Mopping Up With Groovy

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

Mopping Up With Groovy

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