Groovy dynamic stateless mixins

Last week I wrote about per-instance meta-classes and ExpandoMetaClass DSL. At that point it was just prototyped code in my working copy and I was not even sure if it will come to Groovy 1.6 trunk. Now it is there. If you didn’t read this previous post I recommend to have quick look on it as it might help to understand current one.

Today I want to talk about another part of the same commit, which is kind of side-effect of ability to have per-instance meta classes. Now we able to have per instance dynamic stateless mixins.

But let me start we explanation of what is mixin for me.

Mixin is either static or dynamic way to extend class or instance behavior by adding methods and/or state defined in another class.

Static here means defined at compile time and dynamic means defined on runtime. This two types of mixins are totally different.

• Static mixins applied to class but dynamic one to meta class (either class one or instance one)
• Static mixins always effect hierarchy, dynamic ones may or may not effect it depending on nature of meta class under effect (if meta class is the one associated with the class it does, if not it does not
• Static mixins always effect type information, dynamic ones never does that.

Stateless or stateless mixins means ability of mixins to extend object behavior not only with methods but with state as well. Stateful compile time mixins can be understand as kind of multiple inheritance.

Groovy has no compile time mixins yet. I think Groovy has to have something similar to Scala traits but it is subject to different discussion.

Groovy has no stateful mixins yet as well and below I will talk a bit about my idea how they should look like. So main subject for today is dynamic stateless mixins

Let me show simple example. First of all we define usual category class, which will be used to mix in.

class DeepFlattenToCategory {
    static Set flattenTo(element) {
        LinkedHashSet set = new LinkedHashSet()
        element.flattenTo(set)
        return set
    }
    // Object - put to result set
    static void flattenTo(element, Set addTo) {
        addTo <
           element.flattenTo(set);
        }
    }
    // Map - flatten each value
    static void flattenTo(Map elements, Set addTo) {
       elements.values().flattenTo(addTo);
    }
    // Array - flatten each element
    static void flattenTo(Object [] elements, Set addTo) {
        elements.each { element ->
           element.flattenTo(set);
        }
    }
}

This category does pretty obvious thing – deep flattening of complex data structure as demonstrated by following code snippet.

        // mixin Category to meta class
        Object.metaClass.mixin DeepFlattenToCategory
        // and here we are
        assertEquals ([8,9,3,2,1,4], [[8,9] as Object [], [3,2,[2:1,3:4]],[2,3]].flattenTo () as List)

Careful reader may notice probably that we modify only meta class for Object but methods for Collection, Object [] and Map becomes available as well. Such behavior is usual for categories but a bit uncommon for ExpandMetaClass – in the past he had to call ExpandoMetaClass.enableGlobally () to ask meta classes to look for missed methods. Such approach had several downsides – to name a few: unnecessary memory footprint for creation ExpandoMetaClass for each class (both modified and non-modified) and lack of “good place” where to call ExpandoMetaClass.enableGlobally (). It means that if you developed library, which requires it – you should call it somewhere, because nobody can guarantee that user application will do that. Fortunately, now ExpandoMetaClass can define methods for sub classes and if sub class missed some method he will try to find it in hierarchy. Obviously such behavior is must for mixins – otherwise our sample will look much less nicer(which is still to be valid code but creating four ExpandoMetaClasses instead of one)

        // mixin Category to meta classes
        Object.metaClass.mixin DeepFlattenToCategory
        Collection.metaClass.mixin DeepFlattenToCategory
        Object[].metaClass.mixin DeepFlattenToCategory
        Map.metaClass.mixin DeepFlattenToCategory
        // and here we are
        assertEquals ([8,9,3,2,1,4], [[8,9] as Object [], [3,2,[2:1,3:4]],[2,3]].flattenTo () as List)

        Object.metaClass.foo(ArrayList){ -> .... }

BTW, ExpandoMetaClass has new syntax to define methods for subclasses

        Object.metaClass.foo(ArrayList){ -> .... }

OR

        Object.metaClass {
           foo(ArrayList){ -> .... {
        }

OR

        Object.metaClass {
           define(ArrayList){
              foo{ -> .... }
           }
        }

OK, back to mixins. So far, it seems very similar to normal categories, so let us make our sample more interesting

        class NoFlattenArrayListCategory {
           // ArrayList - put to result set
           static void flattenTo(ArrayList element, Set addTo) {
              addTo << element;
           }
        }

        def x = [2,3]
        x.metaClass.mixin NoFlattenArrayListCategory
        assertEquals ([x, 8,9,3,2,1,4], [x, [8,9] as Object [], [3,2,[2:1,3:4]],[2,3]].flattenTo () as List)

What we do here is create category, which does not flatten array lists and mix it in instance meta class. Voila!
Of course the same can be achieved with ExpandoMetaClass without mixing in category

        def x = [2,3]
        x.metaClass.flattenTo = { Set addTo -> addTo << delegate }
        assertEquals ([x, 8,9,3,2,1,4], [x, [8,9] as Object [], [3,2,[2:1,3:4]],[2,3]].flattenTo () as List)

but please remember that category can be easily implemented in Java and provide much better performance compare to closure-based meta methods. And now we can use it as mixins.

The last thing I want to talk about is some features, which is missing and which I would love to see in Groovy. All below is just imagination and I don’t know exact implementation details and underwater stones but here are problems to be addressed.

• Stateful mixins
• Mixing in objects
• Better syntax to define categories in Groovy
• Compile time mixins

Stateful mixins

Imagine that category class defines not only static methods but also some fields and these fields magically becomes available as properties when we mix these category in. Probably if object already has such properties the one defined by category should be ignored, but if not… we have extremely powerful tool.

The main technical problem here is that we don’t want to define these new properties via ExpandoMetaClass, which will be very slow, but if we created instance of category class to associated it with original object… well, we have no good place to store this association. For Groovy compiled objects we of course can store refernence(s) directly in the object and for Java one in WeakHashMap. But what will happen if this mixed in instance keeps strong reference for original Java object… It will never be collected. Believe me, it will happen very often. So far I don’t know any good solution for that.

Mixing in objects

If we had solution for stateful mixin most probably it will lead us to ability to mix in not only category class but any object. I can imagine fantastic possibilities for that.

Better syntax to define categories in Groovy

Well static methods with additional parameter ’self’ are so ugly. We definitely need something better. So far we have @Category annotation, which solves only part of the problem.

Compile time mixin

Having stateful mixins with good syntax to define category methods will lead us to ability mixin category classes on runtime. As result we will have probably the strongest mixin story between all dynamic languages and as powerful as Scala traits in the world of staticly compiled ones.

So it goes.

Groovy per instance meta classes and ExpandoMetaClass DSL

So far Groovy allow per instance meta classes only for objects, which implements GroovyObject interface (like any class defined in Groovy). It can be illustrated by following simple test case:

   // for java.lang.String
   "test string".metaClass.toString = { "modified string" }
   assertEquals( "modified string", "test string".toString() )
   // but any other string will have the same behavior
   assertEquals( "modified string", "any other string".toString() )

At the same time

   // for GroovyObject
   class MyBean {
      String toString () { "bean" }
   }

   def bean = new MyBean ()
   def emc = new ExpandoMetaClass(MyBean,false,true)
   emc.toString = { -> "modified bean" }
   bean.metaClass = emc
   assertEquals("modified bean", bean.toString())
   // but any other bean will keep default behavior
   assertEquals("bean", new MyBean().toString())
   // if we want to modify behavior of all MyBean objects
   MyBean.metaClass = emc
   assertEquals("modified bean", new MyBean().toString())

Notice that we use different technique to add methods to Java and Groovy objects. As getMetaClass () for Java object always returns the same meta class we can use .metaClass notation, each GroovyObject has it is own meta class (of course, it is the same object by default) so we act differently depending on what we want to modify (whole class or just one Object)

Now I have patch which solves all these problems.

First of all we can assigned per instance meta class for any object. Of course, for Java objects it kept in WeakHashMap which involves some performance penalties. But now our first sample looks much more natural

   // for java.lang.String
   "test string".metaClass.toString = { "modified string" }
   assertEquals( "modified string", "test string".toString() )
   // but any other string will have natural behavior
   assertEquals( "any other string", "any other string".toString() )

Groovy objects also has benefits – we don’t need to create ExpandoMetaClass manually.

   def bean = new MyBean ()
   bean.metaClass.toString = { -> "modified bean" }
   assertEquals("modified bean", bean.toString())
   // but any other bean will keep default behavior
   assertEquals("bean", new MyBean().toString())
   // if we want to modify behavior of all MyBean objects
   MyBean.metaClass = bean.metaClass
   assertEquals("modified bean", new MyBean().toString())

But now, when we have unified behavior for Java and Groovy objects we can use simply and powerful ExpandoMetaClass DSL. Here is example

        // add behavior to meta class for Integer
        Integer.metaClass {
           // block of static methods
           'static' {
                fib { Number n ->
                    n.fib ()
                }
                // we can use both = or << if needed
                unusedStatic << { -> }
            }

            // one more syntax for static methods
            static.unusedStatic2 = { -> }

            // property definition
            ZERO = 0

            // method definition
            fib { ->
                def n = delegate
                if (n == ZERO)
                  return 1;
                if (n == 1)
                  return 1
                else
                  return fib(n-1) + fib(n-2)
            }

            // of course we can use both = or << if needed as well
            unusedMethod << { -> }
        }
        // and now we have it
        assertEquals( 3, 3.fib())

        // but we can also modify meta class for particular number
        4.metaClass {
            fib { ->
                10
            }
        }

        // and it works!
        assertEquals( 13, Integer.fib(5))

You know what… I like it. And thanks a lot for Graeme for inspirations and advices while I worked on that. Hope it will be accepted well and commited to the trunk.