This document discusses various metaprogramming techniques in Groovy including: - Meta Object Protocol (MOP) which allows modifying classes at runtime. - Categories and mixins for adding methods to existing classes. - Traits for reusable pieces of code with state. - Extension modules for globally available extensions that can modify classes. - AST transformations for compile-time modifications by transforming the AST. - Over 70 built-in AST transformations like @ToString and @Immutable.

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Andres
Almiray
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@ Oracle
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github.com/aalmiray

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https://en.wikipedia.org/wiki/Metaprogramming
Metaprogramming is a programming technique in which
computer programs have the ability to treat other programs
as their data.
It means that a program can be designed to read, generate,
analyze or transform other programs, and even modify itself
while running.
In some cases, this allows programmers to minimize the
number of lines of code to express a solution, in turn
reducing development time. It also allows programs greater
flexibility to efficiently handle new situations without
recompilation.

7. M.O.P.

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Meta Object Protocol
• Every Class has a companion Metaclass
• Metaclasses provide additional behavior such as
• Properties
• Methods
• Modifications are visible only to Groovy.

9. Enhancing a Groovy class
1 class Person {
2 String name
3 }
4
5 Person.metaClass.greet = { s ->
6 "Hello $s, my name is $name"
7 }
8
9 p = new Person(name: 'Andres')
10 p.greet('Groovy')
11
12 // Hello Groovy, my name is Andres

10. Enhancing a Java class
1 String.metaClass.spongeBob = {
2 int i = 0
3 delegate.toCharArray()collect { c ->
4 (i++ % 2 != 0) ? c : c.toUpperCase()
5 }.join('')
6 }
7
8 'Metaprogramming is fun'.spongeBob()
9
10 // MeTaPrOgRaMmInG Is fUn

11. DOES NOT UNDERSTAND

12. A catch-all option
1 class Foo {
2 Object methodMissing(String name, Object args) {
3 "What do you mean by ${name}(${args})?"
4 }
5 }
6
7 def foo = new Foo()
8 foo.say('Groovy')
9
10 // What do you mean by say([Groovy])?

13. A catch-all option
1 class Foo {
2 void propertyMissing(String name, Object arg) {
3 println "Can't assign a '$name' property to you!"
4 }
5
6 Object propertyMissing(String name) {
7 "You don't have a property named '$name'!"
8 }
9 }
10
11 def foo = new Foo()
12 println foo.id
13 foo.lang = 'Groovy'
14
15 // You don't have a property named 'id'!
16 // Can't assign a 'lang' property to you!

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Does not Understand
• Method signatures must match the convention
• Object methodMissing(String, Object)
• void propertyMissing(String, Object)
• Object propertyMissing(String)
• May be implemented by both Groovy and Java classes

15. Categories
1 class Greeter {
2 static String greet(String self) {
3 "Hello ${self}!"
4 }
5 }
6
7 use(Greeter) {
8 'Groovy'.greet()
9 }
10
11 // Hello Groovy!

16. Categories
1 @Grab('org.apache.commons:commons-lang3:3.11')
2 import org.apache.commons.lang3.StringUtils
3
4 use(StringUtils) {
5 'Groovy'.chop() == 'Groov'
6 }

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Categories
• Affect classes inside a specific scope.
• Methods must be static.
• Type of first argument becomes the receiver.

18. Mixins
1 @Grab('org.apache.commons:commons-lang3:3.11')
2 import org.apache.commons.lang3.StringUtils
3
4 class Greeter {
5 static String greet(String self) {
6 "Hello ${self}!"
7 }
8 }
9
10 String.mixin Greeter, StringUtils
11
12 'Groovy'.greet()
13 'Groovy'.chop()
14
15 // Hello Groovy!
16 // Groov

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Mixins
• Like categories but their scope is not limited.
• Applies to both Groovy and Java classes.

20. Traits
1 interface NameProvider {
2 String getName()
3 }
4
5 trait WithName implements NameProvider {
6 String name
7 }
8
9 class Person implements WithName { }
10
11 String greet(NameProvider np) {
12 "Hello $np.name"
13 }
14
15 p = new Person(name: 'Groovy')
16 greet(p)
17
18 // Hello Groovy

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Traits
• Java 8 gave us default methods on interfaces
• Groovy gives you that plus a stateful option

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Extension Modules
• Like mixins but they are globally available.
• Can modify Groovy and Java classes.
• Provide instance and static methods.
• Can be type checked by the compiler.
• Require an additional resource (module descriptor).

23. Extension
1 package griffon.plugins.bcrypt;
2
3 public class BCryptExtension {
4 public static String encodeAsBcrypt(String self) {
5 return BCrypt.hashpw(self, BCrypt.gensalt());
6 }
7
8 public static String encodeAsBcrypt(String self, int
9 genSaltRound) {
10 return BCrypt.hashpw(self, BCrypt.gensalt(genSaltRound));
11 }
12 }

24. /META-INF/groovy/
org.codehaus.groovy.runtime.ExtensionModule
1 moduleName=griffon-bcrypt
2 moduleVersion=0.1
3 extensionClasses=griffon.plugins.bcrypt.BCryptExtension
4 staticExtensionClasses=

25. Using the Extension
1 String password = "my password"
2 // should give you a nice bcrypt hash
3 println password.encodeAsBcrypt()

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Compile Time
• Modify the generated byte code.
• Modifications are visible to Groovy and Java classes.
• Modifications can be type checked by the compiler.

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AST Xforms
• Two flavors:
• Local
• Global
• Writing your own requires knowledge of compiler APIs,
AST type hierarchy, Groovy internals, your standard dark
magic ;-)

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Local AST Xforms
• Require two types:
• An interface that defines the entry point.
• An AST forms that’s linked to the interface.

32. @ToString
1 @groovy.transform.ToString
2 class Person {
3 String firstName
4 String lastName
5 }
6
7 p = new Person(firstName: 'Jack', lastName: 'Nicholson')
8
9 // Person(Jack, Nicholson)

33. @ToString
1 @java.lang.annotation.Documented
2 @Retention(RetentionPolicy.RUNTIME)
3 @Target({ElementType.TYPE})
4 @GroovyASTTransformationClass("org.codehaus.groovy.transform.
5 ToStringASTTransformation")
6 public @interface ToString {
7 ...
8 }

34. @ToString
1 @GroovyASTTransformation(phase = CompilePhase.CANONICALIZATION)
2 public class ToStringASTTransformation
3 extends AbstractASTTransformation {
4 public void visit(ASTNode[] nodes, SourceUnit source) {
5 // magic goes here
6 }
7 }

35. @Canonical
1 @groovy.transform.Canonical
2 class Person {
3 String firstName
4 String lastName
5 }
6 def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
7 // Effect of @ToString
8 assert p1.toString() == 'Person(Jack, Nicholson)'
9
10 // Effect of @TupleConstructor
11 def p2 = new Person('Jack','Nicholson')
12 assert p2.toString() == 'Person(Jack, Nicholson)'
13
14 // Effect of @EqualsAndHashCode
15 assert p1==p2
16 // Effect of @EqualsAndHashCode
17 assert p1.hashCode()==p2.hashCode()

36. @Category
1 @groovy.lang.Category(String)
2 class Greeter {
3 String greet() {
4 "Hello ${this}!"
5 }
6 }
7
8 use(Greeter) {
9 'Groovy'.greet()
10 }
11
12 // Hello Groovy!

37. @Immutable
1 @groovy.transform.Immutable
2 class Point {
3 int x, y
4 }
5
6 p1 = new Point(10, 10)
7 p2 = new Point(x: 10, y: 10)
8 p3 = new Point(10, 20)
9
10 assert p1 == p2
11 assert p1 != p3

38. @Sortable
1 @groovy.transform.Sortable
2 class Thing {
3 String id
4 }
5
6 t1 = new Thing(id: '123')
7 t2 = new Thing(id: '456')
8
9 assert t1 < t2

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70+ xforms available
@ASTTest @AnnotationCollector @AnnotationCollectorMode @AutoClone
@AutoCloneStyle @AutoExternalize @AutoFinal @AutoImplement @BaseScript
@Bindable @Canonical @Categoy @Commons @CompilationUnitAware
@CompileDynamic @CompileStatic @ConditionalInterrupt @Delegate
@EqualsAndHashCode @ExternalizeMethods @ExternalizeVerifier @Field
@Generated @Grab @GrabConfig @GrabExclude @GrabResolver @Grapes
@Immutable @ImmutableBase @ImmutableOptions @IndexedProperty
@InheritConstructors @Internal @KnownImmutable @Lazy @ListenerList @Log
@Log4j @Log4j2 @Macro @MapConstructor @Memoized @Mixin @NamedDelegate
@NamedParam @NamedParams @NamedVariant @Newify @NullCheck
@PackageScope @PackageScopeTarget @PlatformLog @PropertyOptions
@RecordBase @RecordType @SelfType @Singleton @Slf4j @Sortable @SourceURI
@Synchronized @TailRecursive @ThreadInterrupt @TimedInterrupt @ToString @Trait
@TupleConstructor @TypeChecked @TypeCheckingMode @Undefined @Vetoable
@VisibilityOptions @WithReadLock @WithWriteLock

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Global AST Xforms
• They only require the AST xform implementation.
• They are always available to all classes during the
compilation step.

41. http://spockframework.org/

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Implementing AST Xforms
• Use of compiler APIs such as Expression, Statement,
ClassNode, etc.
• May use ASTBuilder to create expressions and
statements from text, code, builder nodes.
• May use macro methods to simplify expressions.
• Refer to core AST forms for hints and tricks.

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Resources
• https://groovy-lang.org/metaprogramming.html
• https://www.slideshare.net/paulk_asert/groovy-transforms
• A History of the Groovy Programming Language
• (https://dl.acm.org/doi/pdf/10.1145/3386326)

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