SAX parser doesn't follow references

Question

I am trying to parse an HL7 message definition from xsd. I have my schema definition split up between two files. First file contains actual message definition and the second contains segment definitions within the message.

I am trying to tweak an example code to parse XML from here https://gist.github.com/helderdarocha/8791651. I don't understand why SAX parser doesn't follow references.

Here are two examples of my xsd definitions.

First file has the following definition

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
           targetNamespace="http://www.xsd_porcessor.org/parser" 
           xmlns="http://www.xsd_porcessor.org/parser" 
           elementFormDefault="qualified"
           attributeFormDefault="unqualified">

           <xs:include schemaLocation="segments.xsd"/>

           <xs:complexType name="ADT.01.MESSAGE">
                <xs:sequence>
                    <xs:element maxOccurs="1" minOccurs="1" ref="MSH"/>
                    <xs:element maxOccurs="1" minOccurs="1" ref="EVN"/>
                    <xs:element maxOccurs="1" minOccurs="1" ref="PID"/>
                    <xs:element maxOccurs="1" minOccurs="1" ref="PV1"/>
                    <xs:element maxOccurs="1" minOccurs="1" ref="IN1"/>
                    <xs:element maxOccurs="1" minOccurs="1" ref="IN2"/>
                </xs:sequence>
           </xs:complexType>
           <xs:element name="ADT.A01" type="ADT.01.MESSAGE"/>
</xs:schema>

The second file has the following header

<?xml version="1.1" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
           targetNamespace="http://www.xsd_porcessor.org/parser" 
           xmlns="http://www.xsd_porcessor.org/parser" 
           elementFormDefault="qualified"
           attributeFormDefault="unqualified">

...and a multitude segment definitions represented as complexTypes. Bellow is example of one

   <xs:complexType name="MSH.SEGMENT">
        <xs:sequence>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.1.FieldSeparator"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.2.ServiceString"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.3.SendingApplication"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.4.SendingFacility"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.5.ReceivingApplication"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.6.ReceivingFacility"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.7.DateTimeOfMessage"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.8.Security"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.9.MessageType"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.10.MessageControlID"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.11.ProcessingID"/>
            <xs:element maxOccurs="1" minOccurs="1" ref="MSH.12.VersionID"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.13.SequenceNumber"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.14.ContinuationPointer"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.15.AcceptAcknowledgmentType"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.16.ApplicationAcknowledgmentType"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.17.CountryCode"/>
            <xs:element maxOccurs="unbounded" minOccurs="0" ref="MSH.18.CharacterSet"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.19.PrincipalLanguageOfMessage"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.20.AlternateCharacterSetHandlingScheme"/>
            <xs:element maxOccurs="unbounded" minOccurs="0" ref="MSH.21.MessageProfileIdentifier"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.22.SendingResponsibleOrganization"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.23.ReceivingResponsibleOrganization"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.24.SendingNetworkAddress"/>
            <xs:element maxOccurs="1" minOccurs="0" ref="MSH.25.ReceivingNetworkAddress"/>
        </xs:sequence>
    </xs:complexType>  
    <xs:element name="MSH" type="MSH.SEGMENT"/>

Here is a tweaked parser itself

package ca.parser.xml;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.parsers.*;

import org.xml.sax.*;

import org.xml.sax.helpers.*;

public class SAXReaderExample {

    public static final String PATH = "resources";

    public static void main(String[] args) throws ParserConfigurationException, SAXException, IOException {
        SAXParserFactory spf = SAXParserFactory.newInstance();
        SAXParser sp = spf.newSAXParser();
        XMLReader reader = sp.getXMLReader();
        reader.setContentHandler(new SchemaSaxHandler());
        reader.parse(new InputSource(new FileInputStream(new File(PATH, "messages.xsd"))));
    }
}


class SchemaSaxHandler extends DefaultHandler {

    // temporary - always null when tag closes
    private String currentSimpleTypeName;
    private String currentSimpleTypeBaseType;
    private SchemaElement currentElement;
    private SchemaComplexType currentComplexType;
    private List<SchemaElement> currentSequence;

    // cumulative - will use the data when XML finishes
    private Map<String, String> simpleTypes = new HashMap<>();
    private Map<String, SchemaComplexType> complexTypes = new HashMap<>();
    private SchemaElement rootElement;

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

        if (qName.equals("xs:simpleType")) {
            currentSimpleTypeName = atts.getValue("name");
        }
        if (qName.equals("xs:restriction")) {
            currentSimpleTypeBaseType = atts.getValue("base");
        }

        if (qName.equals("xs:complexType")) {
            currentComplexType = new SchemaComplexType();

            currentComplexType.setName(atts.getValue("name"));
        }

        if (qName.equals("xs:sequence")) {
            currentSequence = new ArrayList<>();
        }

        if (qName.equals("xs:element")) {
            currentElement = new SchemaElement();
            if (atts.getValue("name")==null) {
                currentElement.setName(atts.getValue("ref"));
            }else {
                currentElement.setName(atts.getValue("name"));
            }
            currentElement.setType(atts.getValue("type"));
            currentElement.setReference(atts.getValue("ref"));
             if (currentSequence != null) {
                currentSequence.add(currentElement);
            } else {
                rootElement = currentElement;
            }
        }

        if (qName.equals("xs:attribute")) {
            currentComplexType.addAttribute(atts.getValue("name"), atts.getValue("type"));
        }
    }

    @Override
    public void endElement(String uri, String localName, String qName) throws SAXException {
        if (qName.equals("xs:simpleType")) {
            simpleTypes.put(currentSimpleTypeName, currentSimpleTypeBaseType);
            currentSimpleTypeName = null;
            currentSimpleTypeBaseType = null;
        }
        if (qName.equals("xs:complexType")) {
            complexTypes.put(currentComplexType.getName(), currentComplexType);
            currentComplexType = null;
        }
        if (qName.equals("xs:sequence")) {
            if (currentComplexType != null) {
                currentComplexType.setChildren(currentSequence);
            }
            currentSequence = null;
        }
    }

    @Override
    public void endDocument() throws SAXException {
        makeTree(rootElement);
        printTree(rootElement, "");
    }

    public void makeTree(SchemaElement element) {
        SchemaComplexType type = complexTypes.get(element.getType());
        if (type != null) {
            List<SchemaElement> children = type.getChildren();
            element.setChildren(children);
            for (SchemaElement child : children) {
                makeTree(child);
            }
            element.setAttributes(type.getAttributes());
        } else {
            element.setType(simpleTypes.get(element.getType()));
        }
    }

    private void printTree(SchemaElement element, String indent) {
        System.out.println(indent + element.getName() + " : " + element.getType());
        Map<String, String> attributes = element.getAttributes();
        if (attributes != null) {
            for (Map.Entry<String, String> entry : attributes.entrySet()) {
                System.out.println("    @" + entry.getKey() + " : " + simpleTypes.get(entry.getValue()));
            }
        }
        List<SchemaElement> children = element.getChildren();
        if (children != null) {
            for (SchemaElement child : children) {
                printTree(child, indent + "    ");
            }
        }
    }
    class SchemaElement {

        private String name;
        private String type;
        private String reference;
        public String getReference() {
            return reference;
        }
        public void setReference(String reference) {
            this.reference = reference;
        }
        private List<SchemaElement> children;
        private Map<String, String> attributes;
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public String getType() {
            return type;
        }
        public void setType(String type) {
            this.type = type;
        }
        public List<SchemaElement> getChildren() {
            return children;
        }
        public void setChildren(List<SchemaElement> children) {
            this.children = children;
        }
        public Map<String, String> getAttributes() {
            return attributes;
        }
        public void setAttributes(Map<String, String> attributes) {
            this.attributes = attributes;
        }


    }

    class SchemaComplexType {        
        private String name;
        private String reference;
        private List<SchemaElement> children;
        private Map<String, String> attributes = new HashMap<>();
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public List<SchemaElement> getChildren() {
            return children;
        }
        public void setChildren(List<SchemaElement> children) {
            this.children = children;
        }
        public Map<String, String> getAttributes() {
            return attributes;
        }
        public void setAttributes(Map<String, String> attributes) {
            this.attributes = attributes;
        }

        public String getReference() {
            return reference;
        }

        public void setReference(String reference) {
            this.reference=reference;
        }

        public void addAttribute(String name,String type) {
            attributes.put(name, type);
        }
    }

Any ideas what is going? You help is appreciated.

Thank you.

how familiar are you with sax parsing? Are you a first time user of it? — vtd-xml-author, Apr 03 '18 at 19:28
I used it a LONG time ago and never with XSD. Why does it matter? — svager, Apr 03 '18 at 19:53

df778899 · Accepted Answer · 2018-04-08T15:32:03.417

It sounds like there are two separate concepts at work here.

If a validating SAX parser is being used to parse a piece of XML, and validate it against its schema:

<xmlRootElement
    xmlns="http://www.xsd_porcessor.org/parser"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.xsd_porcessor.org/parser messages.xsd">

... etc, then clearly when that schema was resolved behind the scenes, the parser would need to follow any references and imports in there.

However if the .xsd itself is the XML being parsed, then as you've already found, it's elements will be directly passed into the ContentHandler. The SchemaSaxHandler above will need to do some more work to learn each xs:element - like you're already doing for the simpleTypes and complexTypes Maps - so they can later be resolved from a ref.

If what you need is the model of the resolved elements and types in the XML schema though, it would be worth exploring that behind the scenes schema model - in an XML Parser such as Xerces. As a starting point, this is using XNI - the Xerces Native Interface:

    File baseDir = new File("/myschemas");

    XMLEntityResolver entityResolver = new XMLEntityResolver() {

        @Override
        public XMLInputSource resolveEntity(
                XMLResourceIdentifier resourceIdentifier)
                throws XNIException, IOException {
            // E.g. resourceIdentifier.getLiteralSystemId() will be segments.xsd
            String uri = new File(baseDir,
                    resourceIdentifier.getLiteralSystemId()).toURI()
                    .toString();
            return new XMLInputSource(null, uri, null);
        }
    };
    XMLSchemaLoader loader = new XMLSchemaLoader();
    loader.setEntityResolver(entityResolver);

    XSModel model = loader
            .loadURI(new File(baseDir, "messages.xsd").toURI()
                    .toString());

    System.out.println(model.getComponents(XSConstants.ELEMENT_DECLARATION));

This outputs such as:

{http://www.xsd_porcessor.org/parser}ADT.A01="http://www.xsd_porcessor.org/parser":ADT.A01

I am using Java 8. I am trying to avoid using extra libraries. Is there a way to accomplish what I need with just parsers provided by Java 8 API? — svager, Apr 09 '18 at 14:56
In fact the built-in XML parser in Java 8 etc is an old fork of Xerces, just that the internal package names are different to avoid colliding with the 'real' Xerces. E.g. `com.sun.org.apache.xerces.internal.impl.xs.XMLSchemaLoader` instead of `org.apache.xerces.impl.xs.XMLSchemaLoader`. The example above works fine unmodified, just with the alternative `import` statements. For reference, I should mention that common guidance is to use Xerces instead of the built-in XML parser, as the JDK fork is reputed to be a bit buggy. I've not personally hit any problems with it though. — df778899, Apr 09 '18 at 18:36

SAX parser doesn't follow references

1 Answers1