection object using this code: Collection col = DatabaseManager.getCollection( "xmldb:exist://localhost:8080 /exist/xmlrpc/db" http://exist.sourceforge.net http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - December 2007

Dr. Dobb's Journal - December 2007 Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook Computer Books: Reading Between the Lines Conversations Query Anything with SQLite XQuery Web Maps with the Google Map API OpenALM and Its Manifesto Transactional Programming Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - December 2007

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