Right XML technology-library

Question

I have some experience with XML but get easily lost in the jungle of acronyms and XML technologies out there. I have a problem to solve and would like to know to which technologies or libraries to focus. That is, if it's possible at all. The problem is as follows:
I have a big XML document, although not so big as to not to fit in memory, with structure rather flexible. I want to be able to make queries to it of the type:
Give me all elements containing element (or attribute) XXX
Give me all elements of type YYY inside elements containing element (or attribute) XXX
Give me all elements of type YYY inside elements where element (or attribute) XXX has value VVV (or less than, greater than...)
For example having document:
CD>
<TITLE>Empire Burlesque</TITLE>
<ARTIST>Bob Dylan</ARTIST>
<COUNTRY>USA</COUNTRY>
<COMPANY>Columbia</COMPANY>
<PRICE>10.90</PRICE>
<YEAR>1985</YEAR>
</CD>
<CD>
<TITLE>Hide your heart</TITLE>
<ARTIST>Bonnie Tyler</ARTIST>
<COUNTRY>UK</COUNTRY>
<COMPANY>CBS Records</COMPANY>
<PRICE>9.90</PRICE>
<YEAR>1988</YEAR>
</CD>
<CD>
<TITLE>Greatest Hits</TITLE>
<ARTIST>Dolly Parton</ARTIST>
<COUNTRY>USA</COUNTRY>
<COMPANY>RCA</COMPANY>
<PRICE>9.90</PRICE>
<YEAR>1982</YEAR>
</CD>
Would like to ask for all TITLEs from COMPANY = 'RCA' or all CD info from all CDs where YEAR < 1960 and PRICE < 10, etc.
I realize that this is using XML as a database, but if possible I would like to avoid having to put the data inside a database for querying, even if the database would be as flexible as the XML (I'll be adding or removing element types in the future). Adding a database to my program would be some stretch. I would love to have a java library that could be queried for that kind of info in a flexible way (the query should ideally be a string, not hard coded). I don't need complete guidelines, just names of technologies or libraries to look at. It's simply that I don't know where to start.
Thanks in advance.
Julen.

Answer 1

I recommend using XPath queries as in the following example:import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
Document doc = <your XML document>;
Node rootNode = doc.getDocumentElement();
XPathFactory xpf = XPathFactory.newInstance();
XPath xp = xpf.newXPath();
String query = <your query string>;
NodeList nodeList = (NodeList)xp.evaluate(query, rootNode, XPathConstants.NODESET);This code fragment retrieves the matching nodes in the form of a NodeList collection, which is capable of iterating through the nodes it contains. Though XPath queries are inherently select queries (they do not modify but only return the underlying data), the nodes in the NodeList can be processed further as needed, so you can even simulate action queries. This is because any modifications made to the nodes in the NodeList will be automatically reflected in the corresponding nodes in the parent XML document. Since the efficiency of using XPath queries is largely a matter of how familiar you are with the XPath language, it's best to read up on it first in the tutorials on the Internet. To start you off, here are two examples:
1. This query selects all the <TITLE> elements that have a <CD> parent element and a <COMPANY> sibling element containing "RCA".String query = "//CD/TITLE[../COMPANY='RCA']"2. This query selects all <CD> elements that have a <YEAR> child element containing less than "1987" and a <PRICE> child element containing less than "10".String query = "//CD[YEAR<1987 and PRICE<10]"