Custom validation

This section explains how to customize validation process of value in XML document with SASAX.

Validation of value

SASAX provides the mechanism to customize validation like as constraint in XML Schema definition.

In the parsing XML document, ValueElement(classes derived from it, in fact) creates Java object of the corresponded type from text in XML document at the end of element (= invocation of "endElement()" of itself) at first. For example, IntElement creates java.lang.Integer, DateElement creates java.util.Date, and so on.

Then, ValueElement invokes validate() method on "Validation" instance, which is registered by addValidation() method of ValueElement itself. Validation is the interface to represent custom validation strategy, and has only this method declared as shown below.

public Object validate(ParseContext context, Object value)
    throws SAXException;

ValueElement expects Validation to (1) return appropriate object if the specified value satisfies with condition of custom validation, or (2) throw exception otherwise.

ValueElement repeates to invoke validate() on another Validation with value returned by previous one, if there are some registered Validations. So, you can not only represent complex validation by combination of some Validation instances, but also re-use your custom Validation.

Define custom validation

For example, Validation implementation to examine even-ness of specified value for IntElement is shown below.

public Object validate(ParseContext context, 
                       Object value)
    throws SAXException
{
    int i = ((Integer)(value)).intValue();
    if(0 != (i % 2)){
        String message = value.toString();
        throw new SAXNotRecognizedException(message);
    }
    return value;
}

XML Schema does not support this kind (logical) restriction directly, though you may describe it by "[02468]$" as "pattern" constraint, which means that last digit of even-number should be one of 0, 2, 4, 6 or 8.

Parse document

You can parse XML document with 'even-ness restriction' by the code shown in "Parse document" as you know. "ValidationDemo" is implementation of Validation.

String uri = "http://www.lares.dti.ne.jp/~foozy/";

IntElement intElement =
    new IntElement(null, uri, "int");

intElement.addValidation(new ValidationDemo());

ElementDrivenHandler handler =
    new ElementDrivenHandler(intElement);

handler.parse(reader);

This invocation can accept XML documents which:

• satisfy XML Schema defined in the former section, and
• have even-number value

Complete code for this tutorial section is jp.ne.dti.lares.foozy.sasax.ValidationDemo under src/demo/sasax/src in distribution. This class is included in demo.jar of binary distribution.

Detailed information

Class diagram

Class diagram in this section is shown below:

Class diagram (click for large figure)

Classes which you must define are colored, and other are already defined.

Class names

In this tutorial, abbreviated class names are used. Complete names are shown below.