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Meta-programming in Groovy
Lars Blumberg
Christoph Hartmann
Arvid Heise

29.02.2008

James Strachan wrote

The Groovy Story

Meta-programming in Groovy 2

“Groovy is an agile dynamic language for the Java
Platform with many features that are inspired by
languages like Python, Ruby and Smalltalk, making
them available to Java developers using a
Java-like syntax.”
The Groovy web site


My class is your class

Groovy

Java

Java Runtime Environment

Adopted from Gina, p. 5


Precompiled vs. direct mode

Code.groovy

groovyc

Code.class Code.groovy

Java class loader Groovy class loader

Loaded class Loaded class

Adopted from Gina, p. 48


Groovy programming concepts
Beauty through brevity


Expose the Magic


Running Example

class Dog {
String name = 'dog'

void bark() {
System.out.println "$name: woof"
}

String toString() {
name
}
}


Metaclasses in Groovy


Creating Objects
static void main(args) { ScriptBytecodeAdapter.invokeNewN(
new Dog() DogExample.class, Dog.class,
} new Object[0])


Getting Metaclass for Classes
static void main(args) { ScriptBytecodeAdapter.invokeNewN(
new Dog() DogExample.class, Dog.class,
} new Object[0])


Example for Custom Metaclass

class WaldiMeta extends MetaClassImpl {
WaldiMeta() {
super(GroovySystem.getMetaClassRegistry(), Dog.class)
initialize()
}
}

// Instance-based MetaClass
waldi = new Dog(name: 'Waldi')
waldi.metaClass = new WaldiMeta()

// Class-based MetaClass
GroovySystem.getMetaClassRegistry().setMetaClass(Dog.class, new WaldiMeta())
waldi = new Dog(name: 'Waldi')


Method Invocation
static void main(args) {
dog = new Dog() ScriptBytecodeAdapter. invokeMethodN(
dog.bark() DogExample.class, dog, "bark",
} new Object[0])


Intercepting Method Calls
dog = new Dog() ScriptBytecodeAdapter. invokeMethodN(
dog.bark() DogExample.class, dog, "bark",
} new Object[0])


Interception in GroovyInterceptable

class InterceptingDog extends Dog implements GroovyInterceptable {
Object invokeMethod(name, args) {
System.out.println "$this is about to $name"
metaClass.invokeMethod(this, name, args)
}
}

dog = new InterceptingDog(name: 'Waldi')
dog.bark()

Waldi is about to bark
Waldi: woof


Interception using Interceptor
class InterceptingNeighbor implements Interceptor {
String action
Object beforeInvoke(object, methodName, arguments) {
action = methodName
}
boolean doInvoke() {
if(action != 'bark') return true
println "Neighbor intercepted barking"
false
}
}
proxy = ProxyMetaClass.getInstance(Dog.class)
proxy.interceptor = new InterceptingNeighbor()
proxy.use {
dog = new Dog() Neighbor intercepted barking
dog.bark()
}


Interception with MetaClass
class BrunoMeta extends MetaClassImpl {
Object invokeMethod(sender, object, methodName, originalArguments,
isCallToSuper, fromInsideClass) {
println "$object is about to $methodName"
super.invokeMethod(sender, object, methodName, originalArguments,
isCallToSuper, fromInsideClass)
}

Object invokeMissingMethod(instance, methodName, arguments) {
println "$instance does not $methodName"
}
}

dog = new Dog(name: 'Waldi') Waldi is about to bark
dog.metaClass = new BrunoMeta() Waldi: woof
dog.bark() Waldi is about to speak
dog.speak() Waldi does not speak


Evaluating Expressions
shell = new GroovyShell()
shell.evaluate("1+1")
}


Become Magician


Keep It Simple

XML

Class Hibernate Table

Application


Keep It Simple

Class EJB Table

Application


Keep It Simple

Groovy Table

Application


Meta-programming in Groovy

• Introspection: fully integrated
• GroovyObject: getMetaClass, getProperty
• MetaClass: getProperties, getMethods, getMetaMethods

• Intercession:
• Interception:
• GroovyInterceptable: pretend to have function, error handling
• Interceptor: scope-level; useful for AOP, e.g. logging
• MetaClass: change or observe behavior on class-level
• Expando: dynamic behavior and properties on instance-level
• ExpandoMetaClass: most powerful, dynamic on class-level


We love you …


References

• [Gina]: Dierk Koenig: Groovy in Action
• Codehaus Documentation
http://groovy.codehaus.org/Documentation
• Practically Groovy
http://www.ibm.com/developerworks/views/java/li
braryview.jsp?search_by=practically+groovy
• Groovy Source Code and Mailing List


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