london-ggug 2015 - metaprogramming options with groovy

METAPROGRAMMING OPTIONS WITH GROOVYMETAPROGRAMMING OPTIONS WITH GROOVY

Iván López @ilopmarIván López @ilopmar@LondonGGUG@LondonGGUG

Hello!I am Iván López

@ilopmar

http://greachconf.com@madridgug

▷ 1st rule: You DO talk about the London-GGUG

▷ 2nd rule: If it's your first London-GGUG you HAVE to talk

▷3rd rule: You have to be awesome (by David Dawson)

London-GGUG rules

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)

import groovy.time.TimeCategory

use (TimeCategory) { println (20.hours + 10.days.from.now) // Fri Jul 17 20:00:00 CEST 2015}

println TimeCategory.getHours(20).plus(TimeCategory.getDays(10).from.now)// Fri Jul 17 20:00:00 CEST 2015

Categories example (II)

import groovy.time.TimeCategory

use (TimeCategory) { println (20.hours + 10.days.from.now) // Fri Jul 17 20:00:00 CEST 2015}

println TimeCategory.getHours(20).plus(TimeCategory.getDays(10).from.now)// Fri Jul 17 20:00:00 CEST 2015

Extension modules

▷ JAR file that provides extra methods

▷ Meta-information file

▷ Put jar in classpath to enhance classes

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

// 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 = londonggug.StringUtilsExtension

Extension modules example

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

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

// src/main/groovy/londonggug/StringUtilsExtension.groovypackage londonggugclass 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.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/lmivan

Iván López

london-ggug 2015 - metaprogramming options with groovy

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