Practical Groovy DSL
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  • 1. Guillaume Laforge / Groovy Project Manager / SpringSource Practical Domain-Specific Languages with Groovy All the techniques to create your own DSL. 1
  • 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, JAX, Dynamic Language World, IJTC, and more...
  • 3. 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. 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
  • 5. The context
  • 6. Subject Matter Experts, Business analysts...
  • 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. Lots of languages...
  • 9. And in the end... ...nobody understands each other
  • 10. Expressing requirements...
  • 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
  • 12. Towards more readibility (1)
  • 13. Towards more readibility (1)
  • 14. Towards more readibility (1) 20%
  • 15. Towards more readibility (2)
  • 16. Towards more readibility (2)
  • 17. Towards more readibility (2) 80%
  • 18. 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
  • 19. A collection of DSLs • In our everyday life, we’re surrounded by DSLs • Technical dialects • Notations • Business languages
  • 20. Technical dialects
  • 21. SQL
  • 22. ^[w-.]+@([w-]){2,4}$
  • 23. Notations
  • 24. 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6
  • 25. L2 U F-1 B L2 F B -1 U L2
  • 26. Visual!
  • 27. Business languages
  • 28. 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...
  • 29. 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
  • 30. 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
  • 31. 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”
  • 32. 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!
  • 33. 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
  • 34. Optional parens & semis • Make statements and expressions look more like natural languages • move(left); •move left
  • 35. 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)
  • 36. 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)
  • 37. 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)
  • 38. 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()
  • 39. Groovy’s dynamic heart: The MOP! MetaObject Protocol
  • 40. 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
  • 41. 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
  • 42. 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
  • 43. 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
  • 44. The Builder pattern
  • 45. 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!” } } } }
  • 46. A builder for HR • softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... }
  • 47. A builder for HR • softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... }
  • 48. 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...
  • 49. 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
  • 50. 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
  • 51. 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
  • 52. 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
  • 53. Compile-time metaprogramming • Groovy 1.6 introduced AST Transformations • Compile-time == No runtime performance penalty! Transformation
  • 54. AST Transformations • Two kinds of transformations • Global transformations • applicable to all compilation units • Local transformations • applicable to marked program elements • using specific marker annotations
  • 55. 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 {} 
  • 56. 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...
  • 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
  • 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( [quot;fqn.MyTransformationquot;]) public @interface WithLogging {...}
  • 59. 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] } }
  • 60. 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
  • 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
  • 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”);
  • 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=quot;eventsquot; script-source=quot;classpath:dsl/eventsChart.groovyquot; customizer-ref=quot;eventsMetaClassquot; />
  • 64. Groovy’s own mechanisms • Eval • for evaluating simple expressions • GroovyShell • for more complex scripts and DSLs • GroovyClassLoader • the most powerful mechanism
  • 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’)
  • 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”)
  • 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);
  • 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
  • 69. 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
  • 70. Start small, with key concepts Beware overengineering!
  • 71. Grow your language progressively
  • 72. Get your hands dirty Play with the end-users
  • 73. Let your DSL fly, it’s not yours, it’s theirs!
  • 74. Tight feedback loop Iterative process
  • 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. Playing it safe in a sandbox
  • 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
  • 78. Test, test, test! • Don’t just test for nominal cases • Explicitely test for errors! • Ensure end-users get meaninful error messages
  • 79. a d n e g A • Summary • Questions & Answers
  • 80. 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
  • 81. ? I kan haz my cheezburgr naw? Or do ya reely haz keshtionz?
  • 82. Appendix
  • 83. • 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/xcbiker/386876546/sizes/l/ • http://www.flickr.com/photos/pietel/152403711/sizes/o/
  • 84. • 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/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/