Groovy DSL: creating dynamic closures from Strings

Question

There are some other questions on here that are similar but sufficiently different that I need to pose this as a fresh question:

I have created an empty class, lets call it Test. It doesn't have any properties or methods. I then iterate through a map of key/value pairs, dynamically creating properties named for the key and containing the value... like so:

def langMap = [:]
langMap.put("Zero",0)
langMap.put("One",1)
langMap.put("Two",2)
langMap.put("Three",3)
langMap.put("Four",4)
langMap.put("Five",5)
langMap.put("Six",6)
langMap.put("Seven",7)
langMap.put("Eight",8)
langMap.put("Nine",9)

langMap.each { key,val ->
    Test.metaClass."${key}" = val
}

Now I can access these from a new method created like this:

Test.metaClass.twoPlusThree = { return Two + Three }
println test.twoPlusThree()

What I would like to do though, is dynamically load a set of instructions from a String, like "Two + Three", create a method on the fly to evaluate the result, and then iteratively repeat this process for however many strings containing expressions that I happen to have.

Questions: a) First off, is there simply a better and more elegant way to do this (Based on the info I have given) ? b) Assuming this path is viable, what is the syntax to dynamically construct this closure from a string, where the string references variable names valid only within a method on this class?

Thanks!

Answer 1

I think the correct answer depends on what you're actually trying to do. Can the input string be a more complicated expression, like '(Two + Six) / Four'?

If you want to allow more complex expressions, you may want to directly evaluate the string as a Groovy expression. Inside the GroovyConsole or a Groovy script, you can directly call evaluate, which will evaluate an expression in the context of that script:

def numNames = 'Zero One Two Three Four Five Six Seven Eight Nine'.split()

// Add each numer name as a property to the script.
numNames.eachWithIndex { name, i ->
    this[name] = i
}

println evaluate('(Two + Six) / Four') // -> 2

If you are not in one of those script-friendly worlds, you can use the GroovyShell class:

def numNames = 'Zero One Two Three Four Five Six Seven Eight Nine'.split()
def langMap = [:]
numNames.eachWithIndex { name, i -> langMap[name] = i }

def shell = new GroovyShell(langMap as Binding)
println shell.evaluate('(Two + Six) / Four') // -> 2

But, be aware that using eval is very risky. If the input string is user-generated, i would not recommend you going this way; the user could input something like "rm -rf /".execute(), and, depending on the privileges of the script, erase everything from wherever that script is executed. You may first validate that the input string is "safe" (maybe checking it only contains known operators, whitespaces, parentheses and number names) but i don't know if that's safe enough.

Another alternative is defining your own mini-language for those expressions and then parsing them using something like ANTLR. But, again, this really depends on what you're trying to accomplish.