JSP and JSF each has its own expression language. Those languages are sufficiently different to make working with these technologies together hard. A unified expression language recently added to the JSP Standard Tag Library aims to make it easier to use JSP and JSF together.
A java.net article by Krishna Srinivasan describes how the new unified expression language recently added to the JSP Standard Tag Library makes working with JSP and JSF together easier:
The expression language included in JSP provides greater flexibility to the web application developer. But those who are working with JSF have found themselves unsatisfied by the JSP expression language. The reason for the problem is that JSF is used for rendering GUI components, and needs run-time evaluation of expressions. Deferred evaluation means that the technology using the unified EL takes over the responsibility of evaluating the expression from the JSP engine and evaluates the expression at the appropriate time during the page lifecycle. But the JSP EL is designed for immediate evaluation of expressions.
The article outlines several other integration challenges, and then focuses on the solution, a unified expression language aimed to bridge these two technologies:
The new unified EL [expression language] essentially represents a union of the JSP and JSF expression languages and largely benefits JSF technology. The unified EL has the following features:
Deferred evaluation of expressions.
Support for expressions that can set values and expressions that can invoke methods.
Support for using JSTL iteration tags with deferred expressions.
Do you often work with JSPs and JSFs together? Do you see the need for this unified expression language?