creating context for MVEL with different variable types

Question

I am using MVEL to evaluate arithmetic and logical expressions or a combination of the two. The thing is that I do not know beforehand all the variable types, without creating a pretty complex parsing method for the expression itself (which is passed via a settings file). I know their type only when I go through my data and update the context. For instance, consider the expression (a && b) && (c == 10) && (d < 5) I separate the variables from the operators and initialize my context, but I do not know which are booleans and which are integers. I have tried to initialize all variables in the context with null or new Object() but it does not work as expected. See example code below:

import java.util.HashMap;
import java.util.Map;

import org.mvel2.MVEL;

public class Test {

    private static Map<String, Object> context = new HashMap<String, Object>();

    public static void main(String[] args){

        String expression = "(a && b) && (c == 10) && (d < 5)";

        context.put("a", new Object());
        context.put("b", new Object());
        context.put("c", new Object());
        context.put("d", new Object());

        //do some processing and get the right value, replacing it in the context with the right type

        evaluate(expression,context); //just to try to evaluate with nothing set, crashes

        context.put("a", new Boolean(true));
        evaluate(expression,context);//crashes same as above

        context.put("b", new Boolean(true));
        evaluate(expression,context); //works. the expression is not yet true but it does not crash

        context.put("c", new Integer(10));
        context.put("d", new Integer(1));
        evaluate(expression,context); //works. expression is true

}

    private static void evaluate(String expression, Map<String,Object> context){
        if((Boolean)MVEL.eval(expression, context))
            System.out.println("Hooray");
        else
            System.out.println("Boo!"); 
    }

}

when it crashes, I get this message: Exception in thread "main" org.mvel2.ScriptRuntimeException: expected Boolean; but found: java.lang.Object Exception in thread "main" org.mvel2.ScriptRuntimeException: expected Boolean; but found: java.lang.Object If I initialize with null it would crash saying ..but found: null My guess is that it figures out in the MVEL.eval() method it should receive booleans as the first variables, but I do not find the behavior consistent. A second example makes me even more confused. See example code below:

import java.util.HashMap;
import java.util.Map;

import org.mvel2.MVEL;

public class Test {

    private static Map<String, Object> context = new HashMap<String, Object>();

    public static void main(String[] args){

        context.put("a", null);
        context.put("b", null);
        context.put("c", null);

        String expression = "( a > b + 2 ) && ( c < a - 5 )";

        evaluate(expression,context); //this time it does not crash. it evaluates correctly as false

         //do some processing and get the right value, and replace it in the context

        context.put("a",new Integer(25));
        evaluate(expression,context); //crashes. error below
        context.put("b", new Integer(20));
        context.put("c", new Integer(10));

        evaluate(expression,context); //evaluates correctly to true.
}

    private static void evaluate(String expression, Map<String,Object> context){
        if((Boolean)MVEL.eval(expression, context))
            System.out.println("Hooray");
        else
            System.out.println("Boo!"); 
    }

}

The error message for the crash in this second example is: Exception in thread "main" [Error: failed to subEval expression] [Near : {... ( a > b + 2 ) && ( c < a - 5 ) ....}] ^ Isn't there a default initialization of my context variables possible? I can initialize them with new Boolean(false) but that can influence the expression. Do I have to use some Strict Typing or Strong Typing? BTW, I do not find any decent documentation of the classes.. any suggestions are appreciated. Thank you.

Answer 1

First of all, you can Ask MVEL to get you the input variables automatically. Unfortunately, MVEL does not tell you what it thinks the object type is. So, for instance:

Public class MvelVarTest {

public static void main(String[] args) {
    String expression = "( a > b + 2 ) && ( c < a - 5 )";
    ParserContext context = new ParserContext();
    Serializable compiledExpression = MVEL.compileExpression(expression, context);

    //Now the context will have a list of all the inputs.  Unfortunatly it does not tell you what type of Object the input is.
    for (Map.Entry<String,Class> entry : context.getInputs().entrySet()) {
        System.out.println("Variable name : "+entry.getKey()+", Data Type = "+entry.getValue().toString());
    }

    //Now, you can assign values to the data and run the expression.
    Map values = new HashMap();
    values.put("a",25);
    values.put("b",20);
    values.put("c",10);

    //And we can get a boolean answer
    System.out.println("Result of running formula with (a=25, b=20, c=10) = "+MVEL.executeExpression(compiledExpression,values,Boolean.class));
}

}

However, when you set up the HashMap with your values, you could of just put string values in, and MVEL would of "auto converted" it.