Groovy Meta Programming Slides - acidum.de

Meta-programming in Groovy Lars Blumberg Christoph Hartmann Arvid Heise

29.02.2008

James Strachan wrote

The Groovy Story

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“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

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My class is your class

Groovy Java Java Runtime Environment

Adopted from Gina, p. 5

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Precompiled vs. direct mode Code.groovy groovyc

Code.class

Code.groovy

Java class loader Loaded class

Groovy class loader Loaded class

Adopted from Gina, p. 48

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Groovy programming concepts Beauty through brevity

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Expose the Magic

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Running Example

class Dog { String name = 'dog' void bark() { System.out.println "$name: woof" } String toString() { name } }

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Metaclasses in Groovy

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Creating Objects static void main(args) { new Dog() }

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

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Getting Metaclass for Classes static void main(args) { new Dog() }

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

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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')

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

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

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Intercepting Method Calls static void main(args) { dog = new Dog() dog.bark() }

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

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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

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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() }

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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()

Meta-programming in Groovy

Waldi is about to bark Waldi: woof Waldi is about to speak Waldi does not speak

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

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Become Magician

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Keep It Simple

XML

Class

Hibernate

Table

Application

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Keep It Simple

Class

EJB

Table

Application

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Keep It Simple

Groovy

Table

Application

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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/li braryview.jsp?search_by=practically+groovy • Groovy Source Code and Mailing List

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