[jexl] setFunctions and JexlContext

["Kwok, Grace" <gr...@msci.com>]

Hi,
Could someone explain to me what this mean in actual code?
setFunctions
If the entry value is a class that has one contructor taking a JexlContext as argument, an instance of the namespace will be created at evaluation time. It might be a good idea to derive a JexlContext to carry the information used by the namespace to avoid variable space pollution and strongly type the constructor with this specialized JexlContext.

http://commons.apache.org/jexl/apidocs/index.html

I would like my function to be able to access one of the variable set in the JexlContext.
It sound like I should do something like this.

Public class FunctionClass

{
                Public FunctionClass(JexlContext jexlContext)
                {....}

                Public doSomethingWithVariableA()
                {...}
}

JexlContext jc = new MapContext();
Map<String, Object> funcs = new HashMap<String, Object>();
funcs.put(null, new FunctionClass(jc));
CustomJexlEngine jexl = new JexlEngine();
jexl.setFunctions(funcs);
Jc.put("a", "myPreciousValue");
Expression e11 = jexl.createExpression("doSomethingWithVariableA()");
Object result1 = e11.evaluate(jc);

Then my methods in FunctionClass can access the jexlContext and access variable "a".  However, I don't understand the paragraph about "avoiding variable space pollution".


Thanks, Grace




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Re: [jexl] setFunctions and JexlContext

[henrib <he...@apache.org>]

Hi,
The idea of a namespace functor is to create the object carrying the method
at usage time, optionally using the context to initialize the instance.
The functor constructor can use members/methods of the context; of course,
it is possible to use variables to expose values to that constructor but in
that case, they are accessible to the whole script which is pollution.

Think about a user (http) session or a database connection that you don't
want to expose as a variable.

>From the tests, the following code hides the 'factor' as a member of a
specialized context and use it at functor creation time.
{code}
    public static class EnhancedContext extends MapContext {
        int factor = 6;
    }

    public static class ContextualFunctor {
        private final EnhancedContext context;

        public ContextualFunctor(EnhancedContext theContext) {
            context = theContext;
        }

        public int ratio(int n) {
            context.factor -= 1;
            return n / context.factor;
        }
    }
...
        funcs.put("cx", ContextualFunctor.class);
        JEXL.setFunctions(funcs);
        JexlContext jc = new EnhancedContext();
....
        e = JEXL.createExpression("cx:ratio(10) + cx:ratio(20)");
{code}

Hope this helps,
Cheers
Henrib


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