Java SAX parser validation

Question

I'm using a SAX parser with a custom handler to parse some XML files. This works well so far, but I want to check more than only the well-formedness of the given file and use validation via an XSD Scheme, which also contains default values for optional attributes. There are lots of tutorials online on doing this, but I was not able to find a way that satisfies all my constraints, which are as follows:

-I don't know the scheme beforehand, I have a bunch of XML and XSD files and every XML contains information about the XSD it should conform to

-The validatior should alter the stream the handler gets and insert the default values for optional attributes from the XSD if necessary

-The current custom handler should be used

I'm fairly new to this topic, so I can't preclude that I've stumbled over the solution without beeing aware of it, but I'm currently completely confused on how to do this.

Here is a minimum SSCCE, which should show the problem and related parts:

package parserTest;

import java.io.File;
import java.io.IOException;

import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.validation.TypeInfoProvider;
import javax.xml.validation.ValidatorHandler;

import org.w3c.dom.ls.LSResourceResolver;
import org.xml.sax.Attributes;
import org.xml.sax.ContentHandler;
import org.xml.sax.ErrorHandler;
import org.xml.sax.Locator;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

public class ParserTest
{
  public final static void main(String[] args)
  {
    //Initialize SAX parser
    final SAXParserFactory saxFactory = SAXParserFactory.newInstance();
    SAXParser saxParser = null;
    try
    {
      saxParser = saxFactory.newSAXParser();
    }
    catch(ParserConfigurationException confEx){confEx.printStackTrace();}
    catch (SAXException saxEx){saxEx.printStackTrace();}

    //Initialize Handler
    DefaultHandler saxHandler = new CustomHandler();

    ValidatorHandler vh = new ValidatorHandler()
    {
      @Override
      public void startPrefixMapping(String prefix, String uri) throws SAXException{}

      @Override
      public void startElement(String uri, String localName, String qName, Attributes atts) throws SAXException{}

      @Override
      public void startDocument() throws SAXException{}

      @Override
      public void skippedEntity(String name) throws SAXException{}

      @Override
      public void setDocumentLocator(Locator locator){}

      @Override
      public void processingInstruction(String target, String data) throws SAXException{}

      @Override
      public void ignorableWhitespace(char[] ch, int start, int length)throws SAXException{}

      @Override
      public void endPrefixMapping(String prefix) throws SAXException{}

      @Override
      public void endElement(String uri, String localName, String qName) throws SAXException{}

      @Override
      public void endDocument() throws SAXException{}

      @Override
      public void characters(char[] ch, int start, int length) throws SAXException{}

      @Override
      public void setResourceResolver(LSResourceResolver resourceResolver){}

      @Override
      public void setErrorHandler(ErrorHandler errorHandler){}

      @Override
      public void setContentHandler(ContentHandler receiver){}

      @Override
      public TypeInfoProvider getTypeInfoProvider(){return null;}

      @Override
      public LSResourceResolver getResourceResolver(){return null;}

      @Override
      public ErrorHandler getErrorHandler(){return null;}

      @Override
      public ContentHandler getContentHandler(){return null;}
    };

    vh.setContentHandler(saxHandler);

    //Do the parsing
    File input = new File("");
    try
    {
      saxParser.parse(input, saxHandler);
      //saxParser.parse(input, vh);       //<-- First attempt, gives me error message
      //saxParser.setContentHandler(vh);  //<-- Second attempt, but my parser does not seem to know this method
    }
    catch (IOException ioEx){ioEx.printStackTrace();}
    catch (SAXException saxEx){saxEx.printStackTrace();}
  }

  /*
   * This class is the handler to be used only by this class.
   */
  static private final class CustomHandler extends DefaultHandler
  {
    //Handle start of element
    public final void startElement(String namespaceURI, String localName, String qName, Attributes atts){}

    //Handle end of Element
    public final void endElement(String namespaceURI, String localName, String qName){}

    //Handle start of characters
    public final void characters(char[] ch, int start, int length){}
  }
}

Answer 1

The basic principle is to insert a ValidatorHandler between the SAX parser and your ContentHandler

https://xerces.apache.org/xerces2-j/javadocs/api/javax/xml/validation/ValidatorHandler.html

ValidatorHandler vh = new ValidatorHandler();
vh.setContentHandler(originalContentHandler);
parser.setContentHandler(vh);

The tricky bit is that in order to create a ValidatorHandler, you need to know what schema is in use. How is it identified? If it uses the xsi:schemaLocation attribute, then you can (probably) get the ValidatorHandler to pick it up automatically. If it uses some custom mechanism, you may have to do a "prepass" reading (some of) the source file to discover the schema, then reading it again with the ValidatorHandler in place.

Your ContentHandler will be notified of default values for optional attributes.