CODEWITHSUNDEEP
javagroovymetaprogrammingexpandometaclass
user1768830
user1768830

Reputation:

Scope of Groovy's ExpandoMetaClass?

Groovy exposes an ExpandoMetaClass that allows you to dynamically add instance and class methods/properties to a POJO. I would like to use it to add an instance method to one of my Java classes:

public class Fizz {
    // ...etc.
}

Fizz fizz = new Fizz();
fizz.metaClass.doStuff = { String blah -> fizz.buzz(blah) }

This would be the equivalent to refactoring the Fizz class to have:

public class Fizz {
    // ctors, getters/setters, etc...

    public void doStuff(String blah) {
        buzz(blah);
    }
}

My question:

Does this add doStuff(String blah) to only this particular instance of Fizz? Or do all instances of Fizz now have a doStuff(String blah) instance method?

If the former, how do I get all instances of Fizz to have the doStuff instance method? I know that if I made the Groovy:

fizz.metaClass.doStuff << { String blah -> fizz.buzz(blah) }

Then that would add a static class method to Fizz, such as Fizz.doStuff(String blah), but that's not what I want. I just want all instances of Fizz to now have an instance method called doStuff. Ideas?

Upvotes: 7

Views: 866

Answers (1)

bythe4mile
bythe4mile

Reputation: 642

Firstly, when you add to the main class of Fizz, its instances do not get the method as the instances have already been factored out and added to memory.

So one way of approaching this is to use the method signature from the original class. Therefore instead of

fizz.doStuff(blah)

call the method of the class. Therefore

fizz.&doStuff(blah)

This gets the method signature from the originating class, but uses the attributes from the instance. However as you can imagine, since its calling the originating class, this is a slightly heavy call.

Now one alternative of pushing out to every instance is to make the instances ExpandoMetaClass instances of Fizz. Hence...

Fizz.metaClass.doStuff = {return "blah"}
fizz = new Fizz()
Fizz.metaClass.doOtherStuff = {return "more blah"}
assert fizz.doOtherStuff() == "more blah"

Hope this helps

UPDATE:

Full code example:

class Fizz{
}

Fizz.metaClass.doOtherStuff = {return "more blah"}
def fizz = new Fizz()
assert fizz.doOtherStuff() == "more blah"

def fizz1 = new Fizz()
assert fizz1.doOtherStuff() == "more blah"

Upvotes: 3

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