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> ## Registering a Custom Validator

> If the application developer provides an implementation of the `Validator` interface to perform the validation, you must register this custom validator in the application configuration resource file by using the `validator` XML element:

>     <validator>
>       ...
>       <validator-id>FormatValidator</validator-id>
>       <validator-class>
>         com.sun.bookstore6.validators.FormatValidator
>       </validator-class>
>       <attribute>
>         ...
>         <attribute-name>formatPatterns</attribute-name>
>         <attribute-class>java.lang.String</attribute-class>
>       </attribute>
>     </validator>

> The `validator-id` and `validator-class` elements are required subelements. The `validator-id` element represents the identifier under which the `Validator` class should be registered. This ID is used by the tag class corresponding to the custom `validator` tag.

> The `validator-class` element represents the fully qualified class name of the `Validator` class.

> The `attribute` element identifies an attribute associated with the `Validator` implementation. It has required `attribute-name` and `attribute-class` subelements. The `attribute-name` element refers to the name of the attribute as it appears in the `validator` tag. The `attribute-class` element identifies the Java type of the value associated with the attribute.

> Creating a Custom Validator (page 401) explains how to implement the `Validator` interface.

> Using a Custom Validator (page 375) explains how to reference the validator from the page.