Slides from my GeeCON 2014 Prague talk: "Groovy is a dynamic language that provides different types of metaprogramming techniques. In this talk we’ll mainly see runtime metaprogramming. I’ll explain Groovy Meta-Object-Protocol (MOP), the metaclass, how to intercept method calls, how to deal with method missing and property missing, the use of mixins, traits and categories. All of these topics will be explained with examples in order to understand them. Also, I’ll talk a little bit about compile-time metaprogramming with AST Transformations. AST Transformations provide a wonderful way of manipulating code at compile time via modifications of the Abstract Syntax Tree. We’ll see a basic but powerful example of what we can do with AST transformations." The code is available at: https://github.com/lmivan/geecon2014-prague-metaprograming-with-groovy

1. Iván López @ilopmar
METAPROGRAMMING
WITH GROOVY

2. Hello!
I am Iván López
@ilopmar

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. Intercepting methods
using MOP

10. Groovy Interceptable
▷ GroovyObject interface
▷ Implement GroovyInterceptable to
hook into the execution
public interface GroovyObject {
Object invokeMethod(String name, Object args)
Object getProperty(String propertyName)
void setProperty(String propertyName, Object newValue)
MetaClass getMetaClass()
void setMetaClass(MetaClass metaClass)
}

11. GroovyInterceptable example
class Person implements GroovyInterceptable {
String name
Integer age
public Object getProperty(String propertyName) {
println "Getting property '${propertyName}'"
return this.@"${propertyName}"
}
public void setProperty(String propertyName, Object newValue) {
println "Setting property '${propertyName}' with value '${newValue}'"
this.@"${propertyName}" = newValue
}
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '34'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 34
}
def person = new Person()
person.name = "Iván"
person.age = 34
println "Hello ${person.name}, you're ${person.age}"

12. GroovyInterceptable example
class Person implements GroovyInterceptable {
String name
Integer age
public Object getProperty(String propertyName) {
println "Getting property '${propertyName}'"
return this.@"${propertyName}"
}
public void setProperty(String propertyName, Object newValue) {
println "Setting property '${propertyName}' with value '${newValue}'"
this.@"${propertyName}" = newValue
}
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '34'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 34
}
def person = new Person()
person.name = "Iván"
person.age = 34
println "Hello ${person.name}, you're ${person.age}"

13. GroovyInterceptable example
class Person implements GroovyInterceptable {
String name
Integer age
public Object getProperty(String propertyName) {
println "Getting property '${propertyName}'"
return this.@"${propertyName}"
}
public void setProperty(String propertyName, Object newValue) {
println "Setting property '${propertyName}' with value '${newValue}'"
this.@"${propertyName}" = newValue
}
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '34'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 34
}
def person = new Person()
person.name = "Iván"
person.age = 34
println "Hello ${person.name}, you're ${person.age}"

14. GroovyInterceptable example
class Person implements GroovyInterceptable {
String name
Integer age
public Object getProperty(String propertyName) {
println "Getting property '${propertyName}'"
return this.@"${propertyName}"
}
public void setProperty(String propertyName, Object newValue) {
println "Setting property '${propertyName}' with value '${newValue}'"
this.@"${propertyName}" = newValue
}
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '34'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 34
}
def person = new Person()
person.name = "Iván"
person.age = 34
println "Hello ${person.name}, you're ${person.age}"

15. GroovyInterceptable example
class Person implements GroovyInterceptable {
String name
Integer age
public Object getProperty(String propertyName) {
println "Getting property '${propertyName}'"
return this.@"${propertyName}"
}
public void setProperty(String propertyName, Object newValue) {
println "Setting property '${propertyName}' with value '${newValue}'"
this.@"${propertyName}" = newValue
}
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '34'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 34
}
def person = new Person()
person.name = "Iván"
person.age = 34
println "Hello ${person.name}, you're ${person.age}"

16. GroovyInterceptable example (II)
class Hello implements GroovyInterceptable {
public Object invokeMethod(String methodName, Object args) {
System.out.println "Invoking method '${methodName}' with args '${args}'"
def method = metaClass.getMetaMethod(methodName, args)
method?.invoke(this, args)
}
void sayHi(String name) {
System.out.println "Hello ${name}"
}
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

17. GroovyInterceptable example (II)
class Hello implements GroovyInterceptable {
public Object invokeMethod(String methodName, Object args) {
System.out.println "Invoking method '${methodName}' with args '${args}'"
def method = metaClass.getMetaMethod(methodName, args)
method?.invoke(this, args)
}
void sayHi(String name) {
System.out.println "Hello ${name}"
}
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

18. GroovyInterceptable example (II)
class Hello implements GroovyInterceptable {
public Object invokeMethod(String methodName, Object args) {
System.out.println "Invoking method '${methodName}' with args '${args}'"
def method = metaClass.getMetaMethod(methodName, args)
method?.invoke(this, args)
}
void sayHi(String name) {
System.out.println "Hello ${name}"
}
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

19. GroovyInterceptable example (II)
class Hello implements GroovyInterceptable {
public Object invokeMethod(String methodName, Object args) {
System.out.println "Invoking method '${methodName}' with args '${args}'"
def method = metaClass.getMetaMethod(methodName, args)
method?.invoke(this, args)
}
void sayHi(String name) {
System.out.println "Hello ${name}"
}
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

20. MetaClass
▷ MetaClass registry for each class
▷ Collection of methods/properties
▷ We can always modify the metaclass
▷ Intercept methods implementing
invokeMethod on metaclass

21. MetaClass example
class Hello {
void sayHi(String name) {
println "Hello ${name}"
}
}
Hello.metaClass.invokeMethod = { String methodName, args ->
println "Invoking method '${methodName}' with args '${args}'"
def method = Hello.metaClass.getMetaMethod(methodName, args)
method?.invoke(delegate, args)
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

22. MetaClass example
class Hello {
void sayHi(String name) {
println "Hello ${name}"
}
}
Hello.metaClass.invokeMethod = { String methodName, args ->
println "Invoking method '${methodName}' with args '${args}'"
def method = Hello.metaClass.getMetaMethod(methodName, args)
method?.invoke(delegate, args)
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

23. MetaClass example
class Hello {
void sayHi(String name) {
println "Hello ${name}"
}
}
Hello.metaClass.invokeMethod = { String methodName, args ->
println "Invoking method '${methodName}' with args '${args}'"
def method = Hello.metaClass.getMetaMethod(methodName, args)
method?.invoke(delegate, args)
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

24. MetaClass example
class Hello {
void sayHi(String name) {
println "Hello ${name}"
}
}
Hello.metaClass.invokeMethod = { String methodName, args ->
println "Invoking method '${methodName}' with args '${args}'"
def method = Hello.metaClass.getMetaMethod(methodName, args)
method?.invoke(delegate, args)
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

25. MetaClass example
class Hello {
void sayHi(String name) {
println "Hello ${name}"
}
}
Hello.metaClass.invokeMethod = { String methodName, args ->
println "Invoking method '${methodName}' with args '${args}'"
def method = Hello.metaClass.getMetaMethod(methodName, args)
method?.invoke(delegate, args)
}
def hello = new Hello()
hello.sayHi("GeeCon Prague!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[GeeCon Prague!]'
Hello GeeCon Prague!
Invoking method 'anotherMethod' with args '[]'

26. MOP method
injection

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

28. Adding methods using MetaClass
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)

29. Adding methods using MetaClass
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)

30. Adding methods using MetaClass
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)

31. Adding methods using MetaClass
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)

32. Adding methods using MetaClass
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)

33. Adding methods using MetaClass
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)

34. 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

35. 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

36. 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

37. 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

38. Overriding methods using MetaClass
// 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

39. Overriding methods using MetaClass
// 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

40. Overriding methods using MetaClass
// 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

41. Overriding methods using MetaClass
// 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

42. Overriding methods using MetaClass
// 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

43. Overriding methods using MetaClass
// 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

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

45. 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)

46. 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)

47. 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)

48. 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)

49. Categories example (II)
class FileBinaryCategory {
def static leftShift(File file, URL url) {
def input
def output
try {
input = url.openStream()
output = new BufferedOutputStream(new FileOutputStream(file))
output << input
} finally {
input?.close()
output?.close()
}
}
}

50. Categories example (II)
class FileBinaryCategory {
def static leftShift(File file, URL url) {
def input
def output
try {
input = url.openStream()
output = new BufferedOutputStream(new FileOutputStream(file))
output << input
} finally {
input?.close()
output?.close()
}
}
}

51. Categories example (II)
class FileBinaryCategory {
def static leftShift(File file, URL url) {
def input
def output
try {
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
input = url.openStream()
output = new BufferedOutputStream(new FileOutputStream(file))
output << input
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
} finally {
println tmpFile
// Execution
/tmp/tmp_7428855173238452155
input?.close()
output?.close()
}
}
}

52. Categories example (II)
class FileBinaryCategory {
def static leftShift(File file, URL url) {
def input
def output
try {
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
input = url.openStream()
output = new BufferedOutputStream(new FileOutputStream(file))
output << input
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
} finally {
println tmpFile
// Execution
/tmp/tmp_7428855173238452155
input?.close()
output?.close()
}
}
}

53. Categories example (II)
class FileBinaryCategory {
def static leftShift(File file, URL url) {
def input
def output
try {
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
input = url.openStream()
output = new BufferedOutputStream(new FileOutputStream(file))
output << input
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
} finally {
println tmpFile
// Execution
/tmp/tmp_7428855173238452155
input?.close()
output?.close()
}
}
}

54. Categories example (II)
class FileBinaryCategory {
def static leftShift(File file, URL url) {
def input
def output
try {
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
input = url.openStream()
output = new BufferedOutputStream(new FileOutputStream(file))
output << input
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
} finally {
println tmpFile
// Execution
/tmp/tmp_7428855173238452155
input?.close()
output?.close()
}
}
}

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

56. Extension modules example
// src/main/groovy/geecon2014/StringUtilsExtension.groovy
package geecon2014
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 = geecon2014.StringUtilsExtension
package geecon2014
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)

57. Extension modules example
// src/main/groovy/geecon2014/StringUtilsExtension.groovy
package geecon2014
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 = geecon2014.StringUtilsExtension
package geecon2014
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)

58. Extension modules example
// src/main/groovy/geecon2014/StringUtilsExtension.groovy
package geecon2014
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 = geecon2014.StringUtilsExtension
package geecon2014
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)

59. Extension modules example
// src/main/groovy/geecon2014/StringUtilsExtension.groovy
package geecon2014
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 = geecon2014.StringUtilsExtension
package geecon2014
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)

60. Extension modules example
// src/main/groovy/geecon2014/StringUtilsExtension.groovy
package geecon2014
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 = geecon2014.StringUtilsExtension
package geecon2014
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)

61. Mixins
▷ “Bring in” or “mix in” implementations
from multiple classes
▷ Calls first routed to mixed-in class
▷ Last mixin wins
▷ Can't override methods in metaclass
▷ Not easily un-done

62. Mixins example
class SpidermanPower {
String spiderSense() {
"Using spider-sense..."
}
}
class SupermanPower {
String fly() {
"Flying..."
}
}
@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)

63. class SupermanPower {
String fly() {
"Flying..."
}
}
Mixins example
class SpidermanPower {
String spiderSense() {
"Using spider-sense..."
@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)
}
}

64. class SupermanPower {
String fly() {
"Flying..."
}
}
Mixins example
class SpidermanPower {
String spiderSense() {
"Using spider-sense..."
@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)
}
}

65. class SupermanPower {
String fly() {
"Flying..."
}
}
Mixins example
class SpidermanPower {
String spiderSense() {
"Using spider-sense..."
@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)
}
}

66. Mixins example
class SpidermanPower {
String spiderSense() {
"Using spider-sense..."
@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..."
}
}

67. Mixins example
class SpidermanPower {
String spiderSense() {
"Using spider-sense..."
@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..."
}
}

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

69. Traits example
trait SpidermanPower {
String spiderSense() {
"Using spider-sense..."
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
}
}
trait SupermanPower {
String fly() {
"Flying..."
}
}

70. Traits example
trait SpidermanPower {
String spiderSense() {
"Using spider-sense..."
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
}
}
trait SupermanPower {
String fly() {
"Flying..."
}
}

71. Traits example
trait SpidermanPower {
String spiderSense() {
"Using spider-sense..."
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
}
}
trait SupermanPower {
String fly() {
"Flying..."
}
}

72. Traits example
trait SpidermanPower {
String spiderSense() {
"Using spider-sense..."
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
}
}
trait SupermanPower {
String fly() {
"Flying..."
}
}

73. MOP method
synthesis

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

75. Check for methods and properties
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')
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?"
}
}

76. Check for methods and properties
def p = new Person(name: 'Iván', age: 34)
assert p.respondsTo('sayHi')
assert p.respondsTo('sayHiTo', String)
assert !p.respondsTo('goodbye')
assert p.hasProperty('age')
assert !p.hasProperty('country')
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?"
}
}

77. Check for methods and properties
def p = new Person(name: 'Iván', age: 34)
assert p.respondsTo('sayHi')
assert p.respondsTo('sayHiTo', String)
assert !p.respondsTo('goodbye')
assert p.hasProperty('age')
assert !p.hasProperty('country')
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?"
}
}

78. 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: 34)
assert p.respondsTo('sayHi')
assert p.respondsTo('sayHiTo', String)
assert !p.respondsTo('goodbye')
assert p.hasProperty('age')
assert !p.hasProperty('country')

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

80. 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}'"
}
}

81. 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}'"
}
}

82. 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}'"
}
}

83. 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)
}
}

84. 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)
}
}

85. 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(...)

86. 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)
}
}

87. 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)
}
}

88. 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!'

89. 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!'

90. 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!'

91. 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!'

92. 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("class EmailChannel exItveánnd"s, C"hHaenlnleol! "{)
void sendNewFollower(String username, String follower) {…}
void sendNewMessage(String username, String msg) {…}
// Execution
...}
methodMissing called for sendNewFollower with args [John, Peter]
Sending email notification to 'John' for new follower 'Peter'
Sending class mobile MobilePushChannel push notification extends to Channel 'John' {
for new follower 'Peter'
void sendNewFollower(String username, String follower) {…}
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!'

93. 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!'

94. 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!'

95. 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!'

96. 2.
Compile-time
metaprogramming

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

98. 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

99. Groovy AST Transformations
▷ Lot of AST transformations out-of-the-box
▷ @EqualsAndHashCode, @ToString,
@TuppleConstructor, @Canonical, @Grab,
@Immutable, @Delegate, @Singleton,
@Category, @Log4j, @CompileStatic,
@TypeChecked, @Synchronized
...

100. Global AST
transformations

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

102. Local AST
transformations

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

104. Steps to create local AST
Interface AST Enjoy!

105. Local AST example
import geecon2014.Version
@Version('1.0')
class VersionedClass {
}
package geecon2014
import ...
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("geecon2014.VersionASTTransformation")
@interface Version {
String value()
}
class VersionedClass {
public static final String VERSION = "1.0"
}

106. Local AST example
import geecon2014.Version
@Version('1.0')
class VersionedClass {
}
package geecon2014
import ...
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("geecon2014.VersionASTTransformation")
@interface Version {
String value()
}
class VersionedClass {
public static final String VERSION = "1.0"
}

107. Local AST example
import geecon2014.Version
@Version('1.0')
class VersionedClass {
}
package geecon2014
import ...
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("geecon2014.VersionASTTransformation")
@interface Version {
String value()
}
class VersionedClass {
public static final String VERSION = "1.0"
}

108. 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))
}
}
}
}

109. 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))
}
}
}
}

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

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

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

113. 3.
Recap
Why we should use
metaprogramming?

114. Let’s review some concepts
Metaprogramming
out-of-the box
Easy and very
powerfull
Write better code
Add behaviour
easily
Take advantage of
this power
Because Groovy,
it's groovy

115. With great power
comes great
responsibility

116. Thanks!
Any questions?
Iván López
@ilopmar
lopez.ivan@gmail.com
https://github.com/lmivan
http://kcy.me/1bzj7