Practical Groovy DSL

Guillaume Laforge / Groovy Project Manager / SpringSource

Practical Domain-Specific
Languages with Groovy
All the techniques to create your own DSL.

1

Guillaume Laforge
• Groovy Project Manager
• JSR-241 Spec Lead
• Head of Groovy Development
at SpringSource
• Initiator of the Grails framework
• Co-author of Groovy in Action
• Speaker: JavaOne, QCon, JavaZone, Sun TechDays,
Devoxx, The Spring Experience, JAX, Dynamic
Language World, IJTC, and more...

A few words about Groovy
• Groovy is a dynamic language for the JVM
• with a Meta Object Protocol
• compiles directly to bytecode, seamless Java interop
• Open Source ASL 2 project hosted at Codehaus
• Relaxed grammar derived from Java 5
• + borrowed good ideas from Ruby, Python, Smalltalk
• Fast... for a dynlang on the JVM
• Closures, properties, optional typing, BigDecimal
by default, nice wrapper APIs, and more...

a
d
n
e
g
A • The context and
the usual issues we face
• Some real-life examples of
Domain-Speciﬁc Languages
• Groovy’s DSL capabilities
• Integrating a DSL
in your application
• Considerations to remember
when designing your own DSL

Subject Matter Experts,
Business analysts...

Developer producing
LOLCODE

HAI
CAN HAS STDIO?
I HAS A VAR
IM IN YR LOOP
UP VAR!!1
VISIBLE VAR
IZ VAR BIGGER THAN 10?
KTHXBYE
IM OUTTA YR LOOP
KTHXBYE

And in the end...
...nobody understands each other

DSL: a potential solution?
• Use a more expressive language than a general purpose one

• Share a common metaphore of understanding between developers
and subject matter experts

• Have domain experts help with the design of the business logic of
an application

• Avoid cluttering business code with too much boilerplate
technical code

• Cleanly separate business logic from application code

• Let business rules have their own lifecycle

Towards more readibility (1)

20%

Towards more readibility (2)

80%

A collection of DSLs

• In our everyday life, we’re surrounded by DSLs
• Technical dialects
• Notations
• Business languages

Real-life Groovy examples
• Anti-malaria drug resistance simulation
• Human Resources employee skills representation
• Insurance policies risk calculation engine
• Loan acceptance rules engine for a ﬁnancial platform
• Mathematica-like lingua for nuclear safety simulations
• Market data feeds evolution scenarios
• and more...

A ﬂexible & malleable syntax
• No need to write full-blown classes, use scripts
• Optional typing (def)
• in scripts, you can even omit the def keyword
• Native syntax constructs
• Parentheses & semi-colons are optional
• Named arguments
• BigDecimal by default for decimal numbers
• Closures for custom control structures
• Operator overloading

Scripts vs classes
• Hide all the boilerplate technical code
• an end-user doesn’t need to know about classes
• public class Rule {
public static void main(String[] args) {
System.out.println(“Hello”);
}
}

• println “Hello”

Optional typing
• No need to bother with types or even generics
• unless you want to!
• Imagine an interest rate lookup table method
returning some generiﬁed type:

• Rate<LoanType, Duration, BigDecimal>[]
lookupTable() { ... }
def table = lookupTable()

• No need to repeat the horrible generics type info!

Native syntax constructs
• Lists
• [Monday, Tuesday, Wednesday]
• Maps
• [CA: ‘California’, TX: ‘Texas’]
• Ranges
• def bizDays = Monday..Friday
• def allowedAge = 18..65
• You can create your own custom ranges

Optional parens & semis
• Make statements and expressions
look more like natural languages

• move(left);

•move left

Named arguments
• In Groovy you can mix named and unnamed
arguments for method parameters
• named params are actually put in a map parameter
• plus optional parens & semis

• take 1.pill,
of: Chloroquinine,
after: 6.hours

• Corresponds to a method signature like:
• def take(Map m, MedicineQuantity mq)

BigDecimal by default
• Main reason why ﬁnancial institutions often decide to
use Groovy for their business rules!
• Although these days rounding issues are overrated!
• Java vs Groovy for a simple interpolation equation
• BigDecimal uMinusv = c.subtract(a);
BigDecimal vMinusl = b.subtract(c);
BigDecimal uMinusl = a.subtract(b);
return e.multiply(uMinusv)
.add(d.multiply(vMinusl))
.divide(uMinusl, 10, BigDecimal.ROUND_HALF_UP);

• (d * (b - c) + e * (c - a)) / (a - b)

Custom control structures,
thanks to closures
• When closures are last, they can be put “out” of the
parentheses surrounding parameters

• unless (account.balance > 100.euros,
{ account.debit 100.euros })

• unless (account.balance > 100.euros) {
account.debit 100.euros
}

• Signature def unless(boolean b, Closure c)

Operator overloading
• Currency amounts
a+b a.plus(b)

•
a-b a.minus(b)
15.euros + 10.dollars
a*b a.multiply(b)

• Distance handling
a/b a.divide(b)
a%b a.modulo(b)
• 10.kilometers - 10.meters
a ** b a.power(b)
a|b a.or(b)
• Workﬂow, concurrency
a&b a.and(b)

•
a^b a.xor(b) taskA | taskB & taskC
a[b] a.getAt(b)

• Credit an account
a << b a.leftShift(b)
a >> b a.rightShift(b)
• account << 10.dollars
+a a.positive()
account += 10.dollars
-a a.negative()
account.credit 10.dollars
~a a.bitwiseNegate()

Groovy’s dynamic heart:

The MOP!
MetaObject Protocol

Groovy’s MOP
• All the accesses to methods, properties, constructors,
operators, etc. can be intercepted thanks to the MOP
• While Java’s behavior is hard-wired at compile-
time in the class
• Groovy’s runtime behavior is adaptable at
runtime through the metaclass.
• Different hooks for changing the runtime behavior
• GroovyObject, custom MetaClass implementation,
categories, ExpandoMetaClass

GroovyObject
• All instances of classes created in Groovy implement
the GroovyObject interface:
• getProperty(String name)
• setProperty(String name, Object value)
• invokeMethod(String name, Object[]
params)
• getMetaClass()
• setMetaClass(MetaClass mc)

• A GO can have “pretended” methods and properties

MetaClass
• The core of Groovy’s MOP system
• invokeConstructor()
• invokeMethod() and invokeStaticMethod()
• invokeMissingMethod()
• getProperty() and setProperty()
• getAttribute() and setAttribute()
• respondsTo() and hasProperty()

• MetaClasses can change the behavior of existing third-
party classes — even from the JDK

ExpandoMetaClass
• A DSL for MetaClasses!
• MoneyAmount.metaClass.constructor = { ... }
Number.metaClass.getDollars = { ... }
Distance.metaClass.toMeters = { ... }
Distance.metaClass.static.create = { ... }

• To avoid repetition of Type.metaClass, you can pass a
closure to metaClass { ... }
• The delegate variable in closure represents the
current instance, and it the default parameter

The Groovy MarkupBuilder
• def mkp = new MarkupBuilder()
mkp.html {
head {
title “Groovy in Action”
}
body {
div(width: ‘100’) {
p(class: ‘para) {
span “Best book ever!”
}
}
}
}

A builder for HR
• softskills {
ideas {
capture 2
formulate 3
}
...
}
knowhow {
languages {
java 4
groovy 5
}
...
}

Builders
• Builders are...
• a mechanism for creating any tree-structered graph
• the realization of the GoF builder pattern at the
syntax level in Groovy
• simply a clever use of chained method invocation,
closures, parentheses omission, and use of the
GroovyObject methods
• Existing builders
• XML, Object graph, Swing, Ant, JMX, and more...

The clever trick
• GroovyObject#invokeMethod() is used to catch all
non-existing method calls in the context of the
builder
• The nesting of closures visually shows the level of
nesting / depth in the tree
• builder.m1(attr1:1, attr2:2, { builder.m2(..., {...}) }
becomes equivalent to
builder.m1(attr1:1, attr2:2) { m2(...) {...} }
thanks to parens omission

Adding properties to numbers
• Three possible approaches
• create a Category
• a category is a kind of decorator for default MCs
• create a custom MetaClass
• a full-blown MC class to implement and to set on
the POGO instance
• use ExpandoMetaClass
• friendlier DSL approach but with a catch

With a Category
• class DistanceCategory {
static Distance getMeters(Integer self) {
new Distance(self, Unit.METERS)
}
}

use(DistanceCategory) {
100.meters
}

• Interesting scope: thread-bound & lexical
• But doesn’t work across the hierarchy of classes
• ie. subclasses won’t beneﬁt from the new property

With an ExpandoMetaClass
• Number.metaClass.getMeters = {->
new Distance(delegate, Unit.METERS)
}

100.meters

• Works for the class hierarchy for POJOs, and a ﬂag
exists to make it work for POGOs too
• But the catch is it’s really a global change, so beware
EMC enhancements collisions

Compile-time metaprogramming
• Groovy 1.6 introduced AST Transformations
• Compile-time == No runtime performance penalty!

Transformation

AST Transformations
• Two kinds of transformations
• Global transformations
• applicable to all compilation units
• Local transformations
• applicable to marked program elements
• using speciﬁc marker annotations

Example #1: @Singleton
• Let’s revisit this evil{(anti-)pattern
public class Evil


public static final Evil instance = new Evil ();
private Evil () {}
Evil getInstance() { return instance; }
}

• In Groovy
@Singleton() class Evil {}


• Also a “lazy” version
@Singleton(lazy = true) class Evil {}


Example #2: @Delegate
Not just for Managers
• You can delegate to ﬁelds of your classes
• class Employee {
def doTheWork() { “done” }
}
class Manager {
@Delegate
Employee slave = new Employee()
}
def god = new Manager()
assert god.doTheWork() == “done”

• Damn manager who will get all the praiser...

Global transformations
• Implement ASTTransformation
• Annotate the transfo specifying a compilation phase
• @GroovyASTTransformation(phase=CompilePhase.CONVERSION)
public class MyTransformation
implements ASTTransformation {
public void visit(ASTNode[] nodes, SourceUnit unit)
{ ... }
}

• For discovery, create the file META-INF/services/
org.codehaus.groovy.transform.ASTTransformation
• Add the fully qualified name of the class in that file

Local transformations
• Same approach as Globale transformations
• But you don’t need the META-INF ﬁle
• Instead create an annotation to specify on which
element the transformation should apply

• @Retention(RetentionPolicy.SOURCE)
@Target([ElementType.METHOD])
@GroovyASTTransformationClass(
[quot;fqn.MyTransformationquot;])
public @interface WithLogging {...}

Example: the Spock framework
• Changing the semantics of the original code
• But keeping a valid Groovy syntax
• @Speck
class HelloSpock {
def quot;can you figure out what I'm up to?quot;() {
expect:
name.size() == size

where:
name << [quot;Kirkquot;, quot;Spockquot;, quot;Scottyquot;]
size << [4, 5, 6]
}
}

Various integration mechanisms
• Java 6’s javax.script.* APIs (aka JSR-223)
• Spring’s language namespace
• Groovy’s own mechanisms
• But a key idea is to externalize those DSL programs
• DSL programs can have their own lifecycle
• no need to redeploy an application because of a
rule change
• business people won’t see the technical code

Java 6’s javax.script.* API
• Groovy 1.6 provides its own implementation of the
javax.script.* API

• ScriptEngineManager mgr =
new ScriptEngineManager();
ScriptEngine engine =
mgr.getEngineByName(“Groovy”);

String result = (String)engine.eval(“2+3”);

Spring’s lang namespace
• POGOs (Plain Old Groovy Objects) can be pre-
compiled as any POJO and used interchangeably with
POJOs in a Spring application
• But Groovy scripts & classes can be loaded at runtime
through the <lang:groovy/> namespace and tag
• Reloadable on change
• Customizable through a custom MetaClass
• <lang:groovy id=quot;eventsquot;
script-source=quot;classpath:dsl/eventsChart.groovyquot;
customizer-ref=quot;eventsMetaClassquot; />

Groovy’s own mechanisms
• Eval
• for evaluating simple expressions
• GroovyShell
• for more complex scripts and DSLs
• GroovyClassLoader
• the most powerful mechanism

Eval
• Simple mechanism to evaluate math-like formulas
•Eval.me ( ‘3*4’)
Eval.x (1, ‘3*x + 4’)
Eval.xy (1, 2, ‘x + y’)
Eval.xyz(1, 2, 3, ‘x * y - z’)

GroovyShell
• A Binding provides a context of execution
• can implement lazy evaluation if needed
• A base script class can be speciﬁed
• def binding = new Binding()
binding.mass = 22.3
binding.velocity = 10.6
def shell = new GroovyShell(binding)
shell.evaluate(“mass * velocity ** 2 / 2”)

GroovyClassLoader
• Most powerful mechanism
• could also visit or change the AST
• scripts & classes can be loaded from elsewhere
• more control on compilation
• GroovyClassLoader gcl =
new GroovyClassLoader();
Class clazz = gcl.parseClass(
new File(“f.groovy”));
GroovyObject instance =
(GroovyObject)clazz.newInstance();
instance.setMetaClass(customMC);

Externalize business rules
• Although Groovy DSLs can be embedded in normal
Groovy classes, you should externalize them
• Store them elsewhere
• in a database, an XML ﬁle, etc.
• Beneﬁts
• Business rules are not entangled
in technical application code
• Business rules can have their own lifecycle,
without requiring application redeployments

Start small, with key concepts
Beware overengineering!

Grow your language progressively

Get your hands dirty
Play with the end-users

Let your DSL ﬂy,
it’s not yours,
it’s theirs!

Tight feedback loop
Iterative process

Stay humble.
You can’t get it right the ﬁrst time.
Don’t design alone at your desk
Involve the end users from the start

Various levels of sandboxing

• Groovy supports the usual Java Security Managers
• Use metaprogramming tricks to prevent calling /
instanciating certain classes

• Create a special GroovyClassLoader AST code visitor
to ﬁlter only the nodes of the AST you want to keep
• ArithmeticShell in Groovy’s samples

Test, test, test!
• Don’t just test for nominal cases
• Explicitely test for errors!
• Ensure end-users get meaninful error messages

a
d
n
e
g
A
• Summary
• Questions & Answers

Summary
• Groovy’s a great fit for Domain-Specific Languages
• Malleable & flexible syntax
• Full object-orientation
• Metaprogramming capabilities
• Runtime metaprogramming
• Compile-time metaprogramming
• Groovy’s very often used for mission-critical DSLs

?
I kan haz my cheezburgr naw?
Or do ya reely haz keshtionz?

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