java - 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)