Metamodelling & Metaprogramming. Lena Buffoni

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1 Metamodelling & Metaprogramming Lena Buffoni

2 What is a model? A representation of a concept, phenomenon, relationship, structure, system from the real world Used to communicate, test scenarios, predict behavior Is an abstraction Always simplified to some degree Maps, toy- cars, stochastic economic models are all examples of models

3 A building model Renters view Tax collectors view Electricians view Builders view Realtors view Designers view Each view concentrates on modeling specific aspects

4 From model to meta-model - vehicles How can we generalize the concepts? Car Model (in Modelica) Truck model

5 Generalizing through inheritance Super-class vehicle X wheels a steering wheel a clutch

6 What if we throw an airplane in the mix? Super-class vehicle X wheels a steering wheel a clutch

7 Generalization through metamodelling Aerodynamic Model 1 Vehicle Engine 1..n Actuator 1..n Meta means describing in greek Sensor 1..n Reason in terms of categories on a higher abstraction level

8 Another example : Modeling Methodologies Classical model V model Can we deduce a general modeling methodology? Agile model

9 4 layers of meta-modeling Conforms to Meta-meta model (language specification) Conforms to Meta-model (modeling language) Not the only way to view things! Model (car class) Conforms to Represented by System (car instance) Jean Bézivin, On the unification power of models, 2005.

10 UML and Meta-modeling MOF (Meta Object Faculty) a standard for specifying UML meta-models Provides language constructs for specifying a DSL meta-model Mainly based on Object-Oriented constructs: package, classes, properties (attribute and reference), and (multiple) inheritance. Specificities: composition, opposite...

example: A := B + 3; assignment Left-side assignment

11 Meta-programming Operations on the program itself We have a program model abstract syntax tree Simplified example: A := B + 3; assignment Left-side assignment Right-Side expression Variable Expression A Binary Operator +

12 When? At compile time : preprocessing Macros in C optimization annotations templates in C++ Compilers can be seen as metaprograms

13 When? At runtime : interpreters dynamic component configuration and reconfiguration Java reflection API

14 To what extent? Introspection - look at the structure of the program Generate documentation Verify composability Reflection - manipulate the structure of the program Dynamically configure components Adapt component behavior over time

15 How? At a meta-level: A separate language for processing the program At the same level as the program itself (reification) Concepts of a meta-level can be represented at the base level. Ex: Reflection API in Java Class c = A.getClass();

16 MetaObject Protocol An explicit protocol for representing and manipulating the internal code structure Coming from Lisp and the Common Lisp Object System (CLOS) and the need for backward compatibility among many flavors Allows things such as Get the attributes or methods of a class Create new methods or attributes Modify the code of a method In Java: Javassist, Java Reflection API

17 Java Reflection API java.lang Class Class<T> Class <?>[] Method Method[] getinterfaces() Determines the interfaces implemented by the class or interface represented by this object. getmethod(string name, Class<?>... parametertypes) Returns a Method object that reflects the specified public member method of the class or interface represented by this Class object. getmethods() Returns an array containing Method objects reflecting all the public member methods of the class or interface represented by this Class object, including those declared by the class or interface and those inherited from superclasses and superinterfaces.

18 Java reflection API Dynamically select which method to call public class ConfigurablePrint { void printonlinux(){} void printonmac() {} void printonwindows(){} }

19 Java reflection API public static void main(string[] args) { try{ //load the AppTest at runtime Class cls = Class.forName("ConfigurablePrint"); Object obj = cls.newinstance(); Class [] noparams = {}; //call the printit method String methodname = ""; if(getsystem() == SystemType.WINDOWS) methodname = "printonwindows"; else if(getsystem() == SystemType.MAC) methodname = "printonmac"; else methodname = "printonlinux"; Method method = cls.getdeclaredmethod(methodname, noparams); method.invoke(obj, (Object[])null); }catch(exception ex){ ex.printstacktrace(); }}

20 Drawbacks of reflection? Overheard Loss of compile time checks Loss of traceability Eg: when refactoring Violation of language restrictions Eg: allowing access to public attributes Distinguishing between true reflection uses and quick workarounds?

21 Meta-programming one step further A hack to language extension? add a backwards_for to a language change how inheritance works Meta-meta-programming? compilers of compilers

22 Component Markup languages Markup languages distinguish between the basic programming directives and meta instructions LaTeX mixes text and typesetting directives HTML, XML Annotations, Javadoc tags is Java Attributes in C++

23 Metadata: Annotations in public String tostring() { Predefined annotations return "This is String Representation of current object."; } Javadoc annotations /** * The HelloWorld program. * Lena 1.0 */ public class HelloWorld { } }

24 Annotations //on class level PreferencePolicy { public enum Policy { WEAK, MEDIUM, STRONG } Policy priority() default Policy.MEDIUM; }

25 Component composition Markup is important in component composition Mark methods that will be exposed to the client Version compatibility verification Define configuration and initialization parameters

26 Summary Metamodelling : from specific models towards models of models Metaprogramming : operating on the structure of the code itself Meta-information is essential for component composition

Metamodelling & Metaprogramming. Lena Buffoni

Metamodelling & Metaprogramming Lena Buffoni lena.buffoni@liu.se What is a model? A representation of a concept, phenomenon, relationship, structure, system from the real world Used to communicate, test