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Resolving entities found in source XML during parsing E-mail
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Some XML documents have type definitions described as standard, e.g XML schemas or XML stylesheets. DTD-files for them are completely predefined and are stored on W3C host.

For such documents, SAX-parser can avoid references through Internet by using a local copy of DTD-file. In order to perform such substitutions, SAX suggests mechanism of enity resolving. A handler below shows its usage:

<!-- file: purchase-order.dtd -->

<!-- declaration of the root element and its attributes -->
<!ELEMENT purchase-order (purchased-by, order-items)>
<!ATTLIST purchase-order
	date   CDATA #REQUIRED
	number CDATA #REQUIRED
>

<!ELEMENT purchased-by (address)>
<!ATTLIST purchased-by
	name CDATA #REQUIRED
>
<!ELEMENT address (#PCDATA)>

<!-- order-items can contains at least on item -->
<!ELEMENT order-items (item+)>
<!ELEMENT item EMPTY>
<!ATTLIST item
	code  CDATA #REQUIRED
	type  CDATA #REQUIRED
	label CDATA #REQUIRED
>

<!-- file: sample.xml -->
<?xml version="1.0"?>

<!DOCTYPE purchase-order SYSTEM "http://www.my-company.com/order-1.0.dtd">

<!--
   XML document may refer DTD-file that
	is located somewhere on WEB.
-->

<purchase-order date="2005-10-31" number="12345">

	<purchased-by name="My name">
<!--
		we comment it to lead an error:
		<address>My address</address>
	-->
	</purchased-by>

	<order-items>
		<!--
			here is an example of empty element
			i.e. containing no nested elements
		-->
		<item code="687" type="CD" label="Some music" />
		<item code="129851" type="DVD" label="Some video"/>
	</order-items>

</purchase-order>
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.helpers.DefaultHandler;
import org.xml.sax.InputSource;

/**
 * This is a sample of entity-resolvers usage.
 * Resolvers are used to substitute referenced network
 * resourced by local (downloaded) copies.
 */

public class SampleOfResolvingEntities {
    /**
     * Application entry point
     @param args command-line arguments
     */
    public static void main(String[] args) {
        try {
            // creates and returns new instance of SAX-implementation:
            SAXParserFactory factory = SAXParserFactory.newInstance();
            factory.setValidating(true);
            
            // create SAX-parser...
            SAXParser parser = factory.newSAXParser();
            // parser.getXMLReader().setEntityResolver();
            
            // .. define our handler:
            SaxHandler handler = new SaxHandler();
            
            // and parse:
            parser.parse("sample.xml", handler);
            
        catch (Exception ex) {
            ex.printStackTrace(System.out);
        }
    }
    
    /**
     * Our own implementation of SAX handler reading
     * a purchase-order data.
     */
    private static final class SaxHandler extends DefaultHandler {
        
        // handle references occurred in parsed XML:
        public InputSource resolveEntity(String publicId, 
                String systemId) {
            
            // this will resolve network reference to local file:
            if (systemId.equals(
                        "http://www.my-company.com/order-1.0.dtd")) {
                return new InputSource(getClass().getResourceAsStream(
                            "purchase-order.dtd"));
                // return null;
            else {
                // use the default behaviour
                return null;
            }
        }
        
        // we enter to element 'qName':
        public void startElement(String uri, String localName, 
                String qName, Attributes attrsthrows SAXException {
            
            if (qName.equals("purchase-order")) {
            }
        }
        
        // this is called when document is not valid:
        public void error(SAXParseException exthrows SAXException {
            System.out.println("ERROR: [at " 
                    ex.getLineNumber() "] " + ex);
        }
        
        // this is called when document is not well-formed:
        public void fatalError(SAXParseException exthrows SAXException {
            System.out.println("FATAL_ERROR: [at " 
                    ex.getLineNumber() "] " + ex);
        }
        
        public void warning(SAXParseException exthrows SAXException {
            System.out.println("WARNING: [at " 
                    ex.getLineNumber() "] " + ex);
        }
    }
}


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