README.md

groovy258-category-mixin-runtime-metaprogramming-workshop

Reference: https://en.wikipedia.org/wiki/Metaclass
Reference: https://docs.groovy-lang.org/latest/html/api/groovy/lang/Category.html
Reference: https://groovy-lang.org/metaprogramming.html
Reference: https://groovy-lang.org/metaprogramming.html#categories
Reference: https://groovy-lang.org/metaprogramming.html#xform-Category
Reference: https://docs.groovy-lang.org/next/html/documentation/#_differences_with_mixins
Reference: https://docs.groovy-lang.org/latest/html/api/groovy/lang/ExpandoMetaClass.html
Reference: https://www.youtube.com/watch?v=UJhlp5P7Ec0

preface

• goals of this workshop:
  • introducing the concept of runtime metaprogramming
  • introducing the concept of category (plus simple example)
  • introducing the concept of mixin (plus simple example)
• exercises are in workshop package, answers in answers

runtime metaprogramming

• allows altering the class model and the behavior of a program at runtime
• JVM supports very dynamic behaviour (much cannot be easily used by Java)

internals

All problems in computer science can be solved by another level of indirection

David Wheeler

• this level of indirection is MetaClass and GroovyObject (object compiled with groovy compiler)
• for every method invocation from groovy code, Groovy will find the MetaClass for the given object and delegate the method resolution to the metaclass
  • https://docs.groovy-lang.org/latest/html/api/groovy/lang/MetaClass.html
    • should not be confused with GroovyObject which happens to be a methods that the metaclass may eventually call (MetaClass#invokeMethod vs GroovyObject#invokeMethod)
  • https://docs.groovy-lang.org/latest/html/api/org/codehaus/groovy/runtime/DefaultGroovyMethods.html

  '123'.reverse() is '321' // investigate that invocations: reverse() and is
  
  public static String reverse(CharSequence self) {
      return new StringBuilder(self).reverse().toString();
  }
  
  public static boolean is(Object self, Object other) {
      return self == other;
  }
  

• MetaClass, MetaClassImpl defines the behaviour of any given Groovy or Java class

  // Goal = to be able to simply write "1.m + 20.cm - 8.mm"
  Number.metaClass {
      getMm = { delegate          }
      getCm = { delegate *  10.mm }
      getM  = { delegate * 100.cm }
  }
  
  assert (1.m + 20.cm - 8.mm) == 1.192.m
  

  • simple analogy: just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances
  • all method/property calls from Groovy code go through the meta class
    • getProperty​(Class sender, Object receiver, String property, boolean isCallToSuper, boolean fromInsideClass)
    • invokeMethod​(Class sender, Object receiver, String methodName, Object[] arguments, boolean isCallToSuper, boolean fromInsideClass)
• ExpandoMetaClass extends MetaClassImpl implements GroovyObject
  • ExpandoMetaClass is a MetaClass that behaves like an Expando - it allows for dynamically adding or changing methods, constructors, properties and even static methods by using a neat closure syntax
• In Groovy we work with three kinds of objects
  • POJO - a regular Java object
  • POGO - a Groovy object whose class is written in Groovy
    • extends java.lang.Object
    • implements groovy.lang.GroovyObject
    • through GroovyObject we have access to MetaClass
  • Groovy Interceptor - a Groovy object
    • implements groovy.lang.GroovyInterceptable
      • is marker interface that extends GroovyObject and is used to notify the Groovy runtime that all methods should be intercepted through the method dispatcher mechanism of the Groovy runtime
    • has method-interception capability

      Live Demo
      class Example {
         static void main(String[] args) {
            X x = new X();
            x.id = 1;
            println x.id // 1
            x.go(); // called invokeMethod go()
         } 
      }
       
      class X implements GroovyInterceptable {
         protected dynamicProps = [:]  
          
         @Override
         void setProperty(String name, val) {
            dynamicProps[name] = val
         } 
         
         @Override
         def getProperty(String name) {
            dynamicProps[name]
         }
         
         @Override
         def invokeMethod(String name, Object args) {
            return "called invokeMethod $name($args)"
         }
      }
      

order of invocations

mixins

• simple example

  class Student {
      List schedule = []
      def addLecture(String lecture) { schedule << lecture }
  }
  
  class Worker {
      List schedule = []
      def addMeeting(String meeting) { schedule << meeting }
  }
  
  class CollegeStudent { // note that it's better to use `Traits` than to hack multiple inheritance in that way
      static { mixin Student, Worker }
  }
  
  new CollegeStudent().with {
      addMeeting('Performance review with Boss')
      addLecture('Learn about Groovy Mixins')
      println schedule // [Performance review with Boss]
      println mixedIn[Student].schedule // [Learn about Groovy Mixins]
      println mixedIn[Worker].schedule // [Performance review with Boss]
  }
  

• it is useful when the class is not under our control
• in order to enable this capability, Groovy implements a feature called mixin
• let you add a mixin on any type at runtime
• the instances are not modified
  • if you mixin some class into another, there isn’t a third class generated
  • methods which respond to A will continue responding to A even if mixed in

category

• simple example

  class Distance {
      def number
      String toString() { "${number}m" }
  }
  
  @Category(Number)
  class NumberCategory {
      Distance getMeters() {
          new Distance(number: this)
      }
  }
  
  use (NumberCategory)  {
      assert 42.meters.toString() == '42m'
  }
  

• it is useful when the class is not under our control
• in order to enable this capability, Groovy implements a feature called Categories
• internals:
  • during compilation, all methods are transformed to static ones with an additional self parameter
  • properties invoked using this references are transformed so that they are instead invoked on the additional self parameter and not on the Category instance
• @Category AST transformation simplifies the creation of Groovy categories
  • Historically, a Groovy category was written like this:

    class TripleCategory {
        public static Integer triple(Integer self) {
            3*self
        }
    }
    use (TripleCategory) {
        assert 9 == 3.triple()
    }
    

  • @Category transformation lets you write the same using an instance-style class, rather than a static class style

    @Category(Integer)
    class TripleCategory {
        public Integer triple() { 3*this }
    }
    use (TripleCategory) {
        assert 9 == 3.triple()
    }