Immediate and Deferred Evaluation Syntax

The unified EL supports both immediate and deferred evaluation of expressions. Immediate evaluation means that the JSP engine evaluates the expression and returns the result immediately when the page is first rendered. Deferred evaluation means that the technology using the expression language can employ its own machinery to evaluate the expression sometime later during the page’s life cycle, whenever it is appropriate to do so.

Those expressions that are evaluated immediately use the ${} syntax, which was introduced with the JSP 2.0 expression language. Expressions whose evaluation is deferred use the #{} syntax, which was introduced by JavaServer Faces technology.

Because of its multiphase life cycle, JavaServer Faces technology uses deferred evaluation expressions. During the life cycle, component events are handled, data is validated, and other tasks are performed, all done in a particular order. Therefore, it must defer evaluation of expressions until the appropriate point in the life cycle.

Other technologies using the unified EL might have different reasons for using deferred expressions.

Immediate Evaluation

All expressions using the ${} syntax are evaluated immediately. These expressions can only be used within template text or as the value of a JSP tag attribute that can accept runtime expressions.

The following example shows a tag whose value attribute references an immediate evaluation expression that gets the total price from the session-scoped bean named cart:

<fmt:formatNumber value="${sessionScope.cart.total}"/>

The JSP engine evaluates the expression, ${sessionScope.cart.total}, converts it, and passes the returned value to the tag handler.

Immediate evaluation expressions are always read-only value expressions. The expression shown above can only get the total price from the cart bean; it cannot set the total price.

Deferred Evaluation

Deferred evaluation expressions take the form #{expr} and can be evaluated at other phases of a page life cycle as defined by whatever technology is using the expression. In the case of JavaServer Faces technology, its controller can evaluate the expression at different phases of the life cycle depending on how the expression is being used in the page.

The following example shows a JavaServer Faces inputText tag, which represents a text field component into which a user enters a value. The inputText tag’s value attribute references a deferred evaluation expression that points to the name property of the customer bean.

<h:inputText id="name" value="#{customer.name}" />

For an initial request of the page containing this tag, the JavaServer Faces implementation evaluates the #{customer.name} expression during the render response phase of the life cycle. During this phase, the expression merely accesses the value of name from the customer bean, as is done in immediate evaluation.

For a postback, the JavaServer Faces implementation evaluates the expression at different phases of the life cycle, during which the value is retrieved from the request, validated, and propagated to the customer bean.

As shown in this example, deferred evaluation expressions can be value expressions that can be used to both read and write data. They can also be method expressions. Value expressions (both immediate and deferred) and method expressions are explained in the next section.