When the JSP Standard Tag Library (JSTL) was announced, it included an Expression Language for the definition of expressions in conditional tags, in iteration and for other uses. It struck me that it would be useful to have a generalized expression evaluation system, for use in a variety of circumstances going beyond its initial implementation in JSP.

This seemed like an interesting programming challenge, and I quickly produced an initial version of the Expression Language system, written in Java. That was back in 2002, and the project gone through a series of changes since then.

Of course, Sun has more recently made the Expression Language available as a separate subsystem — the javax.el package — and this takes away some of the need for a separate package such as mine. But some people may still find value in my version. I find it easier to use than the Sun package — but then, I wrote it, so I'm thoroughly familiar with it. It may perform better (I haven't run any tests, so this is speculation). And there's always the advantage (to some users) of having a fully-documented open source version that may be modified at will.

The original version of this system implemented the Expression Language specification in “JavaServer Pages™ Standard Tag Library” published by Sun Microsystems, Inc. It was written in Java, using Java SE 1.4.2. Subsequently, at the request of users of the system, I added further functionality to extend its usefulness, but in order to maintain strict compatibility with the basic specification, I made these additional facilities available only to those who explicitly enable them (see the Javadoc for the Expression and Expression.Parser classes for details).

More recently a version of the Expression Language system in JavaScript has been produced. This may seem superfluous since JavaScript allows any expression to be evaluated at run-time, but the use of a separate expression handler allows more control over the objects that may be accessed, and prevents the code injection security exploits that are a significant danger with arbitrary JavaScript expression evaluation.

All versions of the Expression Language translator/evaluator follow a two-pass approach, first compiling the text representation of the expression into a tree structure, and then executing that compiled form.

One common use of the system in its early days was to provide Expression Language processing on JSP custom tags written to coexist with JSTL tags. Prior to JSP 2.0, the JSP system would not recognize EL constructs in custom tag attributes, so this was the responsibility of the tag writer. The EL, with the class JspResolver, provides a simple way for the custom tag writer to provide full expression evaluation and substitution in tag attributes. (This is of less significance since application servers compatible with JSP 2.0 are now widely available, but some corporate users are constrained for various reasons to using earlier versions so this aspect of the Expression Language may still be of some benefit.)

For those people who wish to use the library but not modify it, a JAR file containing the classes is available for download. A ZIP file of the Javadoc for the library is also available. And for those who wish to modify the source or just to peruse it, a ZIP file of the entire project (source, Ant build file etc.) is available.

The JavaScript version is available as a separate download.

File Name	File Size	File Type
EL.jar	90,459	JAR file
ELDoc.zip	328,726	ZIP of Javadoc
ELProject.zip	42,349	ZIP of source etc.
ELJS.zip	16,584	ZIP of JavaScript

Do not deep-link to the download files — the URLs include a numeric IP address which may change from time to time.

To use this library in a JSP context, the JAR file servlet.jar must be obtained from Sun, or from a Java EE application server vendor. Another good source is the Tomcat Project. To build the project, a copy of this JAR file must be placed in the “lib” directory of the project.

Javadoc for the project may be perused online:

• EL Javadoc

Please use this only for occasional reference; for repeated use please download the ZIP of the Javadoc (see above). Do not deep-link to the online Javadoc — the URL includes a numeric IP address which may change from time to time.

This software is copyright © 2002 – 2008 Peter Wall. It is licensed under the GNU Lesser General Public License.