Java Expression Language (JEXL)

今天药忘吃喽~ 2022-06-16 21:10 247阅读 0赞

[Official Website][]

JEXL is a library intended to facilitate the implementation of dynamic and scripting features in applications and frameworks written in Java.

JEXL implements an Expression Language based on some extensions to the JSTL Expression Language supporting most of the constructs seen in shell-script or ECMAScript.  
Its goal is to expose scripting features usable by technical operatives or consultants working with enterprise platforms.

The library exposes a small footprint API - the [core features][] fit in 3 classes and 10 methods - that can be used in various conditions:

*  Scripting features:
    
     *  Your application lets (advanced) users evaluate or define some simple expressions like computation formulas.
 *  Module or component configuration:
    
     *  Your application has configuration files (eventually generated by a design module) consumed by the end-user module that would benefit from variables and expressions.
     *  When it would be convenient to use IOC but overall complexity doesn't require (or can't depend upon) a full-blown library (Spring, Guice...).
 *  Loose-coupling of interfaces and implementations or duck-typing:
    
     *  You have optional classes that your code cant consider as compilation dependencies.
     *  You have to integrate and call "legacy" code or use components that you dont want to strongly depend upon.
 *  Simple template capabilities:
    
     *  Your application has basic template requirements and JSPs or Velocity would be overkill or too inconvenient to deploy.

JEXL name stands for Java EXpression Language, a simple expression language originally inspired by Apache Velocity and the Expression Language defined in the JavaServer Pages Standard Tag Library version 1.1 (JSTL) and JavaServer Pages version 2.0 (JSP). JEXL 2.0 added features inspired by [Unified EL][]. The syntax is now close to a mix of ECMAScript and "shell-script" making it easy to master by technical operatives or consultants. The objects exposed and their behavior obviously need to be documented though...

The API and the expression language exploit Java-beans naming patterns through introspection to expose property getters and setters. It also considers public class fields as properties and allows to invoke any accessible method.

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.

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.

## A Brief Example ##

When evaluating expressions, JEXL merges an [JexlExpression][] or a [JexlScript][] with a [JexlContext][]. An Expression is created using [JexlEngine\#createExpression()][JexlEngine_createExpression], passing a String containing valid JEXL syntax. A simple JexlContext can be created by instantiating a [MapContext][]; a map of variables that will be internally wrapped can be optionally provided through its constructor. The following example, takes a variable named foo, and invokes the bar() method on the property innerFoo:

// Create or retrieve an engine
        JexlEngine jexl = new JexlBuilder().create();
        
        // Create an expression
        String jexlExp = "foo.innerFoo.bar()";
        JexlExpression e = jexl.createExpression( jexlExp );
        
        // Create a context and add data
        JexlContext jc = new MapContext();
        jc.set("foo", new Foo() );
        
        // Now evaluate the expression, getting the result
        Object o = e.evaluate(jc);

## Extensions to JSTL Expression Language ##

While JEXL is similar to the expression language defined in JSTL, it has improved upon the syntax in a few areas:

*  Support for invocation of any accessible method (see example above).
 *  Support for setting/getting any accessible public field.
 *  A general new() method allowing to instantiate objects.
 *  A general size() method, which works on:
    
     *  String \- returns length
     *  Map \- returns number of keys
     *  List \- returns number of elements.
 *  A general empty() method, which works on Collections and Strings.
 *  Support for the ternary operator 'a ? b : c' - and its GNU-C / "Elvis" variant 'a ?: c'.
 *  Support for the Perl-like regex matching operators '=~' and '!~'
 *  Support for the CSS3-inspired 'startsWith' and 'endsWith' operators '=^' and '=$'
 *  Support for user-defined functions.
 *  Misc : '+' has been overloaded to be use as a String concatenation operator

## Related Resources ##

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:

*  [JSP 2.0][] is covered by Java Specification Requests (JSR) [JSR-152: JavaServer Pages 2.0 Specification][JSR-152_ JavaServer Pages 2.0 Specification].
 *  Apache has an implementation of the expression language for JSP 2.0, called [EL][]
 *  [JSTL 1.1][] is covered by [JSR 52: A Standard Tag Library for JavaServer Pages][JSR 52_ A Standard Tag Library for JavaServer Pages]. See the [JSTL API][].
 *  Apache has a [JSTL Implementation][].

### Velocity ###

[Apache 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.

## Anyone Using It Yet? ##

*  [Commons Configuration][]
 *  [Commons SCXML][]
 *  [Jelly][Commons Jelly]

[Official Website]: http://commons.apache.org/proper/commons-jexl/
[core features]: http://commons.apache.org/proper/commons-jexl/apidocs/org/apache/commons/jexl3/package-summary.html#usage
[Unified EL]: https://en.wikipedia.org/wiki/Unified_Expression_Language
[Commons Jelly]: http://commons.apache.org/jelly
[Commons EL]: http://commons.apache.org/el
[JexlExpression]: http://commons.apache.org/proper/commons-jexl/apidocs/org/apache/commons/jexl3/JexlExpression.html
[JexlScript]: http://commons.apache.org/proper/commons-jexl/apidocs/org/apache/commons/jexl3/JexlScript.html
[JexlContext]: http://commons.apache.org/proper/commons-jexl/apidocs/org/apache/commons/jexl3/JexlContext.html
[JexlEngine_createExpression]: http://commons.apache.org/proper/commons-jexl/apidocs/org/apache/commons/jexl3/JexlEngine.html#createExpressionjava.lang.String
[MapContext]: http://commons.apache.org/proper/commons-jexl/apidocs/org/apache/commons/jexl3/MapContext.html
[JSP 2.0]: http://www.oracle.com/technetwork/java/index-jsp-138231.html
[JSR-152_ JavaServer Pages 2.0 Specification]: http://www.jcp.org/en/jsr/detail?id=152
[EL]: http://commons.apache.org/el/index.html
[JSTL 1.1]: http://www.oracle.com/technetwork/java/index-jsp-135995.html
[JSR 52_ A Standard Tag Library for JavaServer Pages]: http://jcp.org/en/jsr/detail?id=52
[JSTL API]: http://docs.oracle.com/javaee/5/jstl/1.1/docs/tlddocs/
[JSTL Implementation]: http://tomcat.apache.org/taglibs/standard/
[Apache Velocity]: http://velocity.apache.org/
[References]: http://velocity.apache.org/engine/devel/user-guide.html#References
[Commons Configuration]: http://commons.apache.org/configuration
[Commons SCXML]: http://commons.apache.org/scxml