How to Validate XML using Java

Configure Java APIs (SAX, DOM, dom4j, XOM) using JAXP 1.3 to validate XML Documents with DTD and Schema(s).

Many Java XML APIs provide mechanisms to validate XML documents, the JAXP API can be used for most of these XML APIs but subtle configuration differences exists. This article shows five ways of how to configure different Java APIs (including DOM, SAX, dom4j and XOM) using JAXP 1.3 for checking and validating XML with DTD and Schema(s).

Setup (Error Handler, Namespace Aware, Input Document, XML Schema, DTD)
Checking Wellformed-ness (DOM, SAX, dom4j, XOM)
Validate using internal DTD (DOM, SAX, dom4j, XOM)
Validate using internal XSD (DOM, SAX, dom4j, XOM)
Validate using external Schema(s) (DOM, SAX, dom4j, XOM)
Validate using internal DTD and external Schema(s) (DOM, SAX, dom4j, XOM)
Conclusion (Sample Code, Resources)

Setup

All underlying examples can be compiled and executed using Java 5.0 (JAXP 1.3) or higher and make use of the following components and settings.

Error Handler

To report errors, it is necessary to provide an ErrorHandler to the underlying implementation. The ErrorHandler used for the examples is a very simple one which reports the error to System.out and continues until the XML document has been fully parsed or until a fatal-error has been reported.

public class SimpleErrorHandler implements ErrorHandler {

    public void warning(SAXParseException e) throws SAXException {

        System.out.println(e.getMessage());

    }

    public void error(SAXParseException e) throws SAXException {

        System.out.println(e.getMessage());

    }

    public void fatalError(SAXParseException e) throws SAXException {

        System.out.println(e.getMessage());

    }

}

Namespace Aware

Namespaces have been introduced to XML after the first specification of XML had received the official W3C Recommendation status. This is the reason why (most of the) XML parser implementations do not support XML Namespaces by default, to handle the validation of XML documents with namespaces correctly it is therefore necessary to configure the underlying parsers to provide support for XML Namespaces.

Input Document

The input document ("contacts.xml") that has been used for all the code examples is shown below.

<!DOCTYPE contacts SYSTEM "contacts.dtd">

<contacts xsi:noNamespaceSchemaLocation="contacts.xsd"

  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

  <contact title="citizen">

    <firstname>Edwin</firstname>

    <lastname>Dankert</lastname>

  </contact>

</contacts>

XML Schema

The XML Schema ("contacts.xsd") as defined below has been used in the code examples to validate the input document. The input document contains an extra attribute which has not been defined in the XML Schema, this shows that the XML Schema has been used for the validation. When using this XML Schema to validate the input XML document, the following error gets reported:

cvc-complex-type.3.2.2: Attribute 'title' is not allowed to appear in element 'contact'.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="contacts">

    <xs:complexType>

      <xs:sequence>

        <xs:element ref="contact"/>

      </xs:sequence>

    </xs:complexType>

  </xs:element>

  <xs:element name="contact">

    <xs:complexType>

      <xs:sequence>

        <xs:element name="firstname" type="xs:NCName"/>

        <xs:element name="lastname" type="xs:NCName"/>

      </xs:sequence>

    </xs:complexType>

  </xs:element>

</xs:schema>

DTD

The DTD ("contacts.dtd") as defined below has been used in the code examples to validate the input document. To highlight that the DTD has been used for the validation, the title attribute in the input document has a value which is not allowed according to this DTD. When using this DTD to validate the input XML document, the following error gets reported:

Attribute "title" with value "citizen" must have a value from the list "MR MS MRS ".

<!ELEMENT contacts (contact*)>

<!ATTLIST contacts xsi:noNamespaceSchemaLocation CDATA #IMPLIED>

<!ATTLIST contacts xmlns:xsi CDATA #IMPLIED>

<!ELEMENT contact (firstname,lastname)>

<!ATTLIST contact title (MR|MS|MRS) "MS">

<!ELEMENT firstname (#PCDATA)>

<!ELEMENT lastname (#PCDATA)>

Checking Wellformed-ness

Before a document can be called XML and not csv, simple text or any other format, it needs to support the basic rules as defined by the XML Recommendation, when it adheres to these rules it is said to be Wellformed XML.

Code Fragments: DOM, SAX, dom4j, XOM

DOM

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

factory.setValidating(false);

factory.setNamespaceAware(true);

DocumentBuilder builder = factory.newDocumentBuilder();

builder.setErrorHandler(new SimpleErrorHandler());

Document document = builder.parse(new InputSource("document.xml"));

SAX

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(false);

factory.setNamespaceAware(true);

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

reader.parse(new InputSource("document.xml"));

dom4j

SAXReader reader = new SAXReader();

reader.setValidation(false);

reader.setErrorHandler(new SimpleErrorHandler());

reader.read("contacts.xml");

XOM

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(false);

factory.setNamespaceAware(true);

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

Builder builder = new Builder(reader);

builder.build("contacts.xml");

Validate using internal DTD

Parse the input document using only the DTD (contacts.dtd), as defined by the DOCTYPE in the input document, for validation.

Code Fragments: DOM, SAX, dom4j, XOM

DOM

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

DocumentBuilder builder = factory.newDocumentBuilder();

builder.setErrorHandler(new SimpleErrorHandler());

Document document = builder.parse(new InputSource("document.xml"));

SAX

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

reader.parse(new InputSource("document.xml"));

dom4j

SAXReader reader = new SAXReader();

reader.setValidation(true);

reader.setErrorHandler(new SimpleErrorHandler());

reader.read("contacts.xml");

XOM

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

Builder builder = new Builder(reader);

builder.build("contacts.xml");

Validate using internal XSD

Parse the input document using only the XML Schema (contacts.xsd), as defined by the noNamespaceSchemaLocation attribute in the input document, for validation.

Code Fragments: DOM, SAX, dom4j, XOM

DOM

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

factory.setAttribute("http://java.sun.com/xml/jaxp/properties/schemaLanguage",

      "http://www.w3.org/2001/XMLSchema");

DocumentBuilder builder = factory.newDocumentBuilder();

builder.setErrorHandler(new SimpleErrorHandler());

Document document = builder.parse(new InputSource("document.xml"));

SAX

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SAXParser parser = factory.newSAXParser();

parser.setProperty("http://java.sun.com/xml/jaxp/properties/schemaLanguage",

      "http://www.w3.org/2001/XMLSchema");

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

reader.parse(new InputSource("document.xml"));

dom4j

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

SAXParser parser = factory.newSAXParser();

parser.setProperty("http://java.sun.com/xml/jaxp/properties/schemaLanguage",

      "http://www.w3.org/2001/XMLSchema");

SAXReader reader = new SAXReader(parser.getXMLReader());

reader.setValidation(true);

reader.setErrorHandler(new SimpleErrorHandler());

reader.read("contacts.xml");

XOM

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SAXParser parser = factory.newSAXParser();

parser.setProperty("http://java.sun.com/xml/jaxp/properties/schemaLanguage",

      "http://www.w3.org/2001/XMLSchema");

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

Builder builder = new Builder(reader);

builder.build("contacts.xml");

Validate using external Schema

Parse the input document using the schema (contacts.xsd), as defined externally by the source-code.

Code Fragments: DOM, SAX, dom4j, XOM

DOM

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

factory.setValidating(false);

factory.setNamespaceAware(true);

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

DocumentBuilder builder = factory.newDocumentBuilder();

builder.setErrorHandler(new SimpleErrorHandler());

Document document = builder.parse(new InputSource("document.xml"));

SAX

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(false);

factory.setNamespaceAware(true);

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

reader.parse(new InputSource("document.xml"));

dom4j

SAXParserFactory factory = SAXParserFactory.newInstance();

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

SAXParser parser = factory.newSAXParser();

SAXReader reader = new SAXReader(parser.getXMLReader());

reader.setValidation(false);

reader.setErrorHandler(new SimpleErrorHandler());

reader.read("contacts.xml");

XOM

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(false);

factory.setNamespaceAware(true);

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

Builder builder = new Builder(reader);

builder.build("contacts.xml");

Validate using internal DTD and external Schema

Parse the input document using the schema (contacts.xsd), as defined externally by the source-code and the DTD (contacts.dtd), as defined by the DOCTYPE in the input document, for validation.

Code Fragments: DOM, SAX, dom4j, XOM

DOM

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

DocumentBuilder builder = factory.newDocumentBuilder();

builder.setErrorHandler(new SimpleErrorHandler());

Document document = builder.parse(new InputSource("document.xml"));

SAX

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

reader.parse(new InputSource("document.xml"));

dom4j

SAXParserFactory factory = SAXParserFactory.newInstance();

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

SAXParser parser = factory.newSAXParser();

SAXReader reader = new SAXReader(parser.getXMLReader());

reader.setValidation(true);

reader.setErrorHandler(new SimpleErrorHandler());

reader.read("contacts.xml");

XOM

SAXParserFactory factory = SAXParserFactory.newInstance();

factory.setValidating(true);

factory.setNamespaceAware(true);

SchemaFactory schemaFactory =

    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");

factory.setSchema(schemaFactory.newSchema(

    new Source[] {new StreamSource("contacts.xsd")}));

SAXParser parser = factory.newSAXParser();

XMLReader reader = parser.getXMLReader();

reader.setErrorHandler(new SimpleErrorHandler());

Builder builder = new Builder(reader);

builder.build("contacts.xml");

Conclusion

Several mechanisms and XML APIs can be used to parse and validate XML, by using JAXP 1.3 the mechanism can mostly stay the same for these different APIs.

Sample Code

Download any of the archives to get the full source-code for the examples above.

The archives consist of the ./contacts.xml input XML file, ./contacts.xsd the XML Schema document, ./contacts.dtd the DTD document and the source-code for the fragments above, located in the ./src directory.

The archive also contains a number XML Hammer validation projects included in the ./xmlhammer-projects directory. To be able to execute these XML Hammer projects, you will need to have the XML Hammer application installed. This can be downloaded from:

http://www.xmlhammer.org/downloads.html.

Resources

Extensible Markup Language (XML) 1.0

http://www.w3.org/TR/REC-xml/
Namespaces in XML
http://www.w3.org/TR/REC-xml-names/
Java 5.0
http://java.sun.com/j2se/1.5.0/
Java 6.0
http://java.sun.com/javase/6/
JAXP
http://jcp.org/en/jsr/detail?id=206
dom4j
http://www.dom4j.org/
XOM
http://www.xom.nu/
JDOM
http://www.jdom.org/