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
Checks whether the local value of a component is within a certain range. The value must be floating-point or convertible to floating-point.
Checks whether the length of a component's local value is within a certain range. The value must be a java.lang.String.
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.

When you want to register a standard validator on a component, you just nest the standard validator's tag inside the tag representing a component that implements EditableValueHolder and provide the necessary constraints, if the validator tag requires it. Validation can be performed only on components that implement EditableValueHolder because these components accept values that can be validated.

Each standard validator supports a binding attribute, which is used to bind a validator implementation to a backing bean property, similarly to the way binding attributes are used with converter and listener tags, as described previously in this chapter.

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.

This section shows you how to use the LongRangeValidator implementation. The other validators work in a similar way.

See The UIMessage and UIMessages Components for information on how to display validation error messages on the page when validation fails.

Requiring a Value

The name inputText tag on the bookcashier.jsp page has a required attribute, which is set to true. Because of this, the JavaServer Faces implementation checks whether the value of the component is null or is an empty String.

If your component must have a non-null value or a String value at least one character in length, you should add a required attribute to your component tag and set it to true. If your tag does have a required attribute that is set to true and the value is null or a zero-length string, no other validators registered on the tag are called. If your tag does not have a required attribute set to true, other validators registered on the tag are called, but those validators must handle the possibility of a null or zero-length string.

Here is the name inputText tag:

<h:inputText id="name" size="50" 
  value="#{}" required="true"> 

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: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}" />