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13.3.2 Parsing with the DOM
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In contrast to the SAX processing model, DOM processes XML via recursion. Uh-oh, this is going to be another difficult topic. No, it really isn t that difficult to understand. Let s first look at how DOM visualizes a document as opposed to how SAX does it by examining figure 13.4.
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Processing XML
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Once they are entirely in memory, DOM documents are viewed as a hierarchical tree.
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With DOM, everything is a node, and it sees an XML document as a hierarchical tree constructed of these nodes. To accomplish this, it s necessary for the parser to read the entire document into memory before any node processing takes place. This is in contrast to SAX in that SAX processes XML serially as a stream. Notice in figure 13.4 that DOM has its own nomenclature for different types of nodes. Once a document is in memory, the DOM parser starts processing at the document node, CD in this example, which is the top level of the tree, as shown in figure 13.4. It then continues until all nodes are processed. Just like traversing up and down through a directory tree, the way to accomplish this document traversal is by utilizing recursion. An example of this recursion should be the best way to demonstrate the process. We will again utilize our CD class, but in this case we have named it CDDOM. This is because we must declare a different bridge class to handle the XML parsing. The only difference is this declaration:
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@Entity @Indexed @Analyzer(impl=StandardAnalyzer.class) @ClassBridge(name = "", index = Index.TOKENIZED, store = Store.YES, impl = DOMExampleBridge.class) public class CDDOM {
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Let s examine listing 13.16, which shows our DOMExampleBridge.
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Don t reinvent the wheel
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Listing 13.16 Parsing XML with DOM via recursion
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public class DOMExampleBridge implements FieldBridge { public void set( String name, Implement FieldBridge Object value, set method Document document, LuceneOptions options ) {
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CDDOM cd = (CDDOM) value; String xml = cd.getPriceData(); if ( xml == null ) { return; } InputSource source = new InputSource( new StringReader( xml ) );
C Create an XML InputSource
DOMParser parser = new DOMParser(); Instantiate the try { Parser and parse parser.parse( source ); org.w3c.dom.Document xmlDocument = parser.getDocument(); new DOMHandler( xmlDocument, document, options ); } catch (SAXException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); }
} private class DOMHandler { List<Field> mFieldList = new ArrayList<Field>(); Field mField; Field.Store mStore; Field.Index mIndex; Field.TermVector mVector; Float mBoost; public DOMHandler( org.w3c.dom.Document xmlDocument, Document document, LuceneOptions options ) { mStore = options.getStore(); mIndex = options.getIndex(); mVector = options.getTermVector(); mBoost = options.getBoost(); traverse( xmlDocument.getDocumentElement() ); for (Field field : mFieldList) { document.add( field ); } } private void traverse( Node node ) { if ( node == null ) {
Start the recursion
finished, add F Whento document fields
If no Node, we are finished
Processing XML
return; } int type = node.getNodeType(); switch (type) { case Node.ELEMENT_NODE: { NamedNodeMap attrs = node.getAttributes();
Process all XML document nodes
Retrieve any element attributes
for (int x = 0; x < attrs.getLength(); x++) { Create a Field Node attrNode = attrs.item( x ); per attribute Field field = new Field( attrNode.getLocalName().toLowerCase(), attrNode.getNodeValue(), mStore, mIndex, mVector ); if ( mBoost != null ) field.setBoost( mBoost ); mFieldList.add( field ); } NodeList children = node.getChildNodes(); if ( children != null ) { int len = children.getLength(); for (int i = 0; i < len; i++) { traverse( children.item( i ) ); } } break; } case Node.TEXT_NODE: {
Recurse through child nodes
Process all XML text nodes
if ( node instanceof TextImpl ) { if ( !( ( (TextImpl) node ) .isIgnorableWhitespace() ) ) {
Ignore whitespace
Field field = new Field( node.getParentNode() .getLocalName().toLowerCase(), node.getNodeValue(), Field.Store.YES, Field.Index.TOKENIZED ); mFieldList.add( field ); } } break; } } } } }
Create a Field per text node
As always, we implement the FieldBridge s set method B. Next, we convert our XML to an InputSource C and get an instance of the DOMParser D. After parsing the XML document E, we start recursing through the nodes by passing the DocumentNode F.
Don t reinvent the wheel
If the next node is null, we are finished G. Remember the recursion principles it always needs a way out. All elementNodes are processed at H. Processing starts by extracting any attributes , converting them to Document fields, and storing them in the Field ArrayList I mFieldList J. The last step in processing an elementNode is to gather its child nodes, if any, and recurse through them 1). All textNodes (the actual contents of elementNodes) are processed at 1!, and after skipping over any ignorable whitespace 1@, we convert the text to Document fields using its parent node (the elementNode) name as the field name. Once we ve processed the last node 1# in the document, we transfer all of the generated fields to the Lucene document, and we re finished.
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