I was looking for a means of parsing a large XML document while retaining the ability to process particular elements of interest. In this post I will demostrate a way of parsing an XML document with a SAX XMLReader and use an XMLFilter implementation that watches for element(s) nominated by name.
The class below performs partial unmarshalling on a DOM node representing a fragment of the overall document. When the start of the element is discovered we start capturing into the DOM Node and when the corresponding element end is discovered the DOM node is unmarshalled using JAXB, the provided listeners are called passing the recognised object to a handler method.
The following parameterised class implements the chunking facility.
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package ie.robb.sax.helpers; import java.util.ArrayList; import java.util.List; import javax.xml.bind.JAXBContext; import javax.xml.bind.JAXBElement; import javax.xml.bind.JAXBException; import javax.xml.bind.Unmarshaller; import javax.xml.parsers.DocumentBuilderFactory; import org.w3c.dom.Attr; import org.w3c.dom.Document; import org.w3c.dom.Node; import org.xml.sax.Attributes; import org.xml.sax.SAXException; import org.xml.sax.helpers.XMLFilterImpl; public class SAXElementToObject<T> extends XMLFilterImpl { public interface Listener<T> { void objectRead(T object); void unmarshalError(String message); } public enum FirstOrAllEnum { FIRST, ALL } private String namespaceUri; private String name; private Document document; private Node current; private JAXBContext ctx; private Unmarshaller unmarshaller; private T currentObject; private Class<T> type; private List<Listener<T>> listeners = new ArrayList<Listener<T>>(); private int depth = 0; private FirstOrAllEnum firstOrAll; private boolean found = false; private boolean useNamespace = false; public SAXElementToObject(String name, Class<T> type) throws Throwable { init(name, "", type, FirstOrAllEnum.ALL); } public SAXElementToObject(String name, FirstOrAllEnum firstOrAll, Class<T> type) throws Throwable { init(name, "", type, firstOrAll); } public SAXElementToObject(String name, Class<T> type, Listener<T> listener) throws Throwable { init(name, "", type, FirstOrAllEnum.ALL); this.listeners.add(listener); } public SAXElementToObject(String name, FirstOrAllEnum firstOrAll, Class<T> type, Listener<T> listener) throws Throwable { init(name, "", type, firstOrAll); this.listeners.add(listener); } public SAXElementToObject(String name, String namespaceUri, Class<T> type) throws Throwable { init(name, namespaceUri, type, FirstOrAllEnum.ALL); } public SAXElementToObject(String name, String namespaceUri, FirstOrAllEnum firstOrAll, Class<T> type) throws Throwable { init(name, namespaceUri, type, firstOrAll); } public SAXElementToObject(String name, String namespaceUri, Class<T> type, Listener<T> listener) throws Throwable { init(name, namespaceUri, type, FirstOrAllEnum.ALL); this.listeners.add(listener); } public SAXElementToObject(String name, String namespaceUri, FirstOrAllEnum firstOrAll, Class<T> type, Listener<T> listener) throws Throwable { init(name, namespaceUri, type, firstOrAll); this.listeners.add(listener); } private void init(String name, String namespaceUri, Class<T> type, FirstOrAllEnum firstOrAll) throws Throwable { this.name = name; this.namespaceUri = namespaceUri; this.type = type; this.firstOrAll = firstOrAll; this.ctx = JAXBContext.newInstance(type); this.unmarshaller = ctx.createUnmarshaller(); // create a DOM document to build fragments DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); factory.setNamespaceAware(true); this.document = factory.newDocumentBuilder().newDocument(); useNamespace = ! this.namespaceUri.equals(""); } public void addListener(Listener<T> listener) { listeners.add(listener); } @Override public void startElement(String uri, String localName, String qName, Attributes atts) throws SAXException { boolean isRoot = false; if(firstOrAll == FirstOrAllEnum.ALL || firstOrAll == FirstOrAllEnum.FIRST && !found) { if( useNamespace && namespaceUri.equals(uri) && name.equals(qName)) { current = document.createElementNS(uri, qName); isRoot = true; depth = 1; found = true; } else if(!useNamespace && name.equals(qName)) { current = document.createElement(qName); isRoot = true; depth = 1; found = true; } } if(depth > 0) { for(int i = 0 ; i < atts.getLength() ; i ++) { Attr att = document.createAttribute(atts.getQName(i)); att.setValue(atts.getValue(i)); current.appendChild(att); } if(!isRoot) { current = current.appendChild(document.createElement(qName)); depth ++; } } super.startElement(uri, localName, qName, atts); } @Override public void endElement(String uri, String localName, String qName) throws SAXException { if(depth == 1) { try { JAXBElement<T> object = unmarshaller.unmarshal(current, type); currentObject = object.getValue(); for(Listener<T> listener : listeners) listener.objectRead(currentObject); } catch(JAXBException e) { for(Listener<T> listener : listeners) listener.unmarshalError(e.getMessage()); } current = null; depth = 0; } else depth --; if(depth > 0 ){ current = current.getParentNode(); } super.endElement(uri, localName, qName); } @Override public void characters(char[] ch, int start, int length) throws SAXException { if(current != null) { current.appendChild(document.createTextNode(new String(ch, start, length))); } super.characters(ch, start, length); } } |
In the next part of the series I will demonstrate how to use the class above.
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