ConFoo 2016 - Metaprogramming with Groovy

1. METAPROGRAMMING WITH GROOVY Iván López @ilopmar

2. Hello! I am Iván López @ilopmar http://greachconf.com@madridgug

3. Groovy is dynamic ▷ “Delay” to runtime some decisions ▷ Add properties/behaviours in runtime ▷ Wide range of applicability

4. What is metaprogramming?

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

6. 1. Runtime metaprogramming

7. Runtime metaprogramming ▷ Groovy provides this through Meta- Object Protocol (MOP) ▷ Use MOP to: – Invoke methods dynamically – Synthesize classes and methods on the fly

8. What is the Meta Object Protocol? Groovy Groovy Java Java MOP

9. MetaClass ▷ MetaClass registry for each class ▷ Collection of methods/properties ▷ We can always modify the metaclass

10. MOP method injection

11. MOP Method Injection ▷ Injecting methods at code-writing time ▷ We can “open” a class any time ▷ Different techniques: – MetaClass – Categories – Extensions – Mixins vs Traits

12. 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) // Execution If you can see Chuck Norris, he can see you. If you can not see Chuck No If 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

18. 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

19. Adding properties 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

22. // Integer assert '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' // Boolean assert false.toBoolean() == false Boolean.metaClass.toBoolean = { !delegate } assert false.toBoolean() == true Overriding methods using MetaClass

28. Categories ▷ MetaClass changes are “persistent” ▷ Change metaclass in confined code ▷ MOP modified only in the closure

29. Categories example class 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 } // Execution Lorem 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)

33. Categories example (II) import groovy.time.TimeCategory use (TimeCategory) { println (20.hours + 10.days.from.now) // Tue Feb 23 20:00:00 CET 2016 } println TimeCategory.getHours(20).plus(TimeCategory.getDays(10).from.now) // Tue Feb 23 20:00:00 CET 2016

34. Categories example (II) import groovy.time.TimeCategory use (TimeCategory) { println (20.hours + 10.days.from.now) // Tue Feb 23 20:00:00 CET 2016 } println TimeCategory.getHours(20).plus(TimeCategory.getDays(10).from.now) // Tue Feb 23 20:00:00 CET 2016

35. Extension modules ▷ JAR file that provides extra methods ▷ Meta-information file ▷ Put jar in classpath to enhance classes

36. // src/main/groovy/demo/StringUtilsExtension.groovy package demo class StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') } } package demo import spock.lang.Specification class 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.ExtensionModule moduleName = string-utils-module moduleVersion = 0.1 extensionClasses = demo.StringUtilsExtension Extension modules example

37. Extension modules example // src/main/groovy/demo/StringUtilsExtension.groovy package demo class StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') } } # src/main/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule moduleName = string-utils-module moduleVersion = 0.1 extensionClasses = demo.StringUtilsExtension package demo import spock.lang.Specification class 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)

38. Extension modules example // src/main/groovy/demo/StringUtilsExtension.groovy package demo class StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') } } package demo import spock.lang.Specification class 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.ExtensionModule moduleName = string-utils-module moduleVersion = 0.1 extensionClasses = demo.StringUtilsExtension

39. Extension modules example // src/main/groovy/demo/StringUtilsExtension.groovy package demo class StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') } } package demo import spock.lang.Specification class 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.ExtensionModule moduleName = string-utils-module moduleVersion = 0.1 extensionClasses = demo.StringUtilsExtension

40. // src/main/groovy/demo/StringUtilsExtension.groovy package demo class StringUtilsExtension { static String truncate(String self, Integer length, Boolean overflow = false) { self.take(length) + (overflow ? '...' : '') } } package demo import spock.lang.Specification class 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) Extension modules example # src/main/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule moduleName = string-utils-module moduleVersion = 0.1 extensionClasses = demo.StringUtilsExtension

41. Mixins ▷ “Bring in” or “mix in” implementations from multiple classes ▷ Calls first routed to mixed-in class ▷ Last mixin wins ▷ Not easily un-done

42. 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 SupermanPower assert person.fly() == "Flying..." assert !(person instanceof SupermanPower) class SupermanPower { String fly() { "Flying..." } }

43. @Mixin([SpidermanPower]) class Person {} def person = new Person() assert person.spiderSense() == "Using spider-sense..." assert !(person instanceof SpidermanPower) Person.mixin SupermanPower assert person.fly() == "Flying..." assert !(person instanceof SupermanPower) class SupermanPower { String fly() { "Flying..." } } class SpidermanPower { String spiderSense() { "Using spider-sense..." } } Mixins example

46. @Mixin([SpidermanPower]) class Person {} def person = new Person() assert person.spiderSense() == "Using spider-sense..." assert !(person instanceof SpidermanPower) Person.mixin SupermanPower assert person.fly() == "Flying..." assert !(person instanceof SupermanPower) class SpidermanPower { String spiderSense() { "Using spider-sense..." } } Mixins example class SupermanPower { String fly() { "Flying..." } }

47. @Mixin([SpidermanPower]) class Person {} def person = new Person() assert person.spiderSense() == "Using spider-sense..." assert !(person instanceof SpidermanPower) Person.mixin SupermanPower assert person.fly() == "Flying..." assert !(person instanceof SupermanPower) class SpidermanPower { String spiderSense() { "Using spider-sense..." } } Mixins example class SupermanPower { String fly() { "Flying..." } }

48. “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)

49. Traits ▷ Groovy 2.3+ ▷ Similar to Java 8 default methods ▷ Supported in JDK 6, 7 and 8 ▷ Stateful ▷ Composition over inheritance ▷ Documentation

50. class Person implements SpidermanPower {} def person = new Person() assert person.spiderSense() == "Using spider-sense..." assert person instanceof SpidermanPower def person2 = person.withTraits SupermanPower assert person2.fly() == "Flying..." assert person2 instanceof SupermanPower Traits example trait SpidermanPower { String spiderSense() { "Using spider-sense..." } } trait SupermanPower { String fly() { "Flying..." } }

54. MOP method synthesis

55. MOP Method Synthesis ▷ Dynamically figure out behaviour upon invocation ▷ It may not exist until it's called/executed ▷ “Intercept, Cache, Invoke” pattern

56. 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?" } }

57. def p = new Person(name: 'Iván', age: 36) assert p.respondsTo('sayHi') assert p.respondsTo('sayHiTo', String) assert !p.respondsTo('goodbye') assert p.hasProperty('age') 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?" } }

58. 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?" } } def p = new Person(name: 'Iván', age: 36) assert p.respondsTo('sayHi') assert p.respondsTo('sayHiTo', String) assert !p.respondsTo('goodbye') assert p.hasProperty('age') assert !p.hasProperty('country')

59. 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?" } } def p = new Person(name: 'Iván', age: 36) assert p.respondsTo('sayHi') assert p.respondsTo('sayHiTo', String) assert !p.respondsTo('goodbye') assert p.hasProperty('age') assert !p.hasProperty('country')

60. MethodMissing example ▷ Requirements: – Send notifications to users by different channels – +50 notifications – Not all notifications by all channels – Extensible and open to future modifications

61. MethodMissing example abstract 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}'" } }

65. MethodMissing example class 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) } }

67. MethodMissing example class 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) } } notificationService.sendNewFollower(...) notificationService.sendNewMessage(...)

70. MethodMissing example // 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!")

74. MethodMissing example // 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!")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) {…} }

77. MethodMissing example 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!") // 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!'

78. 2. Compile-time metaprogramming

79. Compile-time metaprogramming ▷ Advance feature ▷ Analyze/modify program structure at compile time ▷ Cross-cutting features ▷ Write code that generates bytecode

80. 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

81. Global AST transformations

82. Global AST Transformations ▷ No annotation ▷ Meta-information file ▷ Applied to all code during compilation ▷ Any compilation phase ▷ Grails uses intensively in GORM

83. Local AST transformations

84. Local AST Transformations ▷ Annotate code ▷ No meta-information file ▷ Easy to debug

85. Steps to create local AST Interface AST Enjoy!

86. 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 { }

87. Local AST example class VersionedClass { public static final String VERSION = "1.0" } package demo import ... @Retention(RetentionPolicy.SOURCE) @Target([ElementType.TYPE]) @GroovyASTTransformationClass("demo.VersionASTTransformation") @interface Version { String value() } import demo.Version @Version('1.0') class VersionedClass { }

88. Local AST example class VersionedClass { public static final String VERSION = "1.0" } package demo import ... @Retention(RetentionPolicy.SOURCE) @Target([ElementType.TYPE]) @GroovyASTTransformationClass("demo.VersionASTTransformation") @interface Version { String value() } import demo.Version @Version('1.0') class VersionedClass { }

89. 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)) } } } }

90. 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)) } } } }

91. Local AST example // Execute with: // gradle build // groovy -cp build/libs/add-version-1.0.jar LocalASTExample.groovy import demo.Version @Version('1.0') class VersionedClass { } println VersionedClass.VERSION // Execution 1.0

94. 3. Why we should use metaprogramming?

95. 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

96. With great power comes great responsibility

97. Thanks! Any questions? @ilopmar lopez.ivan@gmail.com https://github.com/ilopmar Iván López http://bit.ly/mp-groovy

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