Java Expression Language (EL) is a compact and powerful mechanism that enables dynamic communication in JSP and JSF-based applications (including development frameworks based on JSF such as PrimeFaces, ICEfaces, and RichFaces); we embed expressions in the presentation layer to communicate with the application logic layer. EL provides bidirectional communication, which means that we can expose application logic data to the user, but we also can submit user data to be processes. Generically speaking, EL can be used to populate HTTP requests with user data, to extract and expose data from HTTP responses, to update HTML DOM, to conditionally process data, and much more.
Note
Commonly, EL expressions will be present in JSP and JSF pages, but they can also appear outside, in faces-config.xml, for example.
In this chapter, you will see how to use EL in web pages to communicate with managed beans, which is the most common case in JSF applications. We will cover the following topics:
EL syntax, operators, and reserved words
EL immediate and deferred evaluation
EL value and method expressions
The conditional text in JSF
Write a custom EL resolver