meta-programming in groovy - · pdf filemeta-programming in groovy 3 groovy is an agile...

29.02.2008

Meta-programming in Groovy

Lars Blumberg

Christoph Hartmann

Arvid Heise

2Meta-programming in Groovy

James Strachan wrote

The Groovy Story


“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

Adopted from Gina, p. 48

Code.groovy

Code.groovy

groovyc

Java class loader Groovy class loader

Code.class

Loaded class Loaded class


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) {new Dog()

}

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

Getting Metaclass for Classes


static void main(args) {new Dog()

}

ScriptBytecodeAdapter.invokeNewN( 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()dog.bark()

}

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

Intercepting Method Calls


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

}

ScriptBytecodeAdapter. invokeMethodN( 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()

dog.bark()

}

Neighbor intercepted barking

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')

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


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

}

Become Magician


Keep It Simple


Class

XML

Application

TableHibernate

Keep It Simple


Class

Application

TableEJB

Keep It Simple


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


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/libraryview.jsp?search_by=practically+groovy

• Groovy Source Code and Mailing List


http://groovy.codehaus.org/Documentation



meta-programming in groovy - · pdf filemeta-programming in groovy 3 groovy is an agile...

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