groovy transforms

SPRINGONE2GX
WASHINGTON, DC
Unless otherwise indicated, these slides are © 2013 -2015 ASERT and licensed under a Creative Commons Attribution -NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/
Groovy AST Transformations
Dr Paul King
@paulk_asert
http://slideshare.net/paulk_asert/groovy-transforms
https://github.com/paulk-asert/groovy-transforms

Apache Groovy
Groovy in Action 2nd edition
Manning promo code: ctwspringo2gx
http://www.manning.com/koenig2
2

Unless otherwise indicated, these slides are © 2013 -2015 ASERT and licensed under a
Creative Commons Attribution-NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/
Contents
3
 Introduction
• Built-in AST Transforms
• Writing your own transforms

Parsing Summary
4
public run()
...
L1
ALOAD 1
LDC 1
AALOAD
ALOAD 0
LDC "Howdy Y'all"
INVOKEINTERFACE callCurrent()
ARETURN
...
println "Howdy Y'all"
BlockStatement
-> ReturnStatement
-> MethodCallExpression
-> VariableExpression("this")
-> ConstantExpression("println")
-> ArgumentListExpression
-> ConstantExpression("Howdy Y'all")
MyScript.groovy
> groovy MyScript.groovy
> groovyc MyScript.groovy
> groovysh
> groovyConsole

Creative Commons Attribution-NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/ 5
Parsing Summary
• 9 phase compiler
– Early stages: read source code
and convert into a sparse syntax
tree
– Middle stages: iteratively build up a
more dense and information rich
version of the syntax tree
– Later stages: check the tree and
convert it into byte code/class files
Initialization
Semantic Analysis
Instruction Selection
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing - Early StagesInitialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization
@ToString
class Greeter {
String message = "Howdy Y'all"
void greet() {
println message
}
}
ClassNode: Greeter
MethodNode: greet
Property: message
type: unresolved(String)
AnnotationNode: ToString
type: unresolved(ToString)
methods:
properties:
annotations:
BlockStatement
MethodCall: this.println(message)

Parsing - Middle StagesInitialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization
ClassNode: Greeter
MethodNode: greet
FieldNode: message
type: resolved(String)
methods:
fields:
constructors:
ConstructorNode
MethodNode: getMessage
MethodNode: setMessage
MethodNode: toString
MethodNode: getMetaClass
…

Parsing - Final StagesInitialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization
public greet()V
...
L1
...
ALOAD 0
GETFIELD Greeter.message
INVOKEINTERFACE callCurrent()
POP
...

When are transforms
applied?
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization
Local Transformations
Global Transformations

1 + 2 + 3

1 + 2 + 3
assert 1 + 1 == 2

Contents
13
• Introduction
 Built-in AST Transforms
• Writing your own transforms

@ToString
@groovy.transform.ToString
class Detective {
String firstName, lastName
}
def d = new Detective(firstName: 'Sherlock', lastName: 'Holmes')
assert d.toString() == 'Detective(Sherlock, Holmes)'
class Detective {
String toString() {
def _result = new StringBuilder()
_result.append('Detective(')
_result.append(this.firstName)
_result.append(', ')
_result.append(this.lastName)
_result.append(')')
return _result.toString()
}
}
ToStringASTTransformation

@ToString annotation parameters…
Parameter Name Purpose
excludes
Name of properties (list or comma separated) to
exclude from toString(). By default, all properties
are included. Incompatible with “includes”.
includes Name of properties (list or comma separated) to include
in toString(). Incompatible with “excludes”.
includeSuper Include the toString() for the super class by setting
to true. Default: false.
includeNames Include the names of the properties by setting this
parameter to true. Default: false.
allProperties Whether JavaBean "properties" are included.2.5

…@ToString annotation parameters
includeFields Include fields, not just properties. Default: false.
ignoreNulls Exclude any properties that are null. By default
null values will be included.
includePackage
Print just the simple name of the class without the
package. By default the package name is
included.
cache
Set to true to cache toString() calculations.
Use only for immutable objects. By default the
value is recalculated upon each method call.
includeSuperProperties Include properties from super class. Default: false

@ToString (using parameters)
@ToString(ignoreNulls=true, excludes='lastName', includeNames=true,
includePackage=false, includeFields=true)
class Detective {
List clues
private nemesis = 'Moriarty'
}
def d = new Detective(firstName: 'Sherlock', lastName: 'Holmes')
assert d.toString() ==
'Detective(firstName:Sherlock, nemesis:Moriarty)'

@EqualsAndHashCode
@EqualsAndHashCode
class Actor {
}
def a1 = new Actor(firstName: 'Ian', lastName: 'McKellen')
def a2 = new Actor(firstName: 'Ian', lastName: 'McKellen')
assert !(a1.is(a2))
assert a1 == a2
EqualsAndHashCodeASTTransformation

@EqualsAndHashCode
class Actor {
int hashCode() {
def _result = HashCodeHelper.initHash()
_result = HashCodeHelper.updateHash(_result, this.firstName)
_result = HashCodeHelper.updateHash(_result, this.lastName)
return _result
}
boolean canEqual(other) {
return other instanceof Actor
}
...

...
boolean equals(other) {
if (other == null) return false
if (this.is(other)) return true
if (!( other instanceof Actor)) return false
Actor otherTyped = (Actor) other
if (!(otherTyped.canEqual(this))) return false
if (!(this.getFirstName().is(otherTyped.getFirstName()))) {
if (!(this.getFirstName() == otherTyped.getFirstName())) {
return false
}
}
if (!(this.getLastName().is(otherTyped.getLastName()))) {
if (!(this.getLastName() == otherTyped.getLastName())) {
return false
}
}
return true
}
}
@EqualsAndHashCod
e

@EqualsAndHashCode annotation
parameters…Parameter Name Purpose
excludes
Exclude certain properties from the calculation by specifying
them as a comma separated list or literal list of String name
values. This is commonly used with an object that has an 'id'
field. By default, no properties are excluded. Incompatible with
“includes”
includes
Include only a specified list of properties by specifying them as a
comma separated list or literal list of String name values.
Incompatible with “excludes”.
cache
Set to true to cache hashCode() calculations. Use only for
immutable objects. By default the hashCode() is recalculated
whenever the hashCode() method is called.

…@EqualsAndHashCode annotation
parametersParameter Name Purpose
callSuper
Include properties from the super class by setting this parameter
to true. By default, the super class is not used as part of the
calculation.
includeFields
Include the class's fields, not just the properties, in the
calculation by setting this parameter to true. By default, fields
are not taken into account.
useCanEqual
Set to false to disable generation of a canEqual() method to
be used by equals(). By default the canEqual() method is
generated. The canEqual idiom provides a mechanism for
permitting equality in the presence of inheritance hierarchies.
For immutable classes with no explicit super class, this idiom is
not required.

@MapConstructor
• How does Groovy normally handle "named" arguments
class Athlete {
String first, last
}
def a1 = new Athlete(
first: 'Michael',
last: 'Jordan')
assert a1.first == 'Michael'
2.5
def a1 = new Athlete()
a1.setFirst('Michael')
a1.setLast('Jordan')

@MapConstructor
• With @MapConstructor we have an explicit Map constructor
• Good for Java integration & final fields
@MapConstructor
class Athlete {
String first, last
}
def a1 = new Athlete(
first: 'Michael',
last: 'Jordan')
assert a1.first == 'Michael'
2.5
Athlete(Map m) {
if (m.containsKey('first')) {
first = m.get('first')
}
if (m.containsKey('last')) {
last = m.get('last')
}
}

@MapConstructor annotation parameters…
excludes
Exclude certain properties from the constructor. Incompatible
with “includes”
includes
Include only a specified list of properties in the constructor.
Incompatible with “excludes”
pre
A Closure containing statements which will be prepended to the
generated constructor
post
A Closure containing statements which will be appended to the
end of the generated constructor
2.5

…@MapConstructor annotation parameters
includeFields
includeProperties Include properties in the constructor. Default: true
includeSuperProperties Include properties from super classes in the constructor
useSetters
By default, properties are set directly using their respective
field but a setter can be used instead if desired
2.5

@MapConstructor
import groovy.transform.*
@ToString(includeFields=true, includeNames=true)
@MapConstructor(post={ full = "$first $last" })
class Person {
final String first, last
private final String full
}
assert new Person(first: 'Dierk', last: 'Koenig').toString() ==
'Person(first:Dierk, last:Koenig, full:Dierk Koenig)'
2.5

@TupleConstructor
• A traditional positional arguments constructor
@TupleConstructor
class Athlete {
String first, last
}
def a1 = new Athlete('Michael', 'Jordan')
def a2 = new Athlete('Michael')
def a3 = new Athlete(first: 'Michael')
assert a1.first == a2.first

@TupleConstructor annotation parameters…
excludes
Exclude certain properties from the constructor. Incompatible
with “includes”
includes
Include only a specified list of properties in the constructor.
Incompatible with “excludes”
callSuper
Should super properties be called within a call to the parent
constructor rather than set as properties
force
By default, this annotation becomes a no-op if you provide your
own constructor, this flag forces constructor creation
defaults
Used to set whether default value processing is enabled (the
default) or disabled
2.5

…@TupleConstructor annotation parameters
includeFields
includeProperties Include properties in the constructor. Default: true
includeSuperFields Include fields from super classes in the constructor
includeSuperProperties Include properties from super classes in the constructor
useSetters
By default, properties are set directly using their respective
field but a setter can be used instead if desired
2.5

@TupleConstructor
@TupleConstructor
class Athlete {
String first, last
}

@TupleConstructor
@TupleConstructor
class Athlete {
String first, last
}
class Athlete {
String first, last
Athlete(String first=null, String last=null) {…}
}

@TupleConstructor
@TupleConstructor
class Athlete {
String first, last
}
class Athlete {
String first, last
Athlete(String first=null, String last=null) {…}
}
class Athlete {
String first, last
Athlete(String first, String last) {…}
Athlete(String first) {…}
Athlete() {…}
}

@TupleConstructor
@TupleConstructor(defaults=false)
class Athlete {
String first, last
}
class Athlete {
String first, last
Athlete(String first, String last) {…}
}

@Canonical
• Combines @ToString, @EqualsAndHashCode and
@TupleConstructor
• Actually uses AnnotationCollector in Groovy 2.5
@Canonical
class Inventor {
}
def i1 = new Inventor('Thomas', 'Edison')
def i2 = new Inventor('Thomas')
assert i1 != i2
assert i1.firstName == i2.firstName
assert i1.toString() == 'Inventor(Thomas, Edison)'
2.5

@Canonical
@TupleConstructor
@AnnotationCollector(value=[ToString, TupleConstructor, EqualsAndHashCode],
mode=AnnotationCollectorMode.PREFER_EXPLICIT_MERGED)
public @interface Canonical { }
2.5

@Canonical
@TupleConstructor
@AnnotationCollector(value=[ToString, TupleConstructor, EqualsAndHashCode],
mode=AnnotationCollectorMode.PREFER_EXPLICIT_MERGED)
public @interface Canonical { }
@Canonical(includeNames=false)
@ToString(excludes='last', includeNames=true)
class Person {
String first, last
String getInitials() { "${first[0]}${last[0]}" }
}
assert new Person(first: 'Dierk', last: 'Koenig').toString() ==
'Person(first:Dierk, initials:DK)'
2.5

@AnnotationCollector
@AnnotationCollector
@ToString(excludes='id,first', includeNames=true)
@EqualsAndHashCode
@TupleConstructor(excludes='id')
@interface MyCanonical {}
@MyCanonical
class Person {
UUID id = UUID.randomUUID()
String first, last
}
def agent = new Person('James', 'Bond')
assert agent.toString() == 'Person(last:Bond)'

@Lazy…
• Safe and efficient deferred construction
• Understands double checked-locking and holder class idioms
// nominally expensive resource with stats
class Resource {
private static alive = 0
private static used = 0
Resource() { alive++ }
def use() { used++ }
static stats() { "$alive alive, $used used" }
}

…@Lazy class ResourceMain {
def res1 = new Resource()
@Lazy res2 = new Resource()
@Lazy static res3 = { new Resource() }()
@Lazy(soft=true) volatile Resource res4
}
new ResourceMain().with {
assert Resource.stats() == '1 alive, 0 used'
res2.use()
res3.use()
res4.use()
assert Resource.stats() == '4 alive, 3 used'
assert res4 instanceof Resource
def expected = 'res4=java.lang.ref.SoftReference'
assert it.dump().contains(expected)
}

@InheritConstructors
• Reduced boiler-plate for scenarios where parent classes have multiple
constructors (e.g. Exceptions & PrintWriter)
@InheritConstructors
class MyPrintWriter extends PrintWriter { }
def pw1 = new MyPrintWriter(new File('out1.txt'))
def pw2 = new MyPrintWriter('out2.txt', 'US-ASCII')
[pw1, pw2].each {
it << 'foo'
it.close()
}
assert new File('out1.txt').text == new
File('out2.txt').text
['out1.txt', 'out2.txt'].each{ new File(it).delete() }

@Sortable…
• Reduced boiler-plate for Comparable classes including specialised
Comparators
@Sortable(includes = 'last,initial')
class Politician {
String first
Character initial
String last
String initials() { first[0] + initial + last[0] }
}

…@Sortable
// @Sortable(includes = 'last,initial') class Politician { ... }
def politicians = [
new Politician(first: 'Margaret', initial: 'H', last: 'Thatcher'),
new Politician(first: 'George', initial: 'W', last: 'Bush')
]
politicians.with {
assert sort()*.initials() == ['GWB', 'MHT']
def comparator = Politician.comparatorByInitial()
assert toSorted(comparator)*.initials() == ['MHT', 'GWB']
}

@Builder
• Recent Java idiom is to use "fluent-api" builders
• Often not needed for Groovy
• But if you need Java integration or want improved IDE support…
@Builder
class Chemist {
String first, last
int born
}
def builder = Chemist.builder()
def c = builder.first("Marie").last("Curie").born(1867).build()

@Builder
• Supports customizable building strategies
DefaultStrategy Creates a nested helper class with methods for each property and a
build() method
SimpleStrategy Creates chainable setters, i.e. each setter returns the object itself
ExternalStrategy For creating a builder for a class you don’t have control over, e.g. from
a library or another team in your organization.
InitializerStrateg
y
Like DefaultStrategy but when used with @CompileStatic allows type-
safe object creation

@Delegate
class Clock {
void tellTime() { println '7 am' }
}
class Alarm {
void soundAlarm() { println 'Time to wake up' }
}
class AlarmClock {
@Delegate Clock c = new Clock()
@Delegate Alarm a = new Alarm()
}
new AlarmClock().with { tellTime(); soundAlarm() }

@Delegate
class Clock {
void tellTime() { println '7 am' }
}
class Alarm {
void soundAlarm() { println 'Time to wake up' }
}
class AlarmClock {
@Delegate Clock c = new Clock()
@Delegate Alarm a = new Alarm()
}
new AlarmClock().with { tellTime(); soundAlarm() }
class AlarmClock {
private Clock c = new Clock()
private Alarm a = new Alarm()
void tellTime() {
c.tellTime()
}
void soundAlarm() {
a.soundAlarm()
}
}

@Memoized
• For making pure functions more efficient
class Calc {
def log = []
@Memoized
int sum(int a, int b) {
log << "$a+$b"
a + b
}
}
new Calc().with {
assert sum(3, 4) == 7
assert log.join(' ') == '3+4 4+4'
}

@TailRecursive
• For unravelling recursion
import groovy.transform.TailRecursive
class RecursiveCalc {
@TailRecursive
int accumulate(int n, int sum = 0) {
(n == 0) ? sum : accumulate(n - 1, sum + n)
}
}
new RecursiveCalc().with {
assert accumulate(10) == 55
}

@TailRecursiveclass RecursiveCalc {
int accumulate(int n, int sum) {
int _sum_ = sum
int _n_ = n
while (true) {
if (_n_ == 0) {
return _sum_
} else {
int __n__ = _n_
int __sum__ = _sum_
_n_ = __n__ - 1
_sum_ = __sum__ + __n__
}
}
}
int accumulate(int n) { accumulate(n, 0) }
}

@Immutable
• For unchanging data structures
@Immutable
class Genius {
}
def g1 = new Genius(firstName: 'Albert', lastName: "Einstein")
assert g1.toString() == 'Genius(Albert, Einstein)'
def g2 = new Genius('Leonardo', "da Vinci")
assert g2.firstName == 'Leonardo'
shouldFail(ReadOnlyPropertyException) {
g2.lastName = 'DiCaprio'
}

@Log @Log4j @Log4j2 @Commons @Slf4j
• For easy logging
@groovy.util.logging.Log
class Database {
def search() {
log.fine(runLongDatabaseQuery())
}
def runLongDatabaseQuery() {
println 'Calling database'
/* ... */
return 'query result'
}
}
new Database().search()

@Synchronized
• For safe synchronization
class PhoneBook1 {
private final phoneNumbers = [:]
@Synchronized
def getNumber(key) {
phoneNumbers[key]
}
@Synchronized
void addNumber(key, value) {
phoneNumbers[key] = value
}
}

@WithReadLock @WithWriteLock
• Declarative and efficient synchronization
class PhoneBook2 {
@WithReadLock
phoneNumbers[key]
}
@WithWriteLock
def addNumber(key, value) {
}
}

@AutoClone (Simple example)
• Easier cloning. With multiple styles supported: because one size doesn't fit
all for cloning on the JVM
@AutoClone
class Chef1 {
String name
List<String> recipes
Date born
}
def name = 'Heston Blumenthal'
def recipes = ['Snail porridge', 'Bacon & egg ice cream']
def born = Date.parse('yyyy-MM-dd', '1966-05-27')
def c1 = new Chef1(name: name, recipes: recipes, born: born)
def c2 = c1.clone()
assert c2.recipes == recipes

@AutoClone (Advanced example)
@TupleConstructor
@AutoClone(style=COPY_CONSTRUCTOR)
class Person {
final String name
final Date born
}
@TupleConstructor(includeSuperProperties=true, callSuper=true)
class Chef2 extends Person {
final List<String> recipes
}
def name = 'Jamie Oliver'
def recipes = ['Lentil Soup', 'Crispy Duck']
def c1 = new Chef2(name, born, recipes)
def c2 = c1.clone()
assert c2.name == name && c2.born == born && c2.recipes == recipes

@AutoCloneStyle
Style Description
CLONE
Adds a clone() method to your class. The clone() method will call
super.clone() before calling clone() on each Cloneable property of the class.
Doesn’t provide deep cloning. Not suitable if you have final properties. (Default)
SIMPLE
Adds a clone() method to your class which calls the no-arg constructor then copies
each property calling clone() for each Cloneable property. Handles inheritance
hierarchies. Not suitable if you have final properties. Doesn’t provide deep cloning.
COPY_CONSTRUCTOR
Adds a “copy” constructor, i.e. one which takes your class as its parameter, and a
clone() method to your class. The copy constructor method copies each property
calling clone() for each Cloneable property. The clone() method creates a new
instance making use of the copy constructor. Suitable if you have final properties.
Handles inheritance hierarchies. Doesn’t provide deep cloning.
SERIALIZATION
Adds a clone() method to your class which uses serialization to copy your class.
Suitable if your class already implements the Serializable or Externalizable
interface. Automatically performs deep cloning. Not as time or memory efficient. Not
suitable if you have final properties.

@AutoExternalize
@AutoExternalize
@ToString
class Composer {
String name
int born
boolean married
}
def c = new Composer(name: 'Wolfgang Amadeus Mozart',
born: 1756, married: true)
def baos = new ByteArrayOutputStream()
baos.withObjectOutputStream{ os -> os.writeObject(c) }
def bais = new ByteArrayInputStream(baos.toByteArray())
def loader = getClass().classLoader
def result
bais.withObjectInputStream(loader) { result = it.readObject().toString() }
assert result == 'Composer(Wolfgang Amadeus Mozart, 1756, true)'

@TimedInterrupt
@ThreadInterrupt
@ConditionalInterrupt
• For safer scripting
• Typically applied through compilation customizers
to user scripts rather than directly used

Contents
60
• Introduction
• Built-in AST Transforms
 Writing your own transforms

Parsing Deep Dive
• Purpose
Read source files/streams and
configure compiler
• Key classes
CompilerConfiguration
CompilationUnit
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

• Purpose
Use (ANTLR) grammar to convert source
code into token tree
• Key classes
CompilationUnit GroovyLexer
GroovyRecognizer GroovyTokenTypes
• CST Transforms
– http://java.dzone.com/articles/groovy-
antlr-plugins-better
Parsing Deep DiveInitialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing Deep Dive
• Purpose
Token tree converted into abstract syntax
tree (AST) and is the first place where we
can begin to write AST visitors
• Key classes
AntlrParserPlugin EnumVisitor
• AST Transforms
@Grab (global)
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing Deep Dive
• Purpose
Resolves classes and performs
consistency and validity checks beyond
what the grammar can provide
• Key classes
StaticVerifier ResolveVisitor
StaticImportVisitor InnerClassVisitor,
AnnotationCollector
• AST Transforms
@Lazy @Builder @Field @Log
@Memoized @PackageScope
@TailRecursive @BaseScript
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing Deep Dive
• Purpose
Finalizes the complete abstract syntax tree and typically
the last point at which you want to run a transformation
• Key classes
InnerClassCompletionVisitor EnumCompletionVisitor,
TraitComposer
• AST Transforms
@Bindable @Vetoable @Mixin @AutoClone
@ConditionalInterrupt @ThreadInterrupt
@TimedInterrupt @ListenerList @Canonical @Category
@Delegate @Bindable @Vetoable
@EqualsAndHashCode @AutoExternalize @Immutable
@IndexedProperty @Synchronized
@InheritConstructors @Sortable @WithReadLock
@WithWriteLock @Singleton @Newify @ToString
@TupleConstructor
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

• Purpose
Chooses an instruction set for the
generated bytecode, e.g. Java 5 versus
pre-Java 5
• AST Transforms
@CompileStatic @TypeChecked
Parsing Deep DiveInitialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing Deep Dive
• Purpose
Creates bytecode based Class in memory
• Key classes
OptimizerVisitor GenericsVisitor Verifier
LabelVerifier ExtendedVerifier
ClassCompletionVerifier AsmClassGenerator
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing Deep Dive
• Purpose
Binary output (.class file) written to file
system
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Parsing Deep Dive
• Purpose
Used to cleanup any resources no longer
needed
Initialization
Semantic Analysis
Parsing
Conversion
Canonicalization
Class Generation
Output
Finalization

Visitor Pattern…
• separates the object being walked (the tree) from the behavior of the walker (the
visitor)
70

…Visitor Pattern
• Consider extending ClassCodeVisitorSupport but also consider extending
AbstractASTTransformation or using ClassCodeExpressionTransformer
71

Writing a Local AST Transform…
• Create an Annotation
• Annotated with @GroovyASTTransformationClass
72
class MainTransformation{}
import org.codehaus.groovy.transform.GroovyASTTransformationClass
import java.lang.annotation.*
@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.METHOD])
@GroovyASTTransformationClass(classes = [MainTransformation])
public @interface Main {}

…Writing a Local AST Transform…
• Write your transform class
73
@GroovyASTTransformation(phase = CompilePhase.INSTRUCTION_SELECTION)
class MainTransformation implements ASTTransformation {
private static final ClassNode[] NO_EXCEPTIONS =
ClassNode.EMPTY_ARRAY
private static final ClassNode STRING_ARRAY =
ClassHelper.STRING_TYPE.makeArray()
void visit(ASTNode[] astNodes, SourceUnit sourceUnit) {
// use guard clauses as a form of defensive programming
if (!astNodes) return
if (!astNodes[0] || !astNodes[1]) return
if (!(astNodes[0] instanceof AnnotationNode)) return
if (astNodes[0].classNode?.name != Main.class.name) return
if (!(astNodes[1] instanceof MethodNode)) return
...

…Writing a Local AST Transform…
74
...
MethodNode annotatedMethod = astNodes[1]
ClassNode declaringClass = annotatedMethod.declaringClass
def callMethod = callX(ctorX(declaringClass), annotatedMethod.name)
Statement body = block(stmt(callMethod))
def visibility = ACC_STATIC | ACC_PUBLIC
def parameters = params(param(STRING_ARRAY, 'args'))
declaringClass.addMethod('main', visibility, VOID_TYPE,
parameters, NO_EXCEPTIONS, body)
}
}

…Writing a Local AST Transform
• Use the transform
75
class Greeter {
@Main
def greet() {
println "Hello from the greet() method!"
}
}

…Writing a Local AST Transform
• Use the transform
76
class Greeter {
@Main
def greet() {
}
}
new GroovyShell(getClass().classLoader).evaluate
'''
class Greeter {
@Main
def greet() {
}
}
'''

Creating AST…
• By hand (raw)
• Verbose, full IDE supported
77
import org.codehaus.groovy.ast.*
import org.codehaus.groovy.ast.stmt.*
import org.codehaus.groovy.ast.expr.*
new ReturnStatement(
new ConstructorCallExpression(
ClassHelper.make(Date),
ArgumentListExpression.EMPTY_ARGUMENTS
)
)

…Creating AST…
• By hand (with helper utility methods)
• Concise, not everything has concise form (yet)
78
import static org.codehaus.groovy.ast.tools.GeneralUtils.*
import static org.codehaus.groovy.ast.ClassHelper.*
returnS(ctorX(make(Date)))

…Creating AST…
• With ASTBuilder (from a specification/DSL)
• Requires AST knowledge, limited IDE support
79
import org.codehaus.groovy.ast.builder.AstBuilder
def ast = new AstBuilder().buildFromSpec {
returnStatement {
constructorCall(Date) {
argumentList {}
}
}
}

…Creating AST…
• With ASTBuilder (from a String)
• Concise, intuitive, can't create everything, limited IDE support
80
def ast = new AstBuilder().buildFromString('new Date()')

…Creating AST…
• With ASTBuilder (from code)
• Clear, concise, some entities cannot be created, IDE assistance
81
def ast = new AstBuilder().buildFromCode {
new Date()
}

…Creating AST
• ASTBuilder limitations
• Great for prototyping, not always suitable for production transforms
• Groovy technology, can be slow, subject to global transforms
• Sometimes wasteful, e.g. you might need to create more than you need
such as creating a whole class to then pull out one method
• Some flavors don't support arbitrary node types, don't make it easy to
handle interactions with existing class nodes, don't make it easy to support
redirects or generics, nor allow you to properly set the line/column numbers
resulting in difficult to debug AST transform and cryptic compilation errors
82

Feature Interactions
• Consider Groovy's @ToString annotation transform which runs at the end of the
Canonicalization phase
• Now suppose we want to create a @Trace annotation transform which when
placed on a class will "instrument" each method with "trace" println statements,
e.g. this:
becomes:
• What behaviour should I expect calling toString() if @Trace runs at the end of
Semantic Analysis? Canonicalization? Instruction Selection?
83
def setX(x) {
this.x = x
}
def setX(x) {
println "setX begin"
this.x = x
println "setX end"
}

Testing AST Transforms
• Consider using ASTTest
• Test both the AST tree and the end-to-end behavior
• Consider writing defensive guards
• Use GroovyConsole
84

Design Considerations
• Don't reuse ClassNodes
• Compile-time vs Runtime trade-offs and typing
• Feature Interactions/Fragility
• Beware Complexity
• Risks of introducing bugs
• Avoid global transforms (unless needed)
• GroovyConsole is your friend (AST/bytecode)
• Use addError for errors
• Retain line/column number information when transforming
• Watch variable scoping
85

Further Information
• Documentation
• http://docs.groovy-lang.org/docs/groovy-
latest/html/documentation/#_compile_time_metaprogramming
• Macro Groovy
• https://github.com/bsideup/MacroGroovy
• https://github.com/bsideup/groovy-macro-methods
• AST Workshop
• http://melix.github.io/ast-workshop/
86

More Information: Groovy in Action, 2ed
87
Manning promo code: ctwspringo2gx

Bonus Material

Groovy and "fluent-api" builders
• A common idiom in recent times for Java is to use an inner helper class and
accompanying "fluent-api" to reduce ceremony when creating Java classes
with many parameters
– But Groovy's named parameters greatly reduces this need
class Chemist {
String first
String last
int born
}
def c = new Chemist(first: "Marie", last: "Curie", born: 1867)
assert c.first == "Marie"
assert c.last == "Curie"
assert c.born == 1867

@Builder…
• But if you need Java integration or want improved IDE support…
import groovy.transform.builder.Builder
@Builder
class Chemist {
String first
String last
int born
}
def builder = Chemist.builder()
def c = builder.first("Marie").last("Curie").born(1867).build()
assert c.first == "Marie"
assert c.last == "Curie"
assert c.born == 1867

…@Builder…
• And it supports customizable building strategies
Strategy Description
DefaultStrategy
Creates a nested helper class for instance creation. Each method in the
helper class returns the helper until finally a build() method is called
which returns a created instance.
SimpleStrategy Creates chainable setters, i.e. each setter returns the object itself after
updating the appropriate property.
ExternalStrategy
Allows you to annotate an explicit builder class while leaving some buildee
class being built untouched. This is appropriate when you want to create a
builder for a class you don’t have control over, e.g. from a library or
another team in your organization.
InitializerStrategyCreates a nested helper class for instance creation which when used with
@CompileStatic allows type-safe object creation.

…@Builder
• Type-safe construction using phantom types (*if* you need it)
@Builder(builderStrategy=InitializerStrategy)
@Immutable
class Chemist {
String first, last
int born
}
@CompileStatic
def solution() {
def init = Chemist.createInitializer().first("Marie").last("Curie").born(1867)
new Chemist(init).with {
assert first == "Marie"
assert last == "Curie"
assert born == 1867
}
}
solution()

@Delegate (motivation)
• Anything wrong
with this?
class NoisySet extends HashSet {
@Override
boolean add(item) {
println "adding $item"
super.add(item)
}
@Override
boolean addAll(Collection items) {
items.each { println "adding $it" }
super.addAll(items)
}
}

• Anything wrong
with this?
• Could we fix this
implementation?
@Override
boolean add(item) {
super.add(item)
}
@Override
super.addAll(items)
}
}

• Anything wrong
with this?
• Could we fix this
implementation?
• What about using
the delegate
pattern written
by hand?
@Override
boolean add(item) {
super.add(item)
}
@Override
super.addAll(items)
}
}

@Delegate
• For declarative but flexible use of the delegate pattern
class NoisySet {
@Delegate
Set delegate = new HashSet()
@Override
boolean add(item) {
delegate.add(item)
}
@Override
delegate.addAll(items)
}
}
Set ns = new NoisySet()
ns.add(1)
ns.addAll([2, 3])
assert ns.size() == 3

@Delegate annotation parameters…
interfaces Set this parameter to true to make the owner class implement the same
interfaces as the delegate, which is the default behavior. To make the
owner not implement the delegate interfaces, set this parameter to false.
deprecated
Set this parameter to true to have the owner class delegate methods
marked as @Deprecated in the delegate. By default @Deprecated
methods are not delegated.
methodAnnotations
Set to true if you want to carry over annotations from the methods of the
delegate to your delegating method. By default, annotations are not
carried over. Currently Closure annotation members are not supported.
parameterAnnotations
Set to true if you want to carry over annotations from the method
parameters of the delegate to your delegating method. By default,
annotations are not carried over. Currently Closure annotation members
are not supported.

…@Delegate annotation parameters
Only one of 'includes', 'includeTypes', 'excludes' or 'excludeTypes' should be used
excludes List of method and/or property names to exclude when
delegating.
excludeTypes List of interfaces containing method signatures to exclude
when delegating.
includes List of method and/or property names to include when
delegating.
includeTypes List of interfaces containing method signatures to exclude
when delegating.

@Memoized
• For making pure functions more efficient
class Calc {
def log = []
@Memoized
int sum(int a, int b) {
log << "$a+$b"
a + b
}
}
new Calc().with {
assert log.join(' ') == '3+4 4+4'
}

@TailRecursive
• For unravelling recursion
import groovy.transform.TailRecursive
class RecursiveCalc {
@TailRecursive
int accumulate(int n, int sum = 0) {
(n == 0) ? sum : accumulate(n - 1, sum + n)
}
}
new RecursiveCalc().with {
assert accumulate(10) == 55
}

@TailRecursiveclass RecursiveCalc {
int accumulate(int n, int sum) {
int _sum_ = sum
int _n_ = n
while (true) {
if (_n_ == 0) {
return _sum_
} else {
int __n__ = _n_
int __sum__ = _sum_
_n_ = __n__ - 1
_sum_ = __sum__ + __n__
}
}
}
int accumulate(int n) { accumulate(n, 0) }
}

@Immutable
• For unchanging data structures
@Immutable
class Genius {
}
def g1 = new Genius(firstName: 'Albert', lastName: "Einstein")
assert g1.toString() == 'Genius(Albert, Einstein)'
def g2 = new Genius('Leonardo', "da Vinci")
assert g2.firstName == 'Leonardo'
shouldFail(ReadOnlyPropertyException) {
g2.lastName = 'DiCaprio'
}

@Log @Log4j @Log4j2 @Commons @Slf4j
• For easy logging
@groovy.util.logging.Log
class Database {
def search() {
log.fine(runLongDatabaseQuery())
}
def runLongDatabaseQuery() {
println 'Calling database'
/* ... */
return 'query result'
}
}
new Database().search()

@Synchronized
• For safe synchronization
class PhoneBook1 {
@Synchronized
phoneNumbers[key]
}
@Synchronized
void addNumber(key, value) {
}
}

@WithReadLock @WithWriteLock
• Declarative and efficient synchronization
class PhoneBook2 {
@WithReadLock
phoneNumbers[key]
}
@WithWriteLock
def addNumber(key, value) {
}
}

@AutoClone (Simple example)
• Easier cloning. With multiple styles supported: because one size doesn't fit
all for cloning on the JVM
@AutoClone
class Chef1 {
String name
List<String> recipes
Date born
}
def name = 'Heston Blumenthal'
def recipes = ['Snail porridge', 'Bacon & egg ice cream']
def c1 = new Chef1(name: name, recipes: recipes, born: born)
def c2 = c1.clone()
assert c2.recipes == recipes

@AutoClone (Advanced example)
@TupleConstructor
class Person {
final String name
final Date born
}
@TupleConstructor(includeSuperProperties=true, callSuper=true)
class Chef2 extends Person {
final List<String> recipes
}
def name = 'Jamie Oliver'
def recipes = ['Lentil Soup', 'Crispy Duck']
def c1 = new Chef2(name, born, recipes)
def c2 = c1.clone()
assert c2.name == name && c2.born == born && c2.recipes == recipes

@AutoCloneStyle
Style Description
CLONE
Adds a clone() method to your class. The clone() method will call
super.clone() before calling clone() on each Cloneable property of the class.
Doesn’t provide deep cloning. Not suitable if you have final properties. (Default)
SIMPLE
Adds a clone() method to your class which calls the no-arg constructor then copies
each property calling clone() for each Cloneable property. Handles inheritance
hierarchies. Not suitable if you have final properties. Doesn’t provide deep cloning.
COPY_CONSTRUCTOR
Adds a “copy” constructor, i.e. one which takes your class as its parameter, and a
clone() method to your class. The copy constructor method copies each property
calling clone() for each Cloneable property. The clone() method creates a new
instance making use of the copy constructor. Suitable if you have final properties.
Handles inheritance hierarchies. Doesn’t provide deep cloning.
SERIALIZATION
Adds a clone() method to your class which uses serialization to copy your class.
Suitable if your class already implements the Serializable or Externalizable
interface. Automatically performs deep cloning. Not as time or memory efficient. Not
suitable if you have final properties.

@AutoExternalize
@AutoExternalize
@ToString
class Composer {
String name
int born
boolean married
}
def c = new Composer(name: 'Wolfgang Amadeus Mozart',
born: 1756, married: true)
def baos = new ByteArrayOutputStream()
baos.withObjectOutputStream{ os -> os.writeObject(c) }
def bais = new ByteArrayInputStream(baos.toByteArray())
def loader = getClass().classLoader
def result
bais.withObjectInputStream(loader) { result = it.readObject().toString() }
assert result == 'Composer(Wolfgang Amadeus Mozart, 1756, true)'

@TimedInterrupt
@ThreadInterrupt
• For safer scripting
• Typically applied through compilation
customizers to user scripts rather than directly
used

@TimedInterrupt
@TimedInterrupt(value = 520L, unit = MILLISECONDS)
class BlastOff1 {
def log = []
def countdown(n) {
sleep 100
log << n
if (n == 0) log << 'ignition'
else countdown(n - 1)
}
}
def b = new BlastOff1()
Thread.start {
try {
b.countdown(10)
} catch (TimeoutException ignore) {
b.log << 'aborted'
}
}.join()
assert b.log.join(' ') == '10 9 8 7 6 aborted'

@ThreadInterrupt
@ThreadInterrupt
class BlastOff2 {
def log = []
def countdown(n) {
Thread.sleep 100
log << n
if (n == 0) log << 'ignition'
else countdown(n - 1)
}
}
def t1 = Thread.start {
try {
b.countdown(10)
} catch(InterruptedException ignore) {
b.log << 'aborted'
}
}
sleep 570
t1.interrupt()
t1.join()

@ConditionalInterrupt({ count <= 5 })
class BlastOff3 {
def log = []
def count = 10
def countdown() {
while (count != 0) {
log << count
count--
}
log << 'ignition'
}
}
try {
b.countdown()
} catch (InterruptedException ignore) {
b.log << 'aborted'
}

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