The Java XML Validation API

 

Validation is a powerful tool. It enables you to quickly check that input is roughly in the form you expect and quickly reject any document that is too far away from what your process can handle. If there's a problem with the data, it's better to find out earlier than later.

In the context of Extensible Markup Language (XML), validation normally involves writing a detailed specification for the document's contents in any of several schema languages such as the World Wide Web Consortium (W3C) XML Schema Language (XSD), RELAX NG, Document Type Definitions (DTDs), and Schematron. Sometimes validation is performed while parsing, sometimes immediately after. However, it's usually done before any further processing of the input takes place. (This description is painted with broad strokes -- there are exceptions.)

Until recently, the exact Application Programming Interface (API) by which programs requested validation varied with the schema language and parser. DTDs and XSD were normally accessed as configuration options in Simple API for XML (SAX), Document Object Model (DOM), and Java™ API for XML Processing (JAXP). RELAX NG required a custom library and API. Schematron might use the Transformations API for XML(TrAX); and still other schema languages required programmers to learn still more APIs, even though they were performing essentially the same operation.

Java 5 introduced the javax.xml.validation package to provide a schema-language-independent interface to validation services. This package is also available in Java 1.3 and later when you install JAXP 1.3 separately. Among other products, an implementation of this library is included with Xerces 2.8.

Validation

The javax.xml.validation API uses three classes to validate documents:SchemaFactory, Schema, and Validator. It also makes extensive use of thejavax.xml.transform.Source interface from TrAX to represent the XML documents. In brief, aSchemaFactory reads the schema document (often an XML file) from which it creates aSchema object. The Schema object creates a Validator object. Finally, theValidator object validates an XML document represented as a Source.

Listing 1 shows a simple program to validate a URL entered on the command line against the DocBook XSD schema.

Listing 1. Validating an Extensible Hypertext Markup Language (XHTML) document

import java.io.*;import javax.xml.transform.Source;import javax.xml.transform.stream.StreamSource;import javax.xml.validation.*;import org.xml.sax.SAXException;public class DocbookXSDCheck {    public static void main(String[] args) throws SAXException, IOException {        // 1. Lookup a factory for the W3C XML Schema language        SchemaFactory factory =             SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");                // 2. Compile the schema.         // Here the schema is loaded from a java.io.File, but you could use         // a java.net.URL or a javax.xml.transform.Source instead.        File schemaLocation = new File("/opt/xml/docbook/xsd/docbook.xsd");        Schema schema = factory.newSchema(schemaLocation);            // 3. Get a validator from the schema.        Validator validator = schema.newValidator();                // 4. Parse the document you want to check.        Source source = new StreamSource(args[0]);                // 5. Check the document        try {            validator.validate(source);            System.out.println(args[0] + " is valid.");        }        catch (SAXException ex) {            System.out.println(args[0] + " is not valid because ");            System.out.println(ex.getMessage());        }              }}

Here's some typical output when checking an invalid document using the version of Xerces bundled with Java 2 Software Development Kit (JDK) 5.0:

file:///Users/elharo/CS905/Course_Notes.xml is not valid because cvc-complex-type.2.3: Element 'legalnotice' cannot have character [children], because the type's content type is element-only.

You can easily change the schema to validate against, the document to validate, and even the schema language. However, in all cases, validation follows these five steps:

1. Load a schema factory for the language the schema is written in.
2. Compile the schema from its source.
3. Create a validator from the compiled schema.
4. Create a Source object for the document you want to validate. A StreamSource is usually simplest.
5. Validate the input source. If the document is invalid, the validate() method throws aSAXException. Otherwise, it returns quietly.

You can reuse the same validator and the same schema multiple times in series. However, only the schema is thread safe. Validators and schema factories are not. If you validate in multiple threads simultaneously, make sure each one has its ownValidator and SchemaFactory objects.

Validate against a document-specified schema

Some documents specify the schema they expect to be validated against, typically usingxsi:noNamespaceSchemaLocation and/or xsi:schemaLocation attributes like this:

<document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:noNamespaceSchemaLocation="http://www.example.com/document.xsd">  ...

If you create a schema without specifying a URL, file, or source, then the Java language creates one that looks in the document being validated to find the schema it should use. For example:

SchemaFactory factory = SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");Schema schema = factory.newSchema();

However, normally this isn't what you want. Usually the document consumer should choose the schema, not the document producer. Furthermore, this approach works only for XSD. All other schema languages require an explicitly specified schema location.

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Abstract factories

SchemaFactory is an abstract factory. The abstract factory design pattern enables this one API to support many different schema languages and object models. A single implementation usually supports only a subset of the numerous languages and models. However, once you learn the API for validating DOM documents against RELAX NG schemas (for instance), you can use the same API to validate JDOM documents against W3C schemas.

For example, Listing 2 shows a program that validates DocBook documents against DocBook's RELAX NG schema. It's almost identical toListing 1. The only things that have changed are the location of the schema and the URL that identifies the schema language.

Listing 2. Validating a DocBook document using RELAX NG

import java.io.*;import javax.xml.transform.Source;import javax.xml.transform.stream.StreamSource;import javax.xml.validation.*;import org.xml.sax.SAXException;public class DocbookRELAXNGCheck {    public static void main(String[] args) throws SAXException, IOException {        // 1. Specify you want a factory for RELAX NG        SchemaFactory factory          = SchemaFactory.newInstance("http://relaxng.org/ns/structure/1.0");                // 2. Load the specific schema you want.         // Here I load it from a java.io.File, but we could also use a         // java.net.URL or a javax.xml.transform.Source        File schemaLocation = new File("/opt/xml/docbook/rng/docbook.rng");                // 3. Compile the schema.        Schema schema = factory.newSchema(schemaLocation);            // 4. Get a validator from the schema.        Validator validator = schema.newValidator();                // 5. Parse the document you want to check.        String input          = "file:///Users/elharo/Projects/workspace/CS905/build/Java_Course_Notes.xml";                // 6. Check the document        try {            validator.validate(source);            System.out.println(input + " is valid.");        }        catch (SAXException ex) {            System.out.println(input + " is not valid because ");            System.out.println(ex.getMessage());        }              }}

If you run this program with the stock Sun JDK and no extra libraries, you'll probably see something like this:

Exception in thread "main" java.lang.IllegalArgumentException: http://relaxng.org/ns/structure/1.0at javax.xml.validation.SchemaFactory.newInstance(SchemaFactory.java:186)at DocbookRELAXNGCheck.main(DocbookRELAXNGCheck.java:14)

This is because, out of the box, the JDK doesn't include a RELAX NG validator. When the schema language isn't recognized,SchemaFactory.newInstance() throws an IllegalArgumentException. However, if you install a RELAX NG library such as Jing and a JAXP 1.3 adapter, then it should produce the same answer the W3C schema does.

Identify the schema language

The javax.xml.constants class defines several constants to identify schema languages:

• XMLConstants.W3C_XML_SCHEMA_NS_URI: http://www.w3.org/2001/XMLSchema
• XMLConstants.RELAXNG_NS_URI: http://relaxng.org/ns/structure/1.0
• XMLConstants.XML_DTD_NS_URI: http://www.w3.org/TR/REC-xml

This isn't a closed list. Implementations are free to add other URLs to this list to identify other schema languages. Typically, the URL is the namespace Uniform Resource Identifier (URI) for the schema language. For example, the URLhttp://www.ascc.net/xml/schematron identifies Schematron schemas.

Sun's JDK 5 only supports XSD schemas. Although DTD validation is supported, it isn't accessible through thejavax.xml.validation API. For DTDs, you have to use the regular SAX XMLReader class. However, you can install additional libraries that add support for these and other schema languages.

How schema factories are located

The Java programming language isn't limited to a single schema factory. When you pass a URI identifying a particular schema language toSchemaFactory.newInstance(), it searches the following locations in this order to find a matching factory:

1. The class named by the "javax.xml.validation.SchemaFactory:schemaURL" system property
2. The class named by the "javax.xml.validation.SchemaFactory:schemaURL" property found in the$java.home/lib/jaxp.properties file
3. javax.xml.validation.SchemaFactory service providers found in the META-INF/services directories of any available Java Archive (JAR) files
4. A platform default SchemaFactory, com.sun.org.apache.xerces.internal.jaxp.validation.xs.SchemaFactoryImpl in JDK 5

To add support for your own custom schema language and corresponding validator, all you have to do is write subclasses ofSchemaFactory, Schema, and Validator that know how to process your schema language. Then, install your JAR in one of these four locations. This is useful for adding constraints that are more easily checked in a Turing-complete language like Java than in a declarative language like the W3C XML Schema language. You can define a mini-schema language, write a quick implementation, and plug it into the validation layer.

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Error handlers

The default response from a schema is to throw a SAXException if there's a problem and do nothing if there isn't. However, you can provide a SAXErrorHandler to receive more detailed information about the document's problems. For example, suppose you want to log all validation errors, but you don't want to stop processing when you encounter one. You can install an error handler such as that in Listing 3.

Listing 3. An error handler that merely logs non-fatal validity errors

import org.xml.sax.ErrorHandler;import org.xml.sax.SAXException;import org.xml.sax.SAXParseException;public class ForgivingErrorHandler implements ErrorHandler {    public void warning(SAXParseException ex) {        System.err.println(ex.getMessage());    }    public void error(SAXParseException ex) {        System.err.println(ex.getMessage());    }    public void fatalError(SAXParseException ex) throws SAXException {        throw ex;    }}

To install this error handler, you create an instance of it and pass that instance to theValidator's setErrorHandler() method:

  ErrorHandler lenient = new ForgivingErrorHandler();  validator.setErrorHandler(lenient);

from  http://www.ibm.com/developerworks/xml/library/x-javaxmlvalidapi/index.html