29.02.2008

Meta-programming in Groovy

Lars Blumberg

Christoph Hartmann

Arvid Heise

2Meta-programming in Groovy

James Strachan wrote

The Groovy Story

3Meta-programming in Groovy

“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

4Meta-programming in Groovy

My class is your class

Groovy

Java

Java Runtime Environment

Adopted from Gina, p. 5

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Precompiled vs. direct mode

Adopted from Gina, p. 48

Code.groovy

Code.groovy

groovyc

Java class loader Groovy class loader

Code.class

Loaded class Loaded class

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Groovy programming concepts

Beauty through brevity

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Expose the Magic

Running Example

class Dog {

String name = 'dog'

void bark() {

System.out.println "$name: woof"

}

String toString() {

name

}

}

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Metaclasses in Groovy

9Meta-programming in Groovy

Creating Objects

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static void main(args) {new Dog()

}

ScriptBytecodeAdapter.invokeNewN( DogExample.class, Dog.class, new Object[0])

Getting Metaclass for Classes

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static void main(args) {new Dog()

}

ScriptBytecodeAdapter.invokeNewN( DogExample.class, Dog.class, new Object[0])

Example for Custom Metaclass

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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

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static void main(args) {dog = new Dog()dog.bark()

}

ScriptBytecodeAdapter. invokeMethodN( DogExample.class, dog, "bark", new Object[0])

Intercepting Method Calls

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static void main(args) {dog = new Dog()dog.bark()

}

ScriptBytecodeAdapter. invokeMethodN( DogExample.class, dog, "bark", new Object[0])

Interception in GroovyInterceptable

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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

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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()

dog.bark()

}

Neighbor intercepted barking

Interception with MetaClass

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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')

dog.metaClass = new BrunoMeta()

dog.bark()

dog.speak()

Waldi is about to bark

Waldi: woof

Waldi is about to speak

Waldi does not speak

Evaluating Expressions

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static void main(args) {shell = new GroovyShell()shell.evaluate("1+1")

}

Become Magician

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Keep It Simple

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Class

XML

Application

TableHibernate

Keep It Simple

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Class

Application

TableEJB

Keep It Simple

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Application

TableGroovy

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

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We love you …

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References

• [Gina]: Dierk Koenig: Groovy in Action

• Codehaus Documentation

http://groovy.codehaus.org/Documentation

• Practically Groovy

http://www.ibm.com/developerworks/views/java/libraryview.jsp?search_by=practically+groovy

• Groovy Source Code and Mailing List

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