Common Component Tag Attributes

Most of the component tags support the attributes shown in Table 7–2.

Table 7–2 Common Component Tag Attributes

Attribute	Description
binding	Identifies a bean property and binds the component instance to it.
id	Uniquely identifies the component.
immediate	If set to true, indicates that any events, validation, and conversion associated with the component should happen when request parameter values are applied,
rendered	Specifies a condition under which the component should be rendered. If the condition is not satisfied, the component is not rendered.
style	Specifies a Cascading Style Sheet (CSS) style for the tag.
styleClass	Specifies a CSS class that contains definitions of the styles.
value	Identifies an external data source and binds the component’s value to it.

All the tag attributes (except id) can accept expressions, as defined by the EL, described in Chapter 6, Expression Language.

The id Attribute

The id attribute is not usually required for a component tag but is used when another component or a server-side class must refer to the component. If you don’t include an id attribute, the JavaServer Faces implementation automatically generates a component ID. Unlike most other JavaServer Faces tag attributes, the id attribute takes expressions using only the evaluation syntax described in The immediate Attribute, which uses the ${} delimiters. For more information on expression syntax, see Value Expressions.

The immediate Attribute

Input components and command components (those that implement the ActionSource interface, such as buttons and hyperlinks) can set the immediate attribute to true to force events, validations, and conversions to be processed when request parameter values are applied.

You need to carefully consider how the combination of an input component’s immediate value and a command component’s immediate value determines what happens when the command component is activated.

Assume that you have a page with a button and a field for entering the quantity of a book in a shopping cart. If the immediate attributes of both the button and the field are set to true, the new value entered in the field will be available for any processing associated with the event that is generated when the button is clicked. The event associated with the button as well as the event validation and conversion associated with the field are all handled when request parameter values are applied.

If the button’s immediate attribute is set to true but the field’s immediate attribute is set to false, the event associated with the button is processed without updating the field’s local value to the model layer. The reason is that any events, conversion, or validation associated with the field occurs after request parameter values are applied.

The rendered Attribute

A component tag uses a Boolean EL expression along with the rendered attribute to determine whether the component will be rendered. For example, the commandLink component in the following section of a page is not rendered if the cart contains no items:

<h:commandLink id="check"
    ...
    rendered="#{cart.numberOfItems > 0}">
    <h:outputText
        value="#{bundle.CartCheck}"/>
</h:commandLink>

Unlike nearly every other JavaServer Faces tag attribute, the rendered attribute is restricted to using rvalue expressions. As explained in Value and Method Expressions, these rvalue expressions can only read data; they cannot write the data back to the data source. Therefore, expressions used with rendered attributes can use the arithmetic operators and literals that rvalue expressions can use but lvalue expressions cannot use. For example, the expression in the preceding example uses the > operator.

The style and styleClass Attributes

The style and styleClass attributes allow you to specify CSS styles for the rendered output of your tags. Displaying Error Messages with the h:message and h:messages Tags describes an example of using the style attribute to specify styles directly in the attribute. A component tag can instead refer to a CSS class.

The following example shows the use of a dataTable tag that references the style class list-background:

<h:dataTable id="books"
    ...
    styleClass="list-background"
    value="#{bookDBAO.books}"
    var="book">

The style sheet that defines this class is stylesheet.css, which will be included in the application. For more information on defining styles, see Cascading Style Sheets Specification at http://www.w3.org/Style/CSS/.

The value and binding Attributes

A tag representing an output component uses the value and binding attributes to bind its component’s value or instance, respectively, to an external data source.