Looking for an expression evaluator

Question

I'm looking for an evaluator for simple condition expressions. Expressions should include variables (read only), strings, numbers and some basic operators.

E.g. expressions something like this:

${a} == "Peter" && ( ${b} == null || ${c} > 10 )

So far i implemented a rather "magical" parser that returns an AST that i can evaluate, but i can't believe that i'm the first one to solve that problem.

What existing code could i use instead?

What language/grammar are you looking to evaluate? Is it something you've made up? What does Java have to do with this? — Matt Ball, Oct 24 '10 at 22:01
We've used JEP on a project (http://www.singularsys.com/jep/) but I'd really like a small, adaptable equivalent that could actually produce bytecode for a function for performance reasons. We abandoned JEP for this reason, and ended up handcoding a huge number of Java classes. Maybe an enterprising soul with ANTLR and BCEL could do this? — andersoj, Oct 24 '10 at 23:12
@Matt Ball: The example is a made up syntax that i'm using right now. But when i find a parser that's meets my demands i can go with whatever syntax it accepts. My main aim syntax-wise is to keep it as minimalistic as possible, because we may have many (think up to 200) of these tiny conditions in a single set. So, readability and performance are an issue. — Stroboskop, Oct 27 '10 at 16:24

andersoj · Answer 1 · 2010-10-24T23:50:46.087

16

Have you looked at MVEL? They provide a getting started guide and performance analysis.

Here's one of their simple examples:

// The compiled expression is serializable and can be cached for re-use.
CompiledExpression compiled = MVEL.compileExpression("x * y"); 

Map vars = new HashMap();
vars.put("x", new Integer(5));
vars.put("y", new Integer(10));

// Executes the compiled expression
Integer result = (Integer) MVEL.executeExpression(compiled, vars); 
assert result.intValue() == 50;

Also (answering my own question) MVEL seems to provide some support for bytecode generation.

Other alternatives, culling from the above answers and my own:

Java Expression Parser (JEP) -- and note there is an old version available for free
Apache Commons JEXL
With regard to Rhino, here's a dude who did some arithmetic evaluation in that context (looks messy)

edited Oct 24 '10 at 23:50

answered Oct 24 '10 at 23:44

andersoj

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MVEL project documentation has been moved from codehaus. The link location is http://mvel.documentnode.com/. github location is - https://github.com/mvel/mvel. – Swapnil Apr 08 '19 at 19:07
But MVEL has a problem with the string like this @{'11412Test'} or @{'Test11412'} it wont be able to parse the above text and throws an error Caused by: [Error: invalid number literal: 11412Test] – moh Jul 26 '23 at 05:16

score 8 · Answer 2 · answered Oct 24 '10 at 23:06

8

Sounds like JEXL might work well for you. Check out its syntax reference.

answered Oct 24 '10 at 23:06

Mike Daniels

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score 7 · Accepted Answer · answered May 27 '13 at 05:08

7

What about SPEL (Spring Expression Lang); http://static.springsource.org/spring/docs/3.0.x/reference/expressions.html

answered May 27 '13 at 05:08

NightWolf

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It's too late now, but that looks pretty much what i was looking for. It is focused on evaluating, but you have a getAST method that returns the expression structure. – Stroboskop Nov 07 '13 at 10:45

score 6 · Answer 4 · answered Oct 24 '10 at 22:02

6

Why don't you use Rhino? It's a JavaScript engine already present inside the JDK.

It can evaluate whatever you like to write in JS.. take a look here

answered Oct 24 '10 at 22:02

Jack

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In fact we're using Rhino for larger custom scripts already. But i don't want to give these small expressions the full power of a scripting language. – Stroboskop Oct 27 '10 at 18:02

score 3 · Answer 5 · answered Oct 24 '10 at 23:00

3

This simple recursive descent parser evaluates constants as named functions having no parameters.

answered Oct 24 '10 at 23:00

trashgod

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When i'll replace magic with science i'll go that way. But right now i'm looking for an existing solution. – Stroboskop Oct 27 '10 at 17:54

score 2 · Answer 6 · answered Jan 09 '15 at 05:10

A very simple and easy to use alternative with a lot of built in excel functions for string, date and number formatting.

The library also allows easy addition of custom functions. A lot of examples available on the git page. A simple example using variables

  ExpressionsEvaluator evalExpr = ExpressionsFactory.create("LEFT(City, 3)");
  Map<String, Object> variables = new HashMap<String, Object>();
  variables.put("City", "New York");
  assertEquals("New", evalExpr.eval(variables));

That looks really promising for an out of the box solution. But i eventually went for Antlr and built my own expression language. — Stroboskop, Jan 13 '16 at 12:01

score 0 · Answer 7 · answered Jul 03 '18 at 20:15

Here is a little library I've worked on that supports expression evaluation (including variables, strings, boolean, etc...).

A little example :

String expression = "EXP(var)";
ExpressionEvaluator evaluator = new ExpressionEvaluator();
evaluator.putVariable(new Variable("var", VariableType.NUMBER, new BigDecimal(20)));

System.out.println("Value of exp(var) : " + evaluator.evaluate(expression).getValue());

Looking for an expression evaluator

7 Answers7

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