metaprogramming with groovy

Metaprogrammingwith Groovy

Iván López - @ilopmar

Hello!I am Iván López

@ilopmar

http://greachconf.com@madridgug

Groovy is dynamic

▷ “Delay” to runtime some decisions

▷ Add properties/behaviours in runtime

▷ Wide range of applicability

What is metaprogramming?

“Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data.

- Wikipedia

1.Runtime metaprogramming

Runtime metaprogramming

▷ Groovy provides this through Meta-Object Protocol (MOP)

▷ Use MOP to:– Invoke methods dynamically– Synthesize classes and methods on

the fly

What is the Meta Object Protocol?

Groovy

MetaClass▷ MetaClass registry for each class

▷ Collection of methods/properties

▷ We can always modify the metaclass

MOP method injection

MOP Method Injection▷ Injecting methods at code-writing time

▷ We can “open” a class any time

▷ Different techniques:– MetaClass– Categories– Extensions– Mixins vs Traits

class StringUtils { static String truncate(String text, Integer length, Boolean overflow = false) { text.take(length) + (overflow ? '...' : '') }}

String chuckIpsum = "If you can see Chuck Norris, he can see you.\If you can not see Chuck Norris you may be only seconds away from death"

println StringUtils.truncate(chuckIpsum, 72)println StringUtils.truncate(chuckIpsum, 72, true)

// ExecutionIf you can see Chuck Norris, he can see you. If you can not see Chuck NoIf you can see Chuck Norris, he can see you. If you can not see Chuck No...

String.metaClass.truncate = { Integer length, Boolean overflow = false -> delegate.take(length) + (overflow ? '...' : '')}

assert chuckIpsum.truncate(72, true) == StringUtils.truncate(chuckIpsum, 72, true)

Adding methods using MetaClass

class Utils {}

def utilsInstance = new Utils()

Utils.metaClass.version = "3.0"utilsInstance.metaClass.released = true

assert utilsInstance.version == "3.0"assert utilsInstance.released == true

Adding properties using MetaClass

class Utils {}

// Integerassert '42' == 42.toString()

Integer.metaClass.toString = { delegate == 42 ? 'The answer to life, the universe and everything' : String.valueOf(delegate)}

assert 42.toString() == 'The answer to life, the universe and everything'assert 100.toString() == '100'

// Booleanassert false.toBoolean() == false

Boolean.metaClass.toBoolean = { !delegate }assert false.toBoolean() == true

Overriding methods using MetaClass

Categories

▷ MetaClass changes are “persistent”

▷ Change metaclass in confined code

▷ MOP modified only in the closure

Categories exampleclass StringUtils { static String truncate(String text, Integer length, Boolean overflow = false) { text.take(length) + (overflow ? '...' : '') }}

use (StringUtils) { println "Lorem ipsum".truncate(5)}

try { println "Lorem ipsum".truncate(5)} catch (MissingMethodException mme) { println mme}

// ExecutionLorem

groovy.lang.MissingMethodException: No signature of method: java.lang.String.truncate() is applicable for argument types: (java.lang.Integer) values: [5]Possible solutions: concat(java.lang.String), take(int)

// ExecutionLorem

Categories example (II)

println TimeCategory.getHours(20).plus(TimeCategory.getDays(10).from.now)// Thu Jun 12 20:00:00 CET 2016

import groovy.time.TimeCategory

use (TimeCategory) { println (20.hours + 10.days.from.now) // Thu Jun 12 20:00:00 CET 2016}

Extension modules

▷ JAR file that provides extra methods

▷ Meta-information file

▷ Put jar in classpath to enhance classes

// src/main/groovy/demo/StringUtilsExtension.groovypackage democlass StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') }}

package demoimport spock.lang.Specificationclass StringUtilsExtensionSpec extends Specification { void 'test trucate'() { expect: "Lorem" == "Lorem ipsum".truncate(5) "Lorem..." == "Lorem ipsum".truncate(5, true) }}

// Execute with:// gradle build// groovy -cp build/libs/string-extensions-1.0.jar ExtensionExample1.groovy

assert "Lorem..." == "Lorem ipsum". truncate(5, true)

# src/main/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModulemoduleName = string-utils-modulemoduleVersion = 0.1extensionClasses = demo.StringUtilsExtension

Extension modules example

Extension modules example// src/main/groovy/demo/StringUtilsExtension.groovypackage democlass StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') }}

// src/main/groovy/demo/StringUtilsExtension.groovypackage democlass StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') }}

Extension modules example

Mixins

▷ “Bring in” or “mix in” implementations from multiple classes

▷ Calls first routed to mixed-in class

▷ Last mixin wins

▷ Not easily un-done

class SpidermanPower { String spiderSense() { "Using spider-sense..." }}

Mixins example

@Mixin([SpidermanPower])class Person {}

def person = new Person()assert person.spiderSense() == "Using spider-sense..."assert !(person instanceof SpidermanPower)

Person.mixin SupermanPowerassert person.fly() == "Flying..."assert !(person instanceof SupermanPower)

class SupermanPower { String fly() { "Flying..." }}

Mixins example

Mixins exampleclass SupermanPower { String fly() { "Flying..." }}

“When we started fixing mixin bugs we didn't know if they were a bug or a feature, so we removed mixins and add traits.

- Jochen Theodorou(Greach 2015 Opening Keynote)

Traits▷ Groovy 2.3+

▷ Similar to Java 8 default methods

▷ Supported in JDK 6, 7 and 8

▷ Stateful

▷ Composition over inheritance

▷ Documentation

class Person implements SpidermanPower {}

def person = new Person()assert person.spiderSense() == "Using spider-sense..."assert person instanceof SpidermanPower

def person2 = person.withTraits SupermanPowerassert person2.fly() == "Flying..."assert person2 instanceof SupermanPower

Traits exampletrait SpidermanPower { String spiderSense() { "Using spider-sense..." }}

trait SupermanPower { String fly() { "Flying..." }}

MOP method synthesis

MOP Method Synthesis

▷ Dynamically figure out behaviour upon invocation

▷ It may not exist until it's called/executed

▷ “Intercept, Cache, Invoke” pattern

def p = new Person(name: 'Iván', age: 34)

assert p.respondsTo('sayHi')assert p.respondsTo('sayHiTo', String)assert !p.respondsTo('goodbye')

assert p.hasProperty('name')assert !p.hasProperty('country')

Check for methods and properties

class Person { String name Integer age

String sayHi() { "Hi, my name is ${name} and I'm ${age}" }

String sayHiTo(String name) { "Hi ${name}, how are you?" }}

assert p.hasProperty('age')assert !p.hasProperty('country')

MethodMissing example

▷ Requirements:– Send notifications to users by different

channels– +50 notifications– Not all notifications by all channels– Extensible and open to future

modifications

MethodMissing exampleabstract class Channel { void sendNewFollower(String username, String follower) { } void sendNewMessage(String username, String msg) { } ...}

class EmailChannel extends Channel { void sendNewFollower(String username, String follower) { println "Sending email notification to '${username}' for new follower '${follower}'" } void sendNewMessage(String username, String msg) { println "Sending email notification to '${username}' for new message '${msg}'" }}

class MobilePushChannel extends Channel { void sendNewFollower(String username, String follower) { println "Sending mobile push notification to '${username}' for new follower '${follower}'" }}

MethodMissing exampleclass NotificationService {

List channels = []

def methodMissing(String name, args) { System.out.println "...methodMissing called for ${name} with args ${args}"

// Generate the implementation def implementation = { Object[] methodArgs -> channels.each { channel -> def metaMethod = channel.metaClass.getMetaMethod(name, methodArgs) return metaMethod.invoke(channel, methodArgs) } } // Cache the implementation in the metaClass NotificationService instance = this instance.metaClass."$name" = implementation

// Execute it! implementation(args) }}

List channels = []

notificationService.sendNewFollower(...)notificationService.sendNewMessage(...)

List channels = []

// Execution...methodMissing called for sendNewFollower with args [John, Peter]Sending email notification to 'John' for new follower 'Peter'Sending mobile push notification to 'John' for new follower 'Peter'

Sending email notification to 'Mary' for new follower 'Steve'Sending mobile push notification to 'Mary' for new follower 'Steve'

...methodMissing called for sendNewMessage with args [Iván, Hello!]Sending email notification to 'Iván' for new message 'Hello!'

def notificationService = new NotificationService( channels: [new EmailChannel(), new MobilePushChannel()])

assert !notificationService.respondsTo('sendNewFollower', String, String)notificationService.sendNewFollower("John", "Peter")assert notificationService.respondsTo('sendNewFollower', String, String)

notificationService.sendNewFollower("Mary", "Steve")

notificationService.sendNewMessage("Iván", "Hello!")

notificationService.sendNewMessage("Iván", "Hello!")class EmailChannel extends Channel { void sendNewFollower(String username, String follower) {…} void sendNewMessage(String username, String msg) {…}}

class MobilePushChannel extends Channel { void sendNewFollower(String username, String follower) {…}}

MethodMissing exampledef notificationService = new NotificationService( channels: [new EmailChannel(), new MobilePushChannel()])

2.Compile-time metaprogramming

Compile-time metaprogramming

▷ Advance feature

▷ Analyze/modify program structure at compile time

▷ Cross-cutting features

▷ Write code that generates bytecode

AST and compilation

▷ AST: Abstract Syntax Tree

▷ AST modified during compilation

▷ Hook into the phases

▷ Initialization, Parsing, Conversion, Semantic analysis, Canonicalization, Instruction selection, Class generation, Output, Finalization

Global ASTtransformations

Global AST Transformations

▷ No annotation

▷ Meta-information file

▷ Applied to all code during compilation

▷ Any compilation phase

Local ASTtransformations

Local AST Transformations

▷ Annotate code

▷ No meta-information file

▷ Easy to debug

Steps to create local AST

Interface AST Enjoy!

Local AST example

package demo

import ...

@Retention(RetentionPolicy.SOURCE)@Target([ElementType.TYPE])@GroovyASTTransformationClass("demo.VersionASTTransformation")@interface Version { String value()}

class VersionedClass { public static final String VERSION = "1.0"}

import demo.Version

@Version('1.0')class VersionedClass {}

Local AST example

package demo

import ...

import demo.Version

Local AST example

package demo

import ...

import demo.Version

Local AST example@GroovyASTTransformation(phase = CompilePhase.SEMANTIC_ANALYSIS)class VersionASTTransformation extends AbstractASTTransformation {

@Override public void visit(final ASTNode[] nodes, final SourceUnit source) { if (nodes.length != 2) { return }

if (nodes[0] instanceof AnnotationNode && nodes[1] instanceof ClassNode) { def annotation = nodes[0] def version = annotation.getMember('value')

if (version instanceof ConstantExpression) { nodes[1].addField('VERSION', ACC_PUBLIC | ACC_STATIC | ACC_FINAL, ClassHelper.STRING_TYPE, version) } else { source.addError(new SyntaxException("Invalid value for annotation", annotation.lineNumber, annotation.columnNumber)) } } }}

Local AST example@GroovyASTTransformation(phase = CompilePhase.SEMANTIC_ANALYSIS)class VersionASTTransformation extends AbstractASTTransformation {

@Override public void visit(final ASTNode[] nodes, final SourceUnit source) { if (nodes.length != 2) { return }

if (nodes[0] instanceof AnnotationNode && nodes[1] instanceof ClassNode) { def annotation = nodes[0] def version = annotation.getMember('value')

if (version instanceof ConstantExpression) { nodes[1].addField('VERSION', ACC_PUBLIC | ACC_STATIC | ACC_FINAL, ClassHelper.STRING_TYPE, version) } else { source.addError(new SyntaxException("Invalid value for annotation", annotation.lineNumber, annotation.columnNumber)) } } }}

Local AST example// Execute with:// gradle build// groovy -cp build/libs/add-version-1.0.jar LocalASTExample.groovy

import demo.Version

println VersionedClass.VERSION

// Execution1.0

import demo.Version

// Execution1.0

import demo.Version

// Execution1.0

3.Why we should usemetaprogramming?

Let’s review some concepts

Metaprogramming out-of-the box

Easy and very powerful

Write better code

Add behaviour easily

Take advantage of this power

Because Groovy, it's groovy

With great powercomes great responsibility

Thanks!Any questions?

@ilopmar

[email protected]

https://github.com/ilopmar

Iván López

http://bit.ly/metaprog-groovy

metaprogramming with groovy

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