JAXB / targeting specific XML element

Programming Questions

lukenp

JAXB / targeting specific XML element

in Programming Questions • 2 years ago

This is my first post. Hallo welt!

I'm moving an as3 prototype into processing as all my tests seem to show the render engine betters flash, despite everything I've read. I was really disappointed at processing's native XML functionality but JAXB has solved some problems. I worked through Toxi's tutorial and extended it out with experimentation but now I'm stumped...

I've got a stack of data and need to be able to target specific nodes / elements based on an ID or a string… everything I've read suggests this can be done without traversing the whole dataset. In as3 it was as easy as:

xmlData.objects.type.(@id==reference);

I'm not expecting a like-for-like solutions -- I'd love one! -- but any pointers here would be smashing. I'm diving into processing head first and loving it, but this is really key.

Replies(7)

PhiLho

Re: JAXB / targeting specific XML element

2 years ago

The keyword here is XPath.
See, for example, http://www.vogella.de/articles/JavaXML/article.html#javaxpath_b

lukenp

Re: Re: JAXB / targeting specific XML element

2 years ago

Thanks for your time phi.lho. My silver bullet turned out to be made of bacofoil. It won't run in processing for me. It looks like pure java which I thought processing accepted but all the same, I can't write it to begin with.

I can't believe how complicated it is compared to as3's ecmascript… ho hum.

I think I'm going to have to bite the bullet and loop through JAXB's arraylists. Rubbish.

PhiLho

Re: JAXB / targeting specific XML element

2 years ago

Show what you tried to do (hint: don't use package, enclosing class, main(), use setup()), and we can see how to make it work. It is probably easier than you think.

lukenp

Re: Re: JAXB / targeting specific XML element

2 years ago

Hi again phi-lho!

Edited code from Vogella below. I removed package, the enclosing class, changed main() to setup(). The location of the xml is in the data folder (which probably needs to be explicit in this case). At this point it's misfiring and printing:

No library found for org.w3c.dom

No library found for org.xml.sax

import java.io.IOException;

import javax.xml.parsers.DocumentBuilder;

import javax.xml.parsers.DocumentBuilderFactory;

import javax.xml.parsers.ParserConfigurationException;

import javax.xml.xpath.XPath;

import javax.xml.xpath.XPathConstants;

import javax.xml.xpath.XPathExpression;

import javax.xml.xpath.XPathExpressionException;

import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;

import org.w3c.dom.NodeList;

import org.xml.sax.SAXException;

void setup(String[] args) throws XPathExpressionException, ParserConfigurationException, SAXException, IOException {

QueryXML process = new QueryXML();

process.query();

}

void query() throws ParserConfigurationException, SAXException,

IOException, XPathExpressionException {

// Standard of reading a XML file

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

factory.setNamespaceAware(true);

DocumentBuilder builder;

Document doc = null;

XPathExpression expr = null;

builder = factory.newDocumentBuilder();

doc = builder.parse("person.xml");

// Create a XPathFactory

XPathFactory xFactory = XPathFactory.newInstance();

// Create a XPath object

XPath xpath = xFactory.newXPath();

// Compile the XPath expression

expr = xpath.compile("//person[firstname='Lars']/lastname/text()");

// Run the query and get a nodeset

Object result = expr.evaluate(doc, XPathConstants.NODESET);

// Cast the result to a DOM NodeList

NodeList nodes = (NodeList) result;

for (int i=0; i<nodes.getLength();i++) {

System.out.println(nodes.item(i).getNodeValue());

}

// New XPath expression to get the number of people with name lars

expr = xpath.compile("count(//person[firstname='Lars'])");

// Run the query and get the number of nodes

Double number = (Double) expr.evaluate(doc, XPathConstants.NUMBER);

System.out.println("Number of objects " +number);

// Do we have more then 2 people with name lars?

expr = xpath.compile("count(//person[firstname='Lars']) >2");

// Run the query and get the number of nodes

Boolean check = (Boolean) expr.evaluate(doc, XPathConstants.BOOLEAN);

System.out.println(check);

}

PhiLho

Re: JAXB / targeting specific XML element

2 years ago

The warning you saw is the least of your concern... It seems that recent versions of Processing check the list of imports, and for those not starting with java or javax, try to find a library. But Java comes with some non-standard package names, like this org.w3c.dom, org.xml.sax (it doesn't complain for that?) or elsewhere com.sun.awt.
Annoying, but at least it is only a warning, it doesn't prevent from running.

You had more concerns, like changing the signature of setup() (it must have no parameters) and still invoking the removed class (I just call the function),
I made it a bit more Processing idiomatic (no System.out) and added more information on the output:

import java.io.IOException;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
void setup() {
try {
query();
} catch (Exception e) {
e.printStackTrace();
}
exit(); // No graphics to display
}
void query() throws ParserConfigurationException, SAXException,
IOException, XPathExpressionException {
// Standard of reading a XML file
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(dataPath("persons.xml"));
// Create a XPathFactory
XPathFactory xFactory = XPathFactory.newInstance();
// Create a XPath object
XPath xpath = xFactory.newXPath();
// Compile the XPath expression
XPathExpression expr = xpath.compile("//person[firstname='Lars']/lastname/text()");
// Run the query and get a nodeset
Object result = expr.evaluate(doc, XPathConstants.NODESET);
// Cast the result to a DOM NodeList
NodeList nodes = (NodeList) result;
println("Last names:");
for (int i = 0; i < nodes.getLength(); i++) {
println(nodes.item(i).getNodeValue());
}
// New XPath expression to get the number of people with name lars
expr = xpath.compile("count(//person[firstname='Lars'])");
// Run the query and get the number of nodes
int number = ((Double) expr.evaluate(doc, XPathConstants.NUMBER)).intValue();
println("Number of people named Iars: " + number);
// Do we have more then 2 people with name lars?
expr = xpath.compile("count(//person[firstname='Lars']) >2");
// Run the query and get the number of nodes
Boolean check = (Boolean) expr.evaluate(doc, XPathConstants.BOOLEAN);
println("More than two? " + check);
}

lukenp

Re: Re: JAXB / targeting specific XML element

2 years ago

Super helpful phi-lho! Many thanks. I was banging my head against a brick wall last night.

Now to get experimenting with xpath.compile()

Actually one last thing… would you recommend turning this all into a separate class?

Thanks again!

lukenp

Re: JAXB / targeting specific XML element

2 years ago

For anyone else who found this useful here is a solid introduction to XPath syntax:

http://www.w3schools.com/xpath/xpath_syntax.asp

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