Java Expression Language (JEXL) is an expression language engine which can be embedded in applications and frameworks. JEXL is inspired by Jakarta Velocity and the Expression Language defined in the JavaServer Pages Standard Tag Library version 1.1 (JSTL) and JavaServer Pages version 2.0 (JSP). While inspired by JSTL EL, it must be noted that JEXL is not a compatible implementation of EL as defined in JSTL 1.1 (JSR-052) or JSP 2.0 (JSR-152). For a compatible implementation of these specifications, see the Commons EL project.
JEXL attempts to bring some of the lessons learned by the Velocity community about expression languages in templating to a wider audience. Commons Jelly needed Velocity-ish method access, it just had to have it.
When evaluating expressions, JEXL merges an Expression with a JexlContext. An Expression is created using ExpressionFactory.createExpression(), passing a String containing valid JEXL syntax. A JexlContext is created using JexlHelper.createContext(), and variables are put into a map exposed through the getVars() method on JexlContext. The following example, takes a variable named foo, and invokes the bar() method on the property innerFoo:
// Create an expression object String jexlExp = "foo.innerFoo.bar()"; Expression e = ExpressionFactory.createExpression( jexlExp ); // Create a context and add data JexlContext jc = JexlHelper.createContext(); jc.getVars().put("foo", new Foo() ); // Now evaluate the expression, getting the result Object o = e.evaluate(jc);
While JEXL is similar to the expression language defined in JSTL, it has improved upon the syntax in a few areas:
JEXL is not a product of the Java Community Process (JCP), but it provides a similar expression syntax. For more information about JSP 2.0 EL and JSTL 1.1 EL:
Jakarta Velocity implements a similar expression language.
In particular the References section of the User Guide has some good information on properties and method which correlate directly to JEXL.