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Resolving entities found in source XML during parsing |
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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 attrs) throws SAXException {
if (qName.equals("purchase-order")) {
}
}
// this is called when document is not valid:
public void error(SAXParseException ex) throws SAXException {
System.out.println("ERROR: [at " +
ex.getLineNumber() + "] " + ex);
}
// this is called when document is not well-formed:
public void fatalError(SAXParseException ex) throws SAXException {
System.out.println("FATAL_ERROR: [at " +
ex.getLineNumber() + "] " + ex);
}
public void warning(SAXParseException ex) throws SAXException {
System.out.println("WARNING: [at " +
ex.getLineNumber() + "] " + ex);
}
}
}
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