© 2010 SpringSource, A division of VMware. All rights reserved© 2010 SpringSource, A division of VMware. All rights reserved

Design Your Own Domain-Specific Languagewith Groovy!

Guillaume Laforge

Groovy Project Manager

lundi 22 mars 2010

Guillaume Laforge

• Groovy Project Manager

• JSR-241 Spec Lead

• Head of Groovy Developmentat SpringSource / VMWare

• Initiator of the Grails framework

• Co-author of Groovy in Action

• Speaker: JavaOne, QCon, JavaZone, Sun TechDays, Devoxx, The Spring Experience, SpringOne, JAX, Dynamic Language World, IJTC, and more...

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!Groovy is a dynamic language for the JVM

• with a Meta Object Protocol

• compiles directly to bytecode

• provides seamless Java interoperability

!Groovy was created in 2003, is hosted at Codehaus, and is under the Apache license

!Relaxed grammar deriving from Java 5

• annotations, generics, static imports, enums, varargs...

• borrowed good ideas from Ruby, Python, Smalltalk

• flat learning curve for Java developers

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Groovy in a few words

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The Context

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Subject Matter Expers, Business Analysts...

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HAICAN HAS STDIO?I HAS A VARIM IN YR LOOP UP VAR!!1 VISIBLE VAR IZ VAR BIGGER THAN 10? KTHXBYEIM OUTTA YR LOOPKTHXBYE

Developer producing LOLCODE

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Lots of Languages...

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And in the end... nobody understands

each other

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Expressing Requirements...

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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

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Towards more readibility (1)

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Towards more readibility (1)

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Towards more readibility (1)

20%

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Towards more readibility (2)

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Towards more readibility (2)

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Towards more readibility (2)

80%

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A collection of DSLs

! In our everyday life, we’re surrounded by DSLs

• Technical dialects

• Notations

• Business languages

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Technical dialects

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SQL

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^[\w-\.]+@([\w-]){2,4}$

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Notations

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1. e4 e5

2. Nf3 Nc6 3. Bb5 a6

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L2 U F-1 B L2 F B -1 U L2

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Visual!

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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...

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Three levels of techniques

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Flexible & malleable syntax

• scripts

• optional typing

• native syntax constructs

• parens / semi ommission

• named arguments

• BigDecimal

• operator overloading

• closures

Meta-programming

• POGO

• categories

• builders

• custom MetaClass

• ExpandoMetaClass

AST transformations

• AST traversal

• local transformations

• global transformations

• hooks into Antlr

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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

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!Hide all the boilerplate technical code

• an end-user doesn’t need to know about classes

Scripts vs classes

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public class Rule {

public static void main(String[] args) {

System.out.println(“Hello”);

}}

println “Hello”

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Optional typing

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!No need to bother with types or even generics

• unless you want to!

! Imagine an interest rate lookup table method returning some generified type:

!No need to repeat the horrible generics type info!

Rate<LoanType, Duration, BigDecimal>[]

lookupTable() { ... }

def table = lookupTable()

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Native syntax constructs

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!Lists

!Maps

!Ranges

• You can create your own custom ranges

[Monday, Tuesday, Wednesday]

[CA: ‘California’, TX: ‘Texas’]

def bizDays = Monday..Friday

def allowedAge = 18..65

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!Make statements and expressions look more like natural languages

Optional parens & semis

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move(left);

move left

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! 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

!Corresponds to a method signature like:

Named arguments

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take 1.pill,

of: Chloroquinine,

after: 6.hours

def take(Map m, MedicineQuantity mq)

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!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 by default

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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)

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!When closures are last, they can be put “out” of the parentheses surrounding parameters

!Signature

Custom control structures,thanks to closures

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unless (account.balance > 100.euros,

{ account.debit 100.euros })

unless (account.balance > 100.euros) {

account.debit 100.euros

}

def unless(boolean b, Closure c)

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Operator overloading

•Currency amounts

•Distance handling

•Workflow, concurrency

•Credit an account

a + b a.plus(b)

a - b a.minus(b)

a * b a.multiply(b)

a / b a.divide(b)

a % b a.modulo(b)

a ** b a.power(b)

a | b a.or(b)

a & b a.and(b)

a ^ b a.xor(b)

a[b] a.getAt(b)

a << b a.leftShift(b)

a >> b a.rightShift(b)

+a a.unaryPlus()

-a a.unaryMinus()

~a a.bitwiseNegate()

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15.euros + 10.dollars

10.kilometers - 10.meters

taskA | taskB & taskC

account << 10.dollars

account += 10.dollars

account.credit 10.dollars

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The MOP (Meta-Object Protocol)

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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

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!All instances of classes created in Groovy implement the GroovyObject interface:

!A GO can have “pretended” methods and properties

GroovyObject

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get/setProperty(String name)

invokeMethod(String name, Object[] params)

propertyMissing(String name)

methodMissing(String name, Object[] params)

get/setMetaClass(MetaClass mc)

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MetaClass

!The core of Groovy’s MOP system

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

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invokeConstructor()

invokeMethod() and invokeStaticMethod()

invokeMissingMethod()

getProperty() and setProperty()

getAttribute() and setAttribute()

respondsTo() and hasProperty()

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!A DSL for MetaClasses!

!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

ExpandoMetaClass

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MoneyAmount.metaClass.constructor = { ... }

Number.metaClass.getDollars = { ... }

Distance.metaClass.toMeters = { ... }

Distance.metaClass.static.create = { ... }

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The Builder pattern

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A builder for HR

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softskills {

ideas { capture 2

formulate 3

} ...}knowhow { languages {

java 4

groovy 5

} ...}

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A builder for HR

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softskills {

ideas { capture 2

formulate 3

} ...}knowhow { languages {

java 4

groovy 5

} ...}

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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...

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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

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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

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With a Category

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class DistanceCategory {

static Distance getMeters(Integer self) {

new Distance(self, Unit.METERS)

}}

use(DistanceCategory) {

100.meters

}

! Interesting scope: thread-bound & lexical

!Have to surround with “use”

• but there are ways to hide it

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!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

With an ExpandoMetaClass

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Number.metaClass.getMeters = {->

new Distance(delegate, Unit.METERS)

}

100.meters

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!Groovy 1.6 introduced AST Transformations

!Compile-time == No runtime performance penalty!

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Transformation

Compile-time Metaprogramming

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Compile-time metaprogramming

!With metaprogramming, Groovy’s able to modify the behaviour of programs... at runtime

!Groovy 1.6 introduced AST Transformations

• AST: Abstract Syntax Tree

• Ability to change what’s being compiled at compile-time!

• No runtime impact!

• Lets you change the semantics of your programs!

• Nice way of implementing patterns and removing boiler-plate technical code

!Two kinds of transformations: global and local

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AST Transformations

!Two kinds of transformations

• Global transformations

• applicable to all compilation units

• Local transformations

• applicable to marked program elements

• using specific marker annotations

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AST Transformations in Groovy 1.6

!Several (local) transformations finds their way

•@Singleton — okay, not really a pattern :-)

•@Immutable, @Lazy, @Delegate

•@Newify

•@Category and @Mixin

•@PackageScope

• Swing’s @Bindable and @Vetoable

• Grape’s @Grab

!Let’s have a look at some of them

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The @Singleton anti-pattern

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!The evil Java singleton

!In Groovy now:

!A lazy version also:

public class Evil {

public static final Evil instance = new Evil();

privavte Evil() {}

Evil getInstance() { return instance; }

}

@Singleton class Evil {}

@Singleton(lazy = true) class Evil {}

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@Immutable

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!To properly implement immutable classes

• No mutators (state musn’t change)

• Private final fields

• Defensive copying of mutable components

• Proper equals() / hashCode() / toString() for comparisons, or for keys in maps, etc.

@Immutable class Coordinates {

Double lat, lng

}def c1 = new Coordinates(lat: 48.8, l

ng: 2.5)

def c2 = new Coordinates(48.8, 2.5)

assert c1 == c2

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@Lazy, not just for lazy dudes!

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!When you need to lazily evaluate or instantiate complex data structures for class fields, mark them with the @Lazy annotation

!Groovy will handle the boiler-plate code for you!

class Dude {

@Lazy pets = retrieveFromSlowDB()

}

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@DelegateNot just for managers!

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!You can delegate to fields of your class

•Think multiple inheritance

!Damn manager will get all the praise...

class Employee {

def doTheWork() { "done" }

}class Manager {

@Delegate Employee slave = new Employee()

}def god = new Manager()

assert god.doTheWork() == "done"

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Global transformations

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! Implement ASTTransformation

!Annotate the transfo specifying a compilation phase

!For discovery, create the file META-INF/services/org.codehaus.groovy.transform.ASTTransformation

!Add the fully qualified name of the class in that file

@GroovyASTTransformation(phase=CompilePhase.CONVERSION)

public class MyTransformation

implements ASTTransformation {

public void visit(ASTNode[] nodes, SourceUnit unit)

{ ... }

}

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Local transformations

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!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(

["fqn.MyTransformation"])

public @interface WithLogging {...}

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Example: the Spock framework

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!Changing the semantics of the original code

!But keeping a valid Groovy syntax

!Check out http://www.spockframework.org

@Speckclass HelloSpock {

def "can you figure out what I'm up to?"() {

expect:

name.size() == size

where:

name << ["Kirk", "Spock", "Scotty"]

size << [4, 5, 6]

}}

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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

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Java 6’s javax.script.* API

!Groovy 1.6 provides its own implementation of the javax.script.* API

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ScriptEngineManager mgr =

new ScriptEngineManager();

ScriptEngine engine =

mgr.getEngineByName(“Groovy”);

String result =

(String)engine.eval(“2+3”);

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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

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<lang:groovy id="events"

script-source="classpath:dsl/eventsChart.groovy"

customizer-ref="eventsMetaClass" />

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Groovy’s own mechanisms

!Eval

• for evaluating simple expressions

!GroovyShell

• for more complex scripts and DSLs

!GroovyClassLoader

• the most powerful mechanism

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Eval

!Simple mechanism to evaluate math-like formulas

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Eval.me ( ‘3*4’)

Eval.x (1, ‘3*x + 4’)

Eval.xy (1, 2, ‘x + y’)

Eval.xyz(1, 2, 3, ‘x * y - z’)

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GroovyShell

!A Binding provides a context of execution

• can implement lazy evaluation if needed

!A base script class can be specified

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def binding = new Binding()

binding.mass = 22.3

binding.velocity = 10.6

def shell = new GroovyShell(binding)

shell.evaluate(“mass * velocity ** 2 / 2”)

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GroovyClassLoader

!Most powerful mechanism

• could also visit or change the AST

• scripts & classes can be loaded from elsewhere

• more control on compilation

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GroovyClassLoader gcl =

new GroovyClassLoader();

Class clazz = gcl.parseClass(

new File(“f.groovy”));

GroovyObject instance =

(GroovyObject)clazz.newInstance();

instance.setMetaClass(customMC);

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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

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A few considerations

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Start small, with key concepts Beware over-engineering!

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Grow your language progressively

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Get your hands dirty Play with the end-users

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

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Tight feedback loop Iterative process

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Stay humble, You can’t get it right the 1st time.

Don’t design alone at your desk Involve the end users from the start

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Playing it safe in a sandbox

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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

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Test, test, test!

!Don’t just test for nominal cases

• Explicitely test for errors!

!Ensure end-users get meaninful error messages

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Question & Answers

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Picture credits

http://www.flickr.com/photos/featheredtar/2305070061/

http://www.thedailygreen.com/cm/thedailygreen/images/WT/christmas-tree-with-gifts-flipbook.jpg

http://www.flickr.com/photos/chicoer2001/188468490/

http://www.flickr.com/photos/olibac/4054644737/

http://www.flickr.com/photos/epsos/3384297473/

http://media.techeblog.com/images/clapboard.jpg (clap)

http://www.flickr.com/photos/diegocupolo/3614879332/ (flower power)

http://www.flickr.com/photos/oskay/237442629/sizes/m/ (danger)

http://www.partybox.co.uk/data/partyimages/250x250/6ftcutoutaustinpowers.jpg (austin powers)

http://www.flickr.com/photos/27663074@N07/3413698337/ (jeepers creepers)

http://www.flickr.com/photos/wheatfields/420088151/sizes/l/

http://www.flickr.com/photos/therefromhere/518053737/sizes/l/

http://www.flickr.com/photos/romainguy/230416692/sizes/l/

http://www.flickr.com/photos/addictive_picasso/2874279971/sizes/l/

http://www.flickr.com/photos/huangjiahui/3127634297/sizes/l/

http://www.flickr.com/photos/25831000@N08/3064515804/sizes/o/

http://www.flickr.com/photos/lanier67/3147696168/sizes/l/

http://www.flickr.com/photos/ktb/4916063/sizes/o/

http://www.flickr.com/photos/nathonline/918128338/sizes/l/

http://www.flickr.com/photos/kevinsteele/39300193/sizes/l/

http://commons.wikimedia.org/wiki/File:Brueghel-tower-of-babel.jpg

http://commons.wikimedia.org/wiki/File:Platypus.jpg

http://www.flickr.com/photos/joaomoura/2317171808/sizes/l/

http://www.flickr.com/photos/wiccked/132687067/

http://www.flickr.com/photos/timsamoff/252370986/sizes/l/

http://www.flickr.com/photos/29738009@N08/2975466425/sizes/l/

http://www.flickr.com/photos/howie_berlin/180121635/sizes/o/

http://www.flickr.com/photos/yogi/1281980605/sizes/l/

http://www.flickr.com/photos/dorseygraphics/1336468896/sizes/l/

http://www.flickr.com/photos/xcbiker/386876546/sizes/l/

http://www.flickr.com/photos/pietel/152403711/sizes/o/

http://www.flickr.com/photos/forezt/192554677/sizes/o/

http://keremkosaner.files.wordpress.com/2008/04/softwaredevelopment.gif

http://www.jouy.inra.fr

http://www.flickr.com/photos/ejpphoto/408101818/sizes/o/

http://www.flickr.com/photos/solaro/2127576608/sizes/l/

http://www.flickr.com/photos/biggreymare/2846899405/sizes/l/

http://www.flickr.com/photos/wwworks/2222523978/ (earth)

http://static-p3.fotolia.com/jpg/00/01/64/30/400_F_1643044_YrBQoPnt0SC5gHAueG0bhx20yCSL42.jpg (trafic light)

http://aldaria02.a.l.pic.centerblog.net/lz2levrz.jpg (griffon)

http://www.flickr.com/photos/geishaboy500/104137788/ (soap)

Matriochka http://lang.russe.free.fr/images/matriochka.jpg

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