Design Your Own
Domain-Specific
Language
Guillaume Laforge - SpringSource
Guillaume Laforge

• Groovy Project Manager
• JSR-241 Spec Lead
• Head of Groovy Development
  at SpringSource
• Initiator...
Introduction




         SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
A few words about Groovy

• Groovy is a dynamic language for the JVM
  – with a Meta Object Protocol
  – compiles directly...
The context
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 ...
Lots of languages...
And in the end...
...nobody understands each other
Expressing Requirements...




                                                                                           ...
DSL: a potential solution?

•Use a more expressive language
 than a general purpose one

•Share a common metaphore of unde...
Towards more readibility (1)




                                                                                         ...
Towards more readibility (1)




                                                                                         ...
Towards more readibility (1)




                                                                                         ...
Towards more readibility (2)




                                                                                         ...
Towards more readibility (2)




                                                                                         ...
Towards more readibility (2)




                                                                                         ...
A collection of DSLs


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

  – Technical dialects

  – Notations

  – Busine...
Technical dialects




                                                                                             15
   ...
SQL
^[w-.]+@([w-]){2,4}$




                                                                                         17
     ...
Notations




                                                                                             18
            ...
1. e4 e5
2. Nf3 Nc6
3. Bb5 a6
L2 U F-1 B L2 F B -1 U L2
Visual!
Business languages




                                                                                            22
    ...
Real-life Groovy examples

• Anti-malaria drug resistance simulation
• Human Resources employee skills representation
• In...
The techniques




         SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
Three levels of techniques



   Flexible &                                     Meta-                                     ...
Flexible & malleable




           SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
A flexible & malleable syntax

• No need to write full-blown classes, use scripts
• Optional typing (def)
  – in scripts, ...
Scripts vs classes

• Hide all the boilerplate technical code
  – an end-user doesn’t need to know about classes


  – pub...
Optional typing

• No need to bother with types or even generics
  – unless you want to!


• Imagine an interest rate look...
Native syntax constructs

• Lists
  – [Monday, Tuesday, Wednesday]

• Maps
  – [CA: ‘California’, TX: ‘Texas’]

• Ranges
 ...
Optional parens & semis

• Make statements and expressions
  look more like natural languages




  – move(left);




  – ...
Named arguments

• In Groovy you can mix named and unnamed
  arguments for method parameters
    – named params are actual...
BigDecimal by default

• Main reason why financial institutions often decide
  to use Groovy for their business rules!
  –...
Custom control structures,
thanks to closures

• When closures are last, they can be put “out” of the
  parentheses surrou...
Operator overloading

a + b
a - b
         a.plus(b)
         a.minus(b)
                                                 ...
Meta-programming




        SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
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 th...
GroovyObject

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

  –get/setProperty(Str...
MetaClass

• The core of Groovy’s MOP system
  –invokeConstructor()
  –invokeMethod() and invokeStaticMethod()
  –invokeMi...
ExpandoMetaClass

• A DSL for MetaClasses!
• MoneyAmount.metaClass.constructor = { ... }
  Number.metaClass.getDollars = {...
The Builder pattern
A builder for HR

• softskills {
      ideas {
          capture 2
          formulate 3
      }
      ...
  }
  knowhow {...
A builder for HR

• softskills {
      ideas {
          capture 2
          formulate 3
      }
      ...
  }
  knowhow {...
Builders

• Builders are...
  – a mechanism for creating any tree-structered graph
  – the realization of the GoF builder ...
The clever trick

• GroovyObject#invokeMethod() is used to catch all
  non-existing method calls in the context of the
  b...
Adding properties to numbers

• Three possible approaches

  – create a Category
    •a category is a kind of decorator fo...
With a Category

• class DistanceCategory {
      static Distance getMeters(Integer self) {
          new Distance(self, U...
With an ExpandoMetaClass

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

 100.meters


...
Compile-time
metaprogramming




        SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
Compile-time
metaprogramming

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

• Two kinds of transformations


  – Global transformations
    •applicable to all compilation units
...
Global transformations

• Implement ASTTransformation
• Annotate the transfo specifying a compilation phase

• @GroovyASTT...
Local transformations

• Same approach as Globale transformations
• But you don’t need the META-INF file
• Instead create ...
Example: the Spock
framework

• Changing the semantics of the original code
• But keeping a valid Groovy syntax
• @Speck
 ...
Integrating DSLs




         SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
Various integration
mechanisms

• Java 6’s javax.script.* APIs (aka JSR-223)
• Spring’s language namespace
• Groovy’s own ...
Java 6’s javax.script.* API

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

• ScriptEngineManag...
Spring’s lang namespace

• POGOs (Plain Old Groovy Objects) can be pre-
  compiled as any POJO and used interchangeably
  ...
Groovy’s own mechanisms

• Eval
  – for evaluating simple expressions


• GroovyShell
  – for more complex scripts and DSL...
Eval

• Simple mechanism to evaluate math-like formulas

• Eval.me (                            ‘3*4’)
  Eval.x (1,       ...
GroovyShell

• A Binding provides a context of execution
  – can implement lazy evaluation if needed


• A base script cla...
GroovyClassLoader

• Most powerful mechanism
  – could also visit or change the AST
  – scripts & classes can be loaded fr...
Externalize business rules

• Although Groovy DSLs can be embedded in normal
  Groovy classes, you should externalize them...
A few considerations




           SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
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
Playing it safe
in a sandbox
Various levels
of sandboxing


• Groovy supports the usual
  Java Security Managers

• Use metaprogramming tricks to preve...
Test, test, test!


• Don’t just test for nominal cases
  – Explicitely test for errors!


• Ensure end-users get meaninfu...
Summary

• Groovy’s a great fit for Domain-Specific Languages
  – Malleable & flexible syntax
  – Full object-orientation
...
Q&A




              Laforge elopment
Guill aume vy Dev
        o f Groo are.com
Head @vmw
 g laforge



            Spri...
?
I kan haz my cheezburgr naw?
 Or do ya reely haz keshtionz?
Pictures Used in this
Presentation
•   http://www.flickr.com/photos/wheatfields/420088151/                           •    ...
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Designing Your Own Domain-Specific Language in Groovy by Guillaume Laforge at SpringOne/2GX 2009

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Designing Your Own Domain-Specific Language in Groovy by Guillaume Laforge at SpringOne/2GX 2009

  1. 1. Design Your Own Domain-Specific Language Guillaume Laforge - SpringSource
  2. 2. 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, SpringOne, JAX, Dynamic Language World, IJTC, and more... 2 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  3. 3. Introduction SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  4. 4. 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... 4 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  5. 5. The context
  6. 6. Subject Matter Experts, Business analysts...
  7. 7. 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
  8. 8. Lots of languages...
  9. 9. And in the end... ...nobody understands each other
  10. 10. Expressing Requirements... 10 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  11. 11. 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 11 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  12. 12. Towards more readibility (1) 12 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  13. 13. Towards more readibility (1) 12 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  14. 14. Towards more readibility (1) 20% 12 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  15. 15. Towards more readibility (2) 13 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  16. 16. Towards more readibility (2) 13 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  17. 17. Towards more readibility (2) 80% 13 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  18. 18. A collection of DSLs • In our everyday life, we’re surrounded by DSLs – Technical dialects – Notations – Business languages 14 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  19. 19. Technical dialects 15 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  20. 20. SQL
  21. 21. ^[w-.]+@([w-]){2,4}$ 17 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  22. 22. Notations 18 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  23. 23. 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6
  24. 24. L2 U F-1 B L2 F B -1 U L2
  25. 25. Visual!
  26. 26. Business languages 22 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  27. 27. 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... 23 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  28. 28. The techniques SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  29. 29. Three levels of techniques Flexible & Meta- AST malleable syntax programming transformations • scripts • POGO • AST traversal • optional typing • categories • local transformations • native syntax constructs • builders • global transformations • parens / semi ommission • custom MetaClasses • hooks into Antlr • BigDecimal • ExpandoMetaClass • operator overloading • closures 25 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  30. 30. Flexible & malleable SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  31. 31. 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 27 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  32. 32. 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” 28 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  33. 33. 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! 29 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  34. 34. 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 30 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  35. 35. Optional parens & semis • Make statements and expressions look more like natural languages – move(left); – move left 31 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  36. 36. 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) 32 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  37. 37. 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) 33 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  38. 38. 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) 34 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  39. 39. Operator overloading a + b a - b a.plus(b) a.minus(b) • Currency amounts a * b a.multiply(b) – 15.euros + 10.dollars a / b a.divide(b) a % b a.modulo(b) • Distance handling a ** b a.power(b) – 10.kilometers - 10.meters a | b a.or(b) a & b a.and(b) • Workflow, concurrency a ^ b a.xor(b) a[b] a.getAt(b) – taskA | taskB & taskC a << b a.leftShift(b) a >> b a.rightShift(b) • Credit an account +a a.unaryPlus() – account << 10.dollars -a a.unaryMinus() account += 10.dollars ~a a.bitwiseNegate() account.credit 10.dollars 35 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  40. 40. Meta-programming SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  41. 41. Groovy’s dynamic heart: The MOP! MetaObject Protocol
  42. 42. 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 38 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  43. 43. GroovyObject • All instances of classes created in Groovy implement the GroovyObject interface: –get/setProperty(String name) –invokeMethod( String name, Object[] params) –propertyMissing(String name) –methodMissing( String name, Object[] params) –get/setMetaClass(MetaClass mc) • A GO can have “pretended” methods and properties 39 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  44. 44. 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 40 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  45. 45. 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 41 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  46. 46. The Builder pattern
  47. 47. A builder for HR • softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... } 43 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  48. 48. A builder for HR • softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... } 43 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  49. 49. 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... 44 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  50. 50. 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 45 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  51. 51. 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 46 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  52. 52. With a Category • 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 47 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  53. 53. 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 48 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  54. 54. Compile-time metaprogramming SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  55. 55. Compile-time metaprogramming • Groovy 1.6 introduced AST Transformations • Compile-time == No runtime performance penalty! Transformation 50 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  56. 56. AST Transformations • Two kinds of transformations – Global transformations •applicable to all compilation units – Local transformations •applicable to marked program elements •using specific marker annotations 51 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  57. 57. 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 52 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  58. 58. 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( ["fqn.MyTransformation"]) public @interface WithLogging {...} 53 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  59. 59. Example: the Spock framework • Changing the semantics of the original code • But keeping a valid Groovy syntax • @Speck class HelloSpock { def "can you figure out what I'm up to?"() { expect: name.size() == size where: name << ["Kirk", "Spock", "Scotty"] size << [4, 5, 6] } } • Check out http://www.spockframework.org • 54 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  60. 60. Integrating DSLs SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  61. 61. 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 56 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  62. 62. 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”); 57 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  63. 63. 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="events" script-source="classpath:dsl/eventsChart.groovy" customizer-ref="eventsMetaClass" /> 58 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  64. 64. Groovy’s own mechanisms • Eval – for evaluating simple expressions • GroovyShell – for more complex scripts and DSLs • GroovyClassLoader – the most powerful mechanism 59 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  65. 65. 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’) 60 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  66. 66. 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”) 61 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  67. 67. 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); 62 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  68. 68. 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 63 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  69. 69. A few considerations SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  70. 70. Start small, with key concepts Beware overengineering!
  71. 71. Grow your language progressively
  72. 72. Get your hands dirty Play with the end-users
  73. 73. Let your DSL fly, it’s not yours, it’s theirs!
  74. 74. Tight feedback loop Iterative process
  75. 75. 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
  76. 76. Playing it safe in a sandbox
  77. 77. 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 72 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  78. 78. Test, test, test! • Don’t just test for nominal cases – Explicitely test for errors! • Ensure end-users get meaninful error messages 73 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  79. 79. 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 74 SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  80. 80. Q&A Laforge elopment Guill aume vy Dev o f Groo are.com Head @vmw g laforge SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
  81. 81. ? I kan haz my cheezburgr naw? Or do ya reely haz keshtionz?
  82. 82. Pictures Used in this Presentation • http://www.flickr.com/photos/wheatfields/420088151/ • http://www.flickr.com/photos/timsamoff/252370986/ sizes/l/ sizes/l/ • http://www.flickr.com/photos/therefromhere/518053737/ • http://www.flickr.com/photos/ sizes/l/ 29738009@N08/2975466425/sizes/l/ • http://www.flickr.com/photos/romainguy/230416692/ • http://www.flickr.com/photos/howie_berlin/180121635/ sizes/l/ sizes/o/ • http://www.flickr.com/photos/addictive_picasso/ • http://www.flickr.com/photos/yogi/1281980605/sizes/l/ 2874279971/sizes/l/ • http://www.flickr.com/photos/dorseygraphics/ • http://www.flickr.com/photos/huangjiahui/3127634297/ 1336468896/sizes/l/ sizes/l/ • http://www.flickr.com/photos/xcbiker/386876546/sizes/l/ • http://www.flickr.com/photos/ • http://www.flickr.com/photos/pietel/152403711/sizes/o/ 25831000@N08/3064515804/sizes/o/ • http://www.flickr.com/photos/forezt/192554677/sizes/o/ • http://www.flickr.com/photos/lanier67/3147696168/ sizes/l/ • http://keremkosaner.files.wordpress.com/2008/04/ softwaredevelopment.gif • http://www.flickr.com/photos/ktb/4916063/sizes/o/ • http://www.jouy.inra.fr • http://www.flickr.com/photos/nathonline/918128338/ sizes/l/ • http://www.flickr.com/photos/ejpphoto/408101818/ sizes/o/ • http://www.flickr.com/photos/kevinsteele/39300193/ sizes/l/ • http://www.flickr.com/photos/solaro/2127576608/sizes/ l/ • http://commons.wikimedia.org/wiki/File:Brueghel-tower- of-babel.jpg • http://www.flickr.com/photos/biggreymare/2846899405/ sizes/l/ • http://commons.wikimedia.org/wiki/File:Platypus.jpg • http://www.flickr.com/photos/joaomoura/2317171808/ sizes/l/ • http://www.flickr.com/photos/wiccked/132687067/ SpringOne 2GX 2009. All rights reserved. Do not distribute without permission.
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