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Java dynamic proxy example code: SAX XML handler 

alfa
Joined:
03/07/2011
Posts:
57

June 03, 2011 09:41:03    Last update: June 03, 2011 09:41:03
Dynamic proxy can be used to eliminate the need to stub out unused interface methods. This is an example for a simple SAX content handler for XML parsing.
  1. The org.xml.sax.ContentHandler interface requires 11 methods be implemented but we only need three: 
    import java.io.IOException;
import java.util.Arrays;
import org.xml.sax.helpers.XMLReaderFactory;
import org.xml.sax.XMLReader;
import org.xml.sax.ContentHandler;
import org.xml.sax.Attributes;
import org.xml.sax.Locator;
import org.xml.sax.SAXException;

public class SaxParserDemo {
    public static void main(String[] args) throws SAXException, IOException {
	XMLReader p = XMLReaderFactory.createXMLReader();
	p.setContentHandler(new DemoContentHandler());
	p.parse("demo.xml");
    }
}

class DemoContentHandler implements ContentHandler {
    public void startElement(String namespaceURI, 
			     String localName, 
			     String qName, 
			     Attributes atts) 
		throws SAXException {
	System.out.printf("<%s>", qName);
    }
  
    public void characters(char[] text, 
			   int start, 
			   int length) 
		throws SAXException {
	System.out.print(Arrays.copyOfRange(text, start, start + length));
    }
  
    public void endElement(String namespaceURI, 
			   String localName, 
			   String qName) 
		throws SAXException {
	System.out.printf("</%s>", qName);
    }

    // do nothing methods  
    public void setDocumentLocator(Locator locator) {}
    public void startDocument() throws SAXException {}
    public void endDocument() throws SAXException {}
    public void startPrefixMapping(String prefix, String uri) throws SAXException {}
    public void endPrefixMapping(String prefix) throws SAXException {}
    public void skippedEntity(String name) throws SAXException {}  
    public void ignorableWhitespace(char[] text, int start, int length) throws SAXException {}
    public void processingInstruction(String target, String data) throws SAXException {}
}

  2. With a dynamic proxy, we don't need the empty blocks for the unused methods: 
    import java.io.IOException;
import java.util.Arrays;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.InvocationHandler;
import org.xml.sax.helpers.XMLReaderFactory;
import org.xml.sax.XMLReader;
import org.xml.sax.ContentHandler;
import org.xml.sax.Attributes;
import org.xml.sax.Locator;
import org.xml.sax.SAXException;

public class SaxParserDemo2 {
    public static void main(String[] args) throws SAXException, IOException {
	XMLReader p = XMLReaderFactory.createXMLReader();
	p.setContentHandler((ContentHandler) Proxy.newProxyInstance(
	    SaxParserDemo2.class.getClassLoader(),
	    new Class[] { ContentHandler.class },
	    new SaxDemoInvocationHandler()
	));
	p.parse("demo.xml");
    }
}

class SaxDemoInvocationHandler implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) {
	String methodName = method.getName();
	if (methodName.equals("startElement")) {
	    System.out.printf("<%s>", args[2]);
	}
	else if (methodName.equals("characters")) {
	    char[] text = (char[]) args[0];
	    int start = ((Integer) args[1]).intValue();
	    int length = ((Integer) args[2]).intValue();
	    System.out.print(Arrays.copyOfRange(text, start, start + length));
	}
	else if (methodName.equals("endElement")) {
	    System.out.printf("</%s>", args[2]);
	}
	return null;
    }
}

  3. Equivalently (with anonymous inner class): 
    import java.io.IOException;
import java.util.Arrays;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.InvocationHandler;
import org.xml.sax.helpers.XMLReaderFactory;
import org.xml.sax.XMLReader;
import org.xml.sax.ContentHandler;
import org.xml.sax.Attributes;
import org.xml.sax.Locator;
import org.xml.sax.SAXException;

public class SaxParserDemo2 {
    public static void main(String[] args) throws SAXException, IOException {
	XMLReader p = XMLReaderFactory.createXMLReader();
	p.setContentHandler((ContentHandler) Proxy.newProxyInstance(
	    SaxParserDemo2.class.getClassLoader(),
	    new Class[] { ContentHandler.class },
	    new InvocationHandler() {
		@Override
		public Object invoke(Object proxy, Method method, Object[] args) {
		    String methodName = method.getName();
		    if (methodName.equals("startElement")) {
			System.out.printf("<%s>", args[2]);
		    }
		    else if (methodName.equals("characters")) {
			char[] text = (char[]) args[0];
			int start = ((Integer) args[1]).intValue();
			int length = ((Integer) args[2]).intValue();
			System.out.print(Arrays.copyOfRange(text, start, start + length));
		    }
		    else if (methodName.equals("endElement")) {
			System.out.printf("</%s>", args[2]);
		    }
		    return null;
		}
	    }
	));
	p.parse("demo.xml");
    }
}

  4. demo.xml: 
    <breakfast-menu>
    <food>
	<name>Belgian Waffles</name>
	<price>$5.95</price>
	<description>
	    two of our famous Belgian Waffles with plenty of real maple syrup
	</description>
	<calories>650</calories>
    </food>

    <food>
	<name>Strawberry Belgian Waffles</name>
	<price>$7.95</price>
	<description>
	light Belgian waffles covered with strawberries and whipped cream
	</description>
	<calories>900</calories>
    </food>

    <food>
	<name>Berry-Berry Belgian Waffles</name>
	<price>$8.95</price>
	<description>
	light Belgian waffles covered with an assortment of fresh berries and whipped cream
	</description>
	<calories>900</calories>
    </food>
</breakfast-menu>

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