15.11.1 Implementing the Validator Interface

A Validator implementation must contain a constructor, a set of accessor methods for any attributes on the tag, and a validate method, which overrides the validate method of the Validator interface.

The hypothetical FormatValidator class also defines accessor methods for setting the formatPatterns attribute, which specifies the acceptable format patterns for input into the fields. The setter method calls the parseFormatPatterns method, which separates the components of the pattern string into a string array, formatPatternsList.

public String getFormatPatterns() {
    return (this.formatPatterns);
}
public void setFormatPatterns(String formatPatterns) {
    this.formatPatterns = formatPatterns;
    parseFormatPatterns();
}

In addition to defining accessor methods for the attributes, the class overrides the validate method of the Validator interface. This method validates the input and also accesses the custom error messages to be displayed when the String is invalid.

The validate method performs the actual validation of the data. It takes the FacesContext instance, the component whose data needs to be validated, and the value that needs to be validated. A validator can validate only data of a component that implements javax.faces.component.EditableValueHolder.

Here is an implementation of the validate method:

@FacesValidator
public class FormatValidator implements Validator, StateHolder {
    ...
    public void validate(FacesContext context, UIComponent component, 
                         Object toValidate) {

        boolean valid = false;
        String value = null;
        if ((context == null) || (component == null)) {
            throw new NullPointerException();
        }
        if (!(component instanceof UIInput)) {
            return;
        }
        if ( null == formatPatternsList || null == toValidate) {
            return;
        }
        value = toValidate.toString();
        // validate the value against the list of valid patterns.
        Iterator patternIt = formatPatternsList.iterator();
        while (patternIt.hasNext()) {
            valid = isFormatValid(
                ((String)patternIt.next()), value);
            if (valid) {
                break;
            }
        }
        if ( !valid ) {
            FacesMessage errMsg =
                new FacesMessage(FORMAT_INVALID_MESSAGE_ID);
            FacesContext.getCurrentInstance().addMessage(null, errMsg);
            throw new ValidatorException(errMsg);
        }
    }
}

The @FacesValidator annotation registers the FormatValidator class as a validator with the JavaServer Faces implementation. The validate method gets the local value of the component and converts it to a String. It then iterates over the formatPatternsList list, which is the list of acceptable patterns that was parsed from the formatPatterns attribute of the custom validator tag.

While iterating over the list, this method checks the pattern of the component's local value against the patterns in the list. If the pattern of the local value does not match any pattern in the list, this method generates an error message. It then creates a javax.faces.application.FacesMessage and queues it on the FacesContext for display, using a String that represents the key in the Properties file:

public static final String FORMAT_INVALID_MESSAGE_ID =
     "FormatInvalid";
}

Finally, the method passes the message to the constructor of javax.faces.validator.ValidatorException.

When the error message is displayed, the format pattern will be substituted for the {0} in the error message, which, in English, is as follows:

Input must match one of the following patterns: {0}

You may wish to save and restore state for your validator, although state saving is not usually necessary. To do so, you will need to implement the StateHolder interface as well as the Validator interface. To implement StateHolder, you would need to implement its four methods: saveState(FacesContext), restoreState(FacesContext, Object), isTransient, and setTransient(boolean). See Saving and Restoring State for more information.

15.11.2 Specifying a Custom Tag

If you implemented a Validator interface rather than implementing a managed bean method that performs the validation, you need to do one of the following.

• Allow the page author to specify the Validator implementation to use with the f:validator tag. In this case, the Validator implementation must define its own properties. Using a Custom Validator explains how to use the f:validator tag.

• Specify a custom tag that provides attributes for configuring the properties of the validator from the page.

To create a custom tag, you need to add the tag to the tag library descriptor for the application, bookstore.taglib.xml:

<tag>
    <tag-name>validator</tag-name>
    <validator>
        <validator-id>formatValidator</validator-id>
        <validator-class>
            dukesbookstore.validators.FormatValidator
        </validator-class>
    </validator>
</tag>

The tag-name element defines the name of the tag as it must be used in a Facelets page. The validator-id element identifies the custom validator. The validator-class element wires the custom tag to its implementation class.

Using a Custom Validator explains how to use the custom validator tag on the page.

15.11.3 Using a Custom Validator

To register a custom validator on a component, you must do one of the following.

• Nest the validator's custom tag inside the tag of the component whose value you want to be validated.

• Nest the standard f:validator tag within the tag of the component and reference the custom Validator implementation from the f:validator tag.

Here is a hypothetical custom formatValidator tag for the Credit Card Number field, nested within the h:inputText tag:

<h:inputText id="ccno" size="19"
  ...
  required="true">
  <mystore:formatValidator
  formatPatterns="9999999999999999|9999 9999 9999 9999|9999-9999-9999-9999"/>
</h:inputText>
<h:message styleClass="validationMessage" for="ccno"/>

This tag validates the input of the ccno field against the patterns defined by the page author in the formatPatterns attribute.

You can use the same custom validator for any similar component by simply nesting the custom validator tag within the component tag.

If the application developer who created the custom validator prefers to configure the attributes in the Validator implementation rather than allow the page author to configure the attributes from the page, the developer will not create a custom tag for use with the validator.

In this case, the page author must nest the f:validator tag inside the tag of the component whose data needs to be validated. Then the page author needs to do one of the following.

• Set the f:validator tag's validatorId attribute to the ID of the validator that is defined in the application configuration resource file.

• Bind the custom Validator implementation to a managed bean property using the f:validator tag's binding attribute, as described in Binding Converters, Listeners, and Validators to Managed Bean Properties.

The following tag registers a hypothetical validator on a component using an f:validator tag and references the ID of the validator:

<h:inputText id="name" value="#{CustomerBean.name}"
            size="10" ...>
    <f:validator validatorId="customValidator" />
    ...
</h:inputText>