Mopping Up With Groovy*
Brent Snook
brent@fuglylogic.com *obligatory lame pun

What is a MOP?

Duck Typing
• POJ (Plain Old Java) – you can
interact with an object based on
what it is (Class)
• Groovy – you can interact based on
which properties and methods are
available on the object (MetaClass)

Modifying Class Behaviour at Runtime
• AKA Monkey Patching
• AKA Duck Punching
• see whytheluckystiff's My Complete List Of
Substitute Names For The Maneuver We
Now Know To Be Monkeypatching

Why Do It?
• it has an amusing name
• it makes you more attractive to the opposite
(or same) sex
• it allows you to write more succinct,
expressive code

But I Feel Dirty …
• you may need to change how you
think about things
• the laws are different in the duck
typing universe
• question old attitudes and
conventions

How Does Groovy
Implement a MOP?

The MetaClass
• every Groovy object has one
• richer introspection and invocation interface
than Java Class
• you can set the MetaClass for an object to
change the rules for how it can behave

The Final Shape of an Object
Added Methods
Class ++ Properties
= MetaClass

The ExpandoMetaClass
• core of Groovy metaprogramming
• allows you to customise behaviour on the fly
• when you ask a newly created GroovyObject
for its metaClass, this is what you get back
<<interface>>
ExpandoMetaClass
GroovyObject MetaClass

What Can You Do With It?
• borrow methods from other classes
• add or override constructors
• dynamically create method names
• add methods to interfaces
• add or override instance methods
• add or override properties
• add or override static methods
http://groovy.codehaus.org/ExpandoMetaClass

Adding a Class Method

Adding a Property

Overriding an Instance Method

The Same In Ruby

Invocation/Access Hooks
• invokeMethod
• getProperty
• setProperty
• propertyMissing
• methodMissing

AOP with invokeMethod

Flexible Methods with methodMissing

MOP!!...huh! What is
it Good For?

Syntactic Sugar

Mocking/Stubbing Behaviour

DSL - The Original Class

Adding Some MOP Trickery

Icecream Time

Replacing/Simplifying Frameworks
and Patterns
• Spring, EasyMock/JMock
• circumvent the rigidity of the Java language
• patterns like Builder
• Dependency Injection
• with Groovy these can be simplified or
disappear altogether

Finally

The Pros and Cons
Pros Cons
• flexibility • sometimes clunky
• extensibility syntax
• power • slower performance

With Great Power…
• don’t get wowed by the coolness factor
• remember to KISS
• the end result should be more
understandable, not less
• keep pushing the boundaries but exercise
judgment

Further Reading and References
• http://www.gnu.org/software/guile/docs/goops/Metaobjects-
and-the-Metaobject-Protocol.html#Metaobjects%20and%20the
%20Metaobject%20Protocol
• http://en.wikipedia.org/wiki/Meta-object_protocol
• http://www.grails-exchange.com/files/GraemeRocher%20-
%20Keynote-DynamicGroovyMetaMagic.pdf
• http://groovy.codehaus.org/ExpandoMetaClass
• http://whytheluckystiff.net/articles/
seeingMetaclassesClearly.html
• http://groovy.codehaus.org/ExpandoMetaClass
• http://hackety.org/2007/08/10/
myCompleteListOfSubstitutePhrasesForTheActWeNowKnowTo
BeMonkeypatching.html
• Groovy In Action