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> ## Using the Standard Validators

> JavaServer Faces technology provides a set of standard classes and associated tags that page authors and application developers can use to validate a component's data. Table 10-7 lists all the standard validator classes and the tags that allow you to use the validators from the page.

> Table 10-7 The Validator Classes
> Validator Class | Tag | Function
> --- | --- | ---
> `DoubleRangeValidator` | `validateDoubleRange` | Checks whether the local value of a component is within a certain range. The value must be floating-point or convertible to floating-point.
> `LengthValidator` | `validateLength` | Checks whether the length of a component's local value is within a certain range. The value must be a `java.lang.String`.
> `LongRangeValidator` | `validateLongRange` | Checks whether the local value of a component is within a certain range. The value must be any numeric type or `String` that can be converted to a `long`.

> All these validator classes implement the `Validator` interface. Component writers and application developers can also implement this interface to define their own set of constraints for a component's value.

> Similarly to the standard converters, each of these validators has a one or more standard error messages associated with it. If you have registered one of these validators onto a component on your page, and the validator is not able to validate the component's value, the validator's error message will display on the page. For example, the error message that displays when the component's value exceeds the maximum value allowed by LongRangeValidator is the following:

>     {1}: Validation Error: Value is greater than allowable maximum of `'{0}''

> In this case the {1} substitution parameter is replaced by the component's label or ID, and the {0} substitution parameter is replaced with the maximum value allowed by the validator.

> See section 2.5.4 of the JavaServer Faces specification for the complete list of error messages. See Displaying Error Messages with the message and messages Tags for information on how to display validation error messages on the page when validation fails.

> ### Validating a Component's Value

> In order to validate a component's value using a particular validator, you need to register the validator on the component. You have three ways to do this:

> - Nest the validator's corresponding tag (shown in Table 10-7) inside the component's tag. Using the LongRangeValidator describes how to use the `validateLongRange` tag. You can use the other standard tags in the same way.
> - Refer to a method that performs the validation from the component tag's `validator` attribute.
> - Nest a `validator` tag inside the component tag and use either the validator tag's `validatorId` attribute or its `binding` attribute to refer to the validator.

> See Referencing a Method That Performs Validation for more information on using the `validator` attribute.

> The `validatorId` attribute works similarly to the converterId attribute of the `converter` tag, as described in Converting a Component's Value. See Binding Converters, Listeners, and Validators to Backing Bean Properties for more information on using the `binding` attribute of the `validator` tag.

> Keep in mind that validation can be performed only on components that implement `EditableValueHolder` because these components accept values that can be validated.

> ### Using the LongRangeValidator

> The Duke's Bookstore application uses a `validateLongRange` tag on the `quantity` input field of the `bookshowcart.jsp` page:

>     <h:inputText id="quantity"   size="4"
>       value="#{item.quantity}" >
>       <f:validateLongRange minimum="1"/>
>     </h:inputText>
>     <h:message for="quantity"/>

> This tag requires that the user enter a number that is at least 1. The `size` attribute specifies that the number can have no more than four digits. The `validateLongRange` tag also has a `maximum` attribute, with which you can set a maximum value of the input.

> The attributes of all the standard validator tags accept value expressions. This means that the attributes can reference backing bean properties rather than specify literal values. For example, the `validateLongRange` tag in the preceding example can reference a backing bean property called `minimum` to get the minimum value acceptable to the validator implementation:

>     <f:validateLongRange minimum="#{ShowCartBean.minimum}" />