The document presents an overview of metaprogramming in Groovy, emphasizing its capability to dynamically invoke methods and synthesize classes at runtime using the Meta Object Protocol (MOP). Key examples illustrate how to implement the GroovyObject interface and use metaclasses to modify class behavior, including adding properties and methods. The text provides practical code snippets demonstrating the functionalities of 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. Intercepting methods
using MOP

10. 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)
}
Groovy Interceptable
▷ GroovyObject interface
▷ Implement GroovyInterceptable to hook
into the execution

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
}
}
def person = new Person()
person.name = "Iván"
person.age = 35
println "Hello ${person.name}, you're ${person.age}"
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '35'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 35

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
}
}
def person = new Person()
person.name = "Iván"
person.age = 35
println "Hello ${person.name}, you're ${person.age}"
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '35'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 35

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
}
}
def person = new Person()
person.name = "Iván"
person.age = 35
println "Hello ${person.name}, you're ${person.age}"
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '35'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 35

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
}
}
def person = new Person()
person.name = "Iván"
person.age = 35
println "Hello ${person.name}, you're ${person.age}"
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '35'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 35

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
}
}
def person = new Person()
person.name = "Iván"
person.age = 35
println "Hello ${person.name}, you're ${person.age}"
// Execution
Setting property 'name' with value 'Iván'
Setting property 'age' with value '35'
Getting property 'name'
Getting property 'age'
Hello Iván, you're 35

16. 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("ConFess Vienna!")
hello.anotherMethod()
GroovyInterceptable example (II)
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'

17. 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("ConFess Vienna!")
hello.anotherMethod()
GroovyInterceptable example (II)
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'

18. 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("ConFess Vienna!")
hello.anotherMethod()
GroovyInterceptable example (II)
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'

19. 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("ConFess Vienna!")
hello.anotherMethod()
GroovyInterceptable example (II)
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
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. 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("ConFess Vienna!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'
MetaClass example

22. 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("ConFess Vienna!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'
MetaClass example

23. 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("ConFess Vienna!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'
MetaClass example

24. 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("ConFess Vienna!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'
MetaClass example

25. 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("ConFess Vienna!")
hello.anotherMethod()
// Execution
Invoking method 'sayHi' with args '[ConFess Vienna!]'
Hello ConFess Vienna!
Invoking method 'anotherMethod' with args '[]'
MetaClass example

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. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

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. 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

37. 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

38. // 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

39. // 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

40. // 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

41. // 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

42. // 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

43. // 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

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. 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()
}
}
}
Categories example (II)

50. 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()
}
}
}
Categories example (II)

51. 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()
}
}
}
Categories example (II)
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
println tmpFile
// Execution
/tmp/tmp_7428855173238452155

52. 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()
}
}
}
Categories example (II)
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
println tmpFile
// Execution
/tmp/tmp_7428855173238452155

53. 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()
}
}
}
Categories example (II)
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
println tmpFile
// Execution
/tmp/tmp_7428855173238452155

54. 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()
}
}
}
Categories example (II)
File tmpFile = File.createTempFile('tmp_', '')
use (FileBinaryCategory) {
tmpFile << "http://groovy.codehaus.org/images/groovy-logo-medium.png".toURL()
}
println tmpFile
// Execution
/tmp/tmp_7428855173238452155

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

56. // src/main/groovy/confess2015/StringUtilsExtension.groovy
package confess2015
class StringUtilsExtension {
static String truncate(String self, Integer length, Boolean overflow = false) {
self.take(length) + (overflow ? '...' : '')
}
}
package confess2015
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 = confess2015.StringUtilsExtension
Extension modules example

57. Extension modules example
// src/main/groovy/confess2015/StringUtilsExtension.groovy
package confess2015
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 = confess2015.StringUtilsExtension
package confess2015
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/confess2015/StringUtilsExtension.groovy
package confess2015
class StringUtilsExtension {
static String truncate(String self, Integer length, Boolean overflow = false) {
self.take(length) + (overflow ? '...' : '')
}
}
package confess2015
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 = confess2015.StringUtilsExtension

59. Extension modules example
// src/main/groovy/confess2015/StringUtilsExtension.groovy
package confess2015
class StringUtilsExtension {
static String truncate(String self, Integer length, Boolean overflow = false) {
self.take(length) + (overflow ? '...' : '')
}
}
package confess2015
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 = confess2015.StringUtilsExtension

60. // src/main/groovy/confess2015/StringUtilsExtension.groovy
package confess2015
class StringUtilsExtension {
static String truncate(String self, Integer length, Boolean overflow = false) {
self.take(length) + (overflow ? '...' : '')
}
}
package confess2015
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 = confess2015.StringUtilsExtension

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

62. 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..."
}
}

63. @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

64. @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

65. @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

66. @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..."
}
}

67. @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..."
}
}

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

69. 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..."
}
}

70. 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..."
}
}

71. 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..."
}
}

72. 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..."
}
}

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. 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?"
}
}

76. def p = new Person(name: 'Iván', age: 35)
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?"
}
}

77. 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: 35)
assert p.respondsTo('sayHi')
assert p.respondsTo('sayHiTo', String)
assert !p.respondsTo('goodbye')
assert p.hasProperty('age')
assert !p.hasProperty('country')

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: 35)
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
// 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!")

89. 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!")

90. 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!")

91. 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!")

92. 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) {…}
}

93. 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!")

94. 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!")

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. Global AST
transformations

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

101. Local AST
transformations

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

103. Steps to create local AST
Interface AST Enjoy!

104. Local AST example
package confess2015
import ...
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("confess2015.VersionASTTransformation")
@interface Version {
String value()
}
class VersionedClass {
public static final String VERSION = "1.0"
}
import confess2015.Version
@Version('1.0')
class VersionedClass {
}

105. Local AST example
class VersionedClass {
public static final String VERSION = "1.0"
}
package confess2015
import ...
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("confess2015.VersionASTTransformation")
@interface Version {
String value()
}
import confess2015.Version
@Version('1.0')
class VersionedClass {
}

106. Local AST example
class VersionedClass {
public static final String VERSION = "1.0"
}
package confess2015
import ...
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("confess2015.VersionASTTransformation")
@interface Version {
String value()
}
import confess2015.Version
@Version('1.0')
class VersionedClass {
}

107. 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))
}
}
}
}

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

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

112. 3.
Why we should use
metaprogramming?

113. 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

114. With great power
comes great
responsibility

115. Thanks!
Any questions?
@ilopmar
lopez.ivan@gmail.com
https://github.com/lmivan
Iván López
http://kcy.me/208wq