Reading a big XML file using stax and dom

Question

I need to read several big (200Mb-500Mb) XML files, so I want to use StaX. My system has two modules - one to read the file ( with StaX ); another module ( 'parser' module ) suppose to get a single entry of that XML and parse it using DOM. My XML files don't have a certain structure - so I cannot use JaxB. How can I pass the 'parser' module a specific entry that I want it to parse? For example:

<Items>
   <Item>
        <name> .... </name>
        <price> ... </price>
   </Item>
   <Item>
        <name> .... </name>
        <price> ... </price>
   </Item>
</Items>

I want to use StaX to parse that file - but each 'item' entry will be passed to the 'parser' module.

Edit:
After a little more reading - I think I need a library that reads an XML file using stream - but parse each entry using DOM. Is there such a thing?

Solution 1

You could use a StAX (javax.xml.stream) parser and transform (javax.xml.transform) each section to a DOM node (org.w3c.dom):

import java.io.*;
import javax.xml.stream.*;
import javax.xml.transform.*;
import javax.xml.transform.stax.StAXSource;
import javax.xml.transform.dom.DOMResult;
import org.w3c.dom.*

public class Demo {

    public static void main(String[] args) throws Exception  {
        XMLInputFactory xif = XMLInputFactory.newInstance();
        XMLStreamReader xsr = xif.createXMLStreamReader(new FileReader("input.xml"));
        xsr.nextTag(); // Advance to statements element

        TransformerFactory tf = TransformerFactory.newInstance();
        Transformer t = tf.newTransformer();
        while(xsr.nextTag() == XMLStreamConstants.START_ELEMENT) {
            DOMResult result = new DOMResult();
            t.transform(new StAXSource(xsr), result);
            Node domNode = result.getNode();
        }
    }

}

Also see:

• Split 1GB Xml file using Java

Solution 2

Blaise Doughan's answer fails in clean java 7 and 8 due to https://bugs.openjdk.java.net/browse/JDK-8016914

java.lang.NullPointerException
at com.sun.org.apache.xerces.internal.dom.CoreDocumentImpl.setXmlVersion(CoreDocumentImpl.java:860)
at com.sun.org.apache.xalan.internal.xsltc.trax.SAX2DOM.setDocumentInfo(SAX2DOM.java:144)

Funny thing: if you use jaxb unmarshaller, you don't get the NPE:

package com.common.config;

import java.io.*;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBElement;
import javax.xml.bind.Unmarshaller;
import javax.xml.stream.*;

import org.w3c.dom.*;

public class Demo {


    public static void main(String[] args) throws Exception  {
        XMLInputFactory xif = XMLInputFactory.newInstance();
        XMLStreamReader xsr = xif.createXMLStreamReader(new FileReader("input.xml"));
        // Advance to root element
        xsr.nextTag(); // TODO: nextTag() can't skip DTD
        xsr.next(); // Advance to first item or EOD

        final JAXBContext jaxbContext = JAXBContext.newInstance();
        final Unmarshaller unm = jaxbContext.createUnmarshaller();
        while(true) {
            // previous unmarshal() already did advance to next element or whitespace
            if (xsr.getEventType() == XMLStreamReader.START_ELEMENT) {
                JAXBElement<Object> jel = unm.unmarshal(xsr, Object.class);
                Node domNode = (Node)jel.getValue();
                System.err.println(domNode.getNodeName());
            } else if (!xsr.hasNext()) {
                    break;
            } else {
                xsr.next();
            }
        }
    }

}

The reason is: com.sun.xml.internal.bind.v2.runtime.unmarshaller.StAXConnector$1 does not implement Locator2 therefore it has no getXMLVersion().