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.groovy
Code.class
Java class loader Groovy class loader
Loaded class 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 \"$name: woof\"
}
String toString() {
name
}
}
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Metaclasses in Groovy
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Creating Objects
static void main(args) { ScriptBytecodeAdapter.invokeNewN(
new Dog() DogExample.class, Dog.class,
} new Object[0])
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Getting Metaclass for Classes
static void main(args) { ScriptBytecodeAdapter.invokeNewN(
new Dog() DogExample.class, Dog.class,
} new Object[0])
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Example for Custom Metaclass
class WaldiMeta extends MetaClassImpl {
WaldiMeta() {
super(GroovySystem.getMetaClassRegistry(), Dog.class)
initialize()
}
}
// Instance-based MetaClass
waldi = new Dog(name: 'Waldi')
waldi.metaClass = new WaldiMeta()
// Class-based MetaClass
GroovySystem.getMetaClassRegistry().setMetaClass(Dog.class, new WaldiMeta())
waldi = new Dog(name: 'Waldi')
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Method Invocation
static void main(args) {
ScriptBytecodeAdapter. invokeMethodN(
dog = new Dog()
DogExample.class, dog, \"bark\",
dog.bark()
new Object[0])
}
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Intercepting Method Calls
static void main(args) {
ScriptBytecodeAdapter. invokeMethodN(
dog = new Dog()
DogExample.class, dog, \"bark\",
dog.bark()
new Object[0])
}
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Interception in GroovyInterceptable
class InterceptingDog extends Dog implements GroovyInterceptable {
Object invokeMethod(name, args) {
System.out.println \"$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') Waldi is about to bark
dog.metaClass = new BrunoMeta() Waldi: woof
dog.bark() Waldi is about to speak
dog.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
Hibernate
Class Table
Application
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Keep It Simple
EJB
Class 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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