The Java EE 5 Tutorial

The rendered Attribute

A component tag uses a Boolean JavaServer Faces expression language (EL) expression, along with the rendered attribute, to determine whether or not the component will be rendered. For example, the check commandLink component on the bookcatalog.jsp page is not rendered if the cart contains no items:

<h:commandLink id="check"
    rendered="#{cart.numberOfItems > 0}">

Unlike nearly every other JavaServer Faces tag attribute, the rendered attribute is restricted to using rvalue expressions. As explained in Unified Expression Language, 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.