History
The expression language started out as part of the JavaServer Pages Standard Tag Library (JSTL) and was originally called SPEL (Simplest Possible Expression Language), then just Expression Language (EL). It was a scripting language which allowed access to Java components (JavaBeans) through JSP. Since JSP 2.0, it has been used inside JSP tags to separate Java code from JSP, and to allow easier access to Java components (than in Java code).
Over the years, the expression language has evolved to include more advanced functionality and it was included in the JSP 2.0 specification. Scripting was made easier for web-content designers who have little or practically no knowledge of the core Java Language. This scripting language made JSP a scripting language in the true sense. Before EL, JSP consisted of some special tags like scriptlets, expressions etc. within which Java code was written explicitly. With EL the web-content designer needs only to know how to make proper calls to core Java methods.
EL was, both syntactically and semantically, similar to JavaScript expressions:
- there is no typecasting
- type conversions are usually done implicitly
- double and single quotes are equivalent
object.propertyhas the same meaning asobject
EL also liberated the programmer from having to know the particularities of how the values are actually accessed: object.property can mean (depending on what the object is) either object.get("property") or object.getProperty("property") or object.getProperty etc.
During the development of JSP 2.0, the JavaServer Faces technology was released which also needed an expression language, but the expression language defined in the JSP 2.0 specification didn't satisfy all the needs for development with JSF technology. The most obvious limitation is that its expressions are evaluated immediately. And also, the JSF components need a way to invoke methods on server-side objects. A more powerful language was created with the following new features:
- Deferred expressions, which are not immediately evaluated
- Expressions that can set as well as get data
- Method expressions, which can invoke methods
The new expression language worked well for the purposes of JSF. But developers had problems when integrating the JSP EL with the JSF EL because of conflicts. Because of these incompatibilities, the unified expression language initiative was started to unify these expression languages. As of JSP 2.1, the expression languages of JSP 2.0 and JSF 1.1 have been merged into a single unified expression language (EL 2.1).
Read more about this topic: Unified Expression Language
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