Common Component Tag Attributes

In general, most of the component tags support the following attributes:

• id: Uniquely identifies the component.

• immediate: If set to true, indicates that any events, validation, and conversion associated with the component should happen in the apply request values phase rather than a later phase.

• 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 stylesheet class that contains definitions of the styles.

• value: Identifies an external data source and binds the component’s value to it.

• binding: Identifies a bean property and binds the component instance to it.

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

The id Attribute

The id attribute is not usually required for a component tag. It 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 only takes expressions using 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 ActionSource, such as buttons and hyperlinks) can set the immediate attribute to true to force events, validations, and conversions to be processed during the apply request values phase of the life cycle (a sub phase in the request phase of the JavaServer Faces lifecycle).

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 both the button’s and the field’s immediate attributes 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 and the event, validation, and conversion associated with the field are all handled during the apply request values phase.

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. This is because any events, conversion, or validation associated with the field occurs during its usual phases of the life cycle, which come after the apply request values phase.

For a complete description of JavaServer Faces lifecycle phases, see the JavaServer Faces 2.0 Specification.

The rendered Attribute

A component tag uses a Boolean EL expression, along with the rendered attribute, to determine whether or not 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 Cascading Style Sheets (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 stylesheet 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 stylesheet 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 a Output component or a subclass of Output component class uses value and binding attributes to bind its component’s value or instance respectively to an external data source.