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?

Answer 1

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)

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

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));