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The Art of Metaprogramming
           in Java

     Abdelmonaim Remani
        @PolymathicCoder
About Me
 Software Architect at Just.me Inc.
 Interested in technology evangelism and enterprise software development and
  architecture
 Frequent speaker (JavaOne, JAX, OSCON, OREDEV, etc…)
 Open-source advocate
 President and founder of a number of user groups
 - NorCal Java User Group
 - The Silicon Valley Spring User Group
 - The Silicon Valley Dart Meetup
 Bio:        http://about.me/PolymathicCoder
 Twitter:    @PolymathicCoder
 Email:      abdelmonaim.remani@gmail.com
About Me
 Today is my birthday!




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License




 Creative Commons Attribution Non-Commercial 3.0 Unported
 - http://creativecommons.org/licenses/by-nc/3.0


 Disclaimer: The graphics, logos, and trademarks used this presentation belong to
  their rightful owners.
Metadata
What Is Metadata?
 The term “Metadata” was coined by Philip Bagley in 1986 in “Extension of
  Programming Language Concepts”
 Data about data or data that describes other data
 - Structural (Before there is any data)
   - Its type
   - Its valid values
   - How it relates to other data
   - Its purpose
   - Etc…
 - Descriptive (After data is there)
   - How it was created
   - The context within which it exists
   - Etc…
How Is Metadata Expressed?
 Implicit
  - Adhering to a certain convention
 Explicit
  - External to the code
    - DSL (Domain-Specific Language)
    - Markup: XML, RDF, etc…
    - Etc…
  - Internal to the code
    - As comments on code
      - Javadoc: @author, @version, @since, etc…
      - XDoclet
      - Etc…
    - As code itself
      - HTML meta tags: <meta name=“author” content=“Abdelmonaim Remani”>
      - Java Annotations: @Override, @Deprecated, etc…
      - Embedded DSL
      - Etc…
How Is Metadata Being Used?
 Schema
 - Data Dictionary in RDBMS
 - Check Constraints (JSR 303 Validation, Etc…)
 - Etc…
 Semantics
 - WSDL (Web Services Description Language)
 - RDF (Resource Description Framework)
 - Etc…
 Data Management
 - Build/deployment instructions (.svn files, .git, etc…)
 - Etc…
 Configurations
 Etc…
Metadata in Java: Annotations
Java Annotations
 JSR 175 (A Metadata Facility for the Java Programming Language)
 - Introduced in J2SE 5.0 (September 30, 2004)
 - Standardized how annotations are declared in Java code
 An alternative way to Javadoc comments, externally as XML, etc…
 - More readable
 - Closer to the code
 - Statically-typed
 - Can be retained until runtime
JDK Annotations
 Java Language Spec Annotations
 - @Override
 - @Deprecated
 - @SuppressWarning
 JSR 250 (Common Annotations for the Java Platform)
 - @PostConstruct
 - @Resource
 - @DenyAll
 - Etc…
Write Your Own


 You need to tell the compiler how the annotation is to be treated
 In java.lang.annotation package
 - @Target
   - The element type the annotation can be applied to
     - ElememtType.ANNOTATION_TYPE
     - ElememtType.CONSTRUCTOR
     - ElememtType.FIELD
     - ElememtType.LOCAL_VARIABLE
     - ElememtType.METHOD
     - ElememtType.PACKAGE
     - ElememtType.PARAMETER
     - ElememtType.TYPE
Write Your Own
 In java.lang.annotation package
 - @Retention
   - RetentionPolicy.SOURCE
     - Discarded by the compiler
   - RetentionPolicy.CLASS
     - Included in the class file but ignored the JVM. This is the default
   - RetentionPolicy.RUNTINE
     - Included in the class file and read by the JVM.
 - @Documented
   - Whether it should be shown in the javadoc or not
 - @Inherited
   - Allowed to be inherited by subclasses
Write Your Own
 Attributes
 - Carry additional metadata details
 - May only be
   - Primitives
   - Enums
   - Java.lang.String
   - Java.lang.Class
   - One-dimensional arrays of the above
 - May have a default value
 - No modifiers or parameters
Code
Notes
 Annotating a package
 - Must create a file named package-info.java in the target package
 - Note that “package-info” is not a valid Java identifier
   - Most IDEs you will prevent you from creating a class name “package-info”
   - Make sure you create it as a regular file with .java extension instead
Metaprogramming
What is Metaprogramming?
 Writing programs that write or manipulate other programs or themselves based on
  some metadata
 Metaprogramming
   -!= Generative/Automatic Programming
 Ralph Johnson
 - “It is about programming at the meta level, that is about changing your
   interpreter or changing how code is compiled”
 Black art and a Big-boy’s toy
 An underused feature
 Supported in many languages and across several platforms
 The backbone of many of the most successful frameworks
How is Metaprogramming Supported?
 Exposing the internals of the runtime/compiler as an API
 Dynamic execution of expressions containing programming commands
 - Code as Strings
 - Code as a series if method calls
 A program transformation system
 - A description gets transformed it to a target language
   - The compiler itself
   - YACC takes in grammar, and produces C/C++ code containing yyparse()
   - ANTLR (ANother Tool for Language Recognition)
Concepts
 Metalanguage
 - The language in which the metaprogram is written in
 Object Language
 - The language in which the target (produced) program is written in
 Reflection or Reflexivity
 - When metalanguage == object language
 - No translation necessary
Usage in code
 Static data that can be pre-computed or pre-generated at compile time
 Eliminate boiler-plate
 - Code that cannot be abstracted in functions for DRYness sake
   - Think Aspects in AOP
   - Stereotypes in Spring
   - Etc…
 - Code of common methods
   - Getters/setters, toString(), hashCode(), equals(), etc…
 Etc…
Benefits
 In code
 - Performance gain
 - Flexibility
 - Simplicity
 - Etc…
 Development
 - Minimize the LOC to express a solution
 - Productivity gain
 - Reduced development time/efficiency
 - Etc…
How to?
 Many techniques focused on specific aspects of metaprogramming
 No well-defined best practices


 This presentation is an attempt to bring in some structure through defining a
  process
 - Defining the metadata
 - Processing the metadata
 - Metamodel construction
 - Validating the metamodel
 - Metamodel interpretation
Metadata Processing
Metadata Processing
 Programmatically reading/accessing metadata
 - Parsing raw metadata
 - Call to an API
   - Reflection
   - Query
   - Etc…
 - Tools
Metadata Processing in Java: Annotation Processing
At Runtime: JSR 175
 JSR 175: A Metadata Facility for the Java Programming Language
 - Defined the core reflection API for reading/accessing annotations on annotated
   elements at runtime as long as their retention policy extends to the runtime
 - Reflection
   - Reading annotations is done in reference to the structure of the program
 Libraries
 - Reflections: http://code.google.com/p/reflections/
 - FEST-Reflect: http://fest.easytesting.org/
 - ReflectASM: http://code.google.com/p/reflectasm/
 - Etc…
At Runtime: JSR 175
At Build Time: JSR 269
 Mirror-Based Reflection
 - Reflective capabilities are encapsulated in intermediary objects called mirrors
 - Annotations are accessible through a processor API
 In J2SE 5.0 didn’t standardize a processing API
 - We used apt, a stand-alone tool, along with the Mirror API (com.sun.mirror)
At Build Time: JSR 269
 In Java SE 6 (December 11, 2006)
 - JSR 269: Pluggable Annotation Processing API
       -http://docs.oracle.com/javase/6/docs/technotes/guides/javac/index.htm

   - Leverages JSR 199 (Java Compiler API)
     - Javax.tools
     - Programmatically invoke javac
     - Implements ServiceLoader interface of SPI (JSR 24 - ServiceProvider API))
     - Provides the DiagnosticListener interface to allow listening for warnings and
       error by the compiler
   - Extends javac as a plug-in allowing to write custom annotation processors
   - Seamless integration with javac
     - Finds if there is an annotation registered to process a particular annotation
     - Plugs it into the compiler
At Build Time: JSR 269
 JSR 269 Defines 2 APIs
 - Writing annotation processors and interacting with the processing environment
   - Javax.annotation.processing
 - Modeling the Java Programming Language
   - Javax.lang.model
     - Type declarations and types (Accommodates Generics)
Code
Code
 Implement javax.annotation.processing.Processor or extend
  javax.annotation.processing.AbstractProcessor
 - Process method
   - Returns whether or not the annotations processed are claimed by the
     processor. This determines whether other processors can further process
     them or not
 Configure the processor using annotations
 - @SupportedAnnotationTypes
   - Register the annotations to be processed with their full-qualified name.
     Wildcats are supported as well
 - @SupportedSourceVersion
   - The Java version supported
 - @SupportedOptions
   - Register supported command-line options
Code
 Annotation processor registration
 - Javac command-line options
   - -processor <processor classes>
   - -proc:none or -proc:only. It is enabled by default
   - -processorpath <annotation path>
 - Leverage JSR 24 (ServiceProvider API) to automatically register processors
   - Compile the processor with –proc:none and package it in a jar
   - Include in META-INF/services a file named
     javax.annotation.processing.Processor containing a text entry of the full-
     qualified name of the processor class
   - Optional: Multiple processor can be registered or ordered
Metadata Processing
 Annotated elements might have nested annotated elements and so forth
 Visitor Design Pattern
 - Separation of annotation processing code from the object structure
Metamodel Construction
What is a Metamodel?
 Metadata is processed into a model that can be accessed programmatically
 - Static
 - Dynamic
 A association of the data and its metadata
Static Metamodel
 - A metamodel based on
  - One all-knowing “god object” encapsulating all possible metadata values that
    could be associated with the one annotated element
  - Advantages
    - Simple
    - Statically typed
  - Disadvantages
    - Nulls all over
Static Metamodel
Dynamic Metamodel
 Ravioli Code
 - Metamodel is structured in small and loosely-coupled components
 Decorator Design Pattern
 - Annotated elements are decorated with annotations that bring in metadata
 Advantages
 - Flexible
 Disadvantages
 - Complex
Dynamic Metamodel
Metamodel Validation
Validation?
 Ensuring the validity or correctness of semantics of the metamodel
 - Verification that a set of constraints are satisfied
 Compliance additional consistency constraints outside the Java language
  specification
 - Java only has @Target
 Example
 - Assuming that you are writing your own JSR 318 (Enterprise javabeans 3.1)
   implementation
   - You should not allow a POJO to be annotated with both @Statefull and
     @Stateless
Constraint Satisfaction
 This is anything but simple
  - A Constraint Satisfaction Problem
   - Can be resolved by multiple algorithms (Backtracking, Constrain reparation,
     and local search)
   - These algorithms are out of the scope of this presentation
Validating the Metamodel
 The imperative way
 - In Java
   - A jungle of conditional statements
Validating the Metamodel
     The logical way (Logic Programming)
       - Semantics are represented declaratively as predicates (Facts and Rules)
       - Procedurally interpreted query resolution
     - Constraint Logic Programming (An extension of Logic Programming)
         - Used for digital circuit verification
•     Prolog is King
     - tuProlog: Implementation of the Prolog interpreter in Java
       - http://alice.unibo.it/xwiki/bin/view/Tuprolog/
     - Jlog: Implementation of the Prolog interpreter in Java
       - http://jlogic.sourceforge.net/
     - JPL: Java Interface to SWI Prolog
       - http://www.swi-prolog.org/FAQ/Java.html
     Yeah you’re gonna have to learn that
Validating the Metamodel
 Clojure
 - Lisp-like language on the JVM
 - core.logic supports logic and constraint programming
 Other
 - Prova Rule Language
   - http://www.prova.ws/
 - Mercury
   - http://www.mercury.csse.unimelb.edu.au/
Validating the Metamodel
 Rules Engine
 - Drools Expert
   - A highly performance optimized rules engine
   - Rules are written in mvel (A powerful expression language) or XML
   - Integrates with Java collaborator POJOs
Interpreting the Metamodel
Transformations
 Changing the structure of existing code
 - AST (Abstract Syntax Tree) rewriting
 - Adding/removing behavior
 - Automatic generation of cookie-cutter code
 - Create complex class hierarchies
 Notable byte code manipulation libraries
 - ASM
 - GCLIB
 - Javassist
 - Etc…
Transformations
 ASM
 - Small
 - Fast
 - Low-level
 - Require a good understanding of byte-code and Java Language Spec
 - Good documentation
 - http://asm.ow2.org/
 GCLIB
 - Built on top of ASM
 - High performance
 - Issues with signed jars
 - Dropped by Hibernate in favor of Javassist for lack of support and active
 - http://cglib.sourceforge.net/
Transformations
 Javassist
 - Very easy to use
 - Works with signed jars
 - Good documentation
 - Excellent Javadoc
 - Adopted by hibernate
 - Slower than GCLIB
 - http://www.jboss.org/javassist/
Weaving in Cross-Cutting Concerns
 OOP creates a hierarchical object model by nature
 Certain things are not necessary a part of the application logic
 - Logging
 - Transactions
 - Security
 - Etc…
 AOP (Aspect Oriented Programming)
   - Code weaving
   - Dynamic Proxies
Mapping
 Map one object to another based on the metamodel
 Example
 - Dozer
   - http://dozer.sourceforge.net/
 - ORM Frameworks
Mapping



    Metadata                                   Row
                                               Class

               Metamodel


     POJO
     Class                 Mapper   Creates     Row
                                              Instance


                 POJO
                Instance
Other
 Inversion of Control
 - Dependency Injection (JSR 330)
 Instrumentation
 - Monitor and measure performance
 - Diagnostics
 - Trace info
 - Etc…
 Etc…
Metaprogramming in the Wild
Metaprogramming in the Wild
 Spring
  - Lightweight POJO-based frameworks
  - Non-invasiveness
    - Stereotyping a class in lieu of inheritance to acquire behavior
    - Arbitrary methods signature instead of overriding inherited ones
  - Declarative configuration
    - Dependency Injection
 Rails
  - Rapid Development
  - Convention over configuration
    - Active Record Pattern
 Hibernate/JPA (JSR 317)
  - ORM
 Guice
 Spring AOP
 Etc…
Model-Driven Engineering
  Philosophy
  - In an application
    - Artifacts that are good candidates for auto-generation
    - Why?
      - The architecture is defined by
       - Making abstractions guided by the separation of concerns
      - Most deal with the same concerns
       - Avoid rediscovering the best approach every time
  - Code generation, Scaffolding, etc…
Project Averroes
Averroes
 A metaprogramming framework
 - Auto-discovery of annotations
   - @Discoverable
 - Annotation processing
 - Metamodel construction
 - Metamodel validation
   - Annotations of annotations (meta-metadata)
     - @AccompaniedWith
     - @AccompaniedWithAtLeastOne
     - @ForType
     - @ForTypeAnnotatedWith
     - @ForTypeIncluding
     - @NotAccompaniedWith
 A work in progress…
 - https://github.com/PolymathicCoder/Averroes
Questions?
Thank You!


@PolymathicCoder

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The Art of Metaprogramming in Java

  • 1. The Art of Metaprogramming in Java Abdelmonaim Remani @PolymathicCoder
  • 2. About Me  Software Architect at Just.me Inc.  Interested in technology evangelism and enterprise software development and architecture  Frequent speaker (JavaOne, JAX, OSCON, OREDEV, etc…)  Open-source advocate  President and founder of a number of user groups - NorCal Java User Group - The Silicon Valley Spring User Group - The Silicon Valley Dart Meetup  Bio: http://about.me/PolymathicCoder  Twitter: @PolymathicCoder  Email: abdelmonaim.remani@gmail.com
  • 3. About Me  Today is my birthday! 01001000 01100001 01110000 01110000 01111001 00100000 01000010 01101001 01110010 01110100 01101000 01100100 01100001 01111001
  • 4. License  Creative Commons Attribution Non-Commercial 3.0 Unported - http://creativecommons.org/licenses/by-nc/3.0  Disclaimer: The graphics, logos, and trademarks used this presentation belong to their rightful owners.
  • 6. What Is Metadata?  The term “Metadata” was coined by Philip Bagley in 1986 in “Extension of Programming Language Concepts”  Data about data or data that describes other data - Structural (Before there is any data) - Its type - Its valid values - How it relates to other data - Its purpose - Etc… - Descriptive (After data is there) - How it was created - The context within which it exists - Etc…
  • 7. How Is Metadata Expressed?  Implicit - Adhering to a certain convention  Explicit - External to the code - DSL (Domain-Specific Language) - Markup: XML, RDF, etc… - Etc… - Internal to the code - As comments on code - Javadoc: @author, @version, @since, etc… - XDoclet - Etc… - As code itself - HTML meta tags: <meta name=“author” content=“Abdelmonaim Remani”> - Java Annotations: @Override, @Deprecated, etc… - Embedded DSL - Etc…
  • 8. How Is Metadata Being Used?  Schema - Data Dictionary in RDBMS - Check Constraints (JSR 303 Validation, Etc…) - Etc…  Semantics - WSDL (Web Services Description Language) - RDF (Resource Description Framework) - Etc…  Data Management - Build/deployment instructions (.svn files, .git, etc…) - Etc…  Configurations  Etc…
  • 9. Metadata in Java: Annotations
  • 10. Java Annotations  JSR 175 (A Metadata Facility for the Java Programming Language) - Introduced in J2SE 5.0 (September 30, 2004) - Standardized how annotations are declared in Java code  An alternative way to Javadoc comments, externally as XML, etc… - More readable - Closer to the code - Statically-typed - Can be retained until runtime
  • 11. JDK Annotations  Java Language Spec Annotations - @Override - @Deprecated - @SuppressWarning  JSR 250 (Common Annotations for the Java Platform) - @PostConstruct - @Resource - @DenyAll - Etc…
  • 12. Write Your Own  You need to tell the compiler how the annotation is to be treated  In java.lang.annotation package - @Target - The element type the annotation can be applied to - ElememtType.ANNOTATION_TYPE - ElememtType.CONSTRUCTOR - ElememtType.FIELD - ElememtType.LOCAL_VARIABLE - ElememtType.METHOD - ElememtType.PACKAGE - ElememtType.PARAMETER - ElememtType.TYPE
  • 13. Write Your Own  In java.lang.annotation package - @Retention - RetentionPolicy.SOURCE - Discarded by the compiler - RetentionPolicy.CLASS - Included in the class file but ignored the JVM. This is the default - RetentionPolicy.RUNTINE - Included in the class file and read by the JVM. - @Documented - Whether it should be shown in the javadoc or not - @Inherited - Allowed to be inherited by subclasses
  • 14. Write Your Own  Attributes - Carry additional metadata details - May only be - Primitives - Enums - Java.lang.String - Java.lang.Class - One-dimensional arrays of the above - May have a default value - No modifiers or parameters
  • 15. Code
  • 16. Notes  Annotating a package - Must create a file named package-info.java in the target package - Note that “package-info” is not a valid Java identifier - Most IDEs you will prevent you from creating a class name “package-info” - Make sure you create it as a regular file with .java extension instead
  • 18. What is Metaprogramming?  Writing programs that write or manipulate other programs or themselves based on some metadata  Metaprogramming -!= Generative/Automatic Programming  Ralph Johnson - “It is about programming at the meta level, that is about changing your interpreter or changing how code is compiled”  Black art and a Big-boy’s toy  An underused feature  Supported in many languages and across several platforms  The backbone of many of the most successful frameworks
  • 19. How is Metaprogramming Supported?  Exposing the internals of the runtime/compiler as an API  Dynamic execution of expressions containing programming commands - Code as Strings - Code as a series if method calls  A program transformation system - A description gets transformed it to a target language - The compiler itself - YACC takes in grammar, and produces C/C++ code containing yyparse() - ANTLR (ANother Tool for Language Recognition)
  • 20. Concepts  Metalanguage - The language in which the metaprogram is written in  Object Language - The language in which the target (produced) program is written in  Reflection or Reflexivity - When metalanguage == object language - No translation necessary
  • 21. Usage in code  Static data that can be pre-computed or pre-generated at compile time  Eliminate boiler-plate - Code that cannot be abstracted in functions for DRYness sake - Think Aspects in AOP - Stereotypes in Spring - Etc… - Code of common methods - Getters/setters, toString(), hashCode(), equals(), etc…  Etc…
  • 22. Benefits  In code - Performance gain - Flexibility - Simplicity - Etc…  Development - Minimize the LOC to express a solution - Productivity gain - Reduced development time/efficiency - Etc…
  • 23. How to?  Many techniques focused on specific aspects of metaprogramming  No well-defined best practices  This presentation is an attempt to bring in some structure through defining a process - Defining the metadata - Processing the metadata - Metamodel construction - Validating the metamodel - Metamodel interpretation
  • 25. Metadata Processing  Programmatically reading/accessing metadata - Parsing raw metadata - Call to an API - Reflection - Query - Etc… - Tools
  • 26. Metadata Processing in Java: Annotation Processing
  • 27. At Runtime: JSR 175  JSR 175: A Metadata Facility for the Java Programming Language - Defined the core reflection API for reading/accessing annotations on annotated elements at runtime as long as their retention policy extends to the runtime - Reflection - Reading annotations is done in reference to the structure of the program  Libraries - Reflections: http://code.google.com/p/reflections/ - FEST-Reflect: http://fest.easytesting.org/ - ReflectASM: http://code.google.com/p/reflectasm/ - Etc…
  • 29. At Build Time: JSR 269  Mirror-Based Reflection - Reflective capabilities are encapsulated in intermediary objects called mirrors - Annotations are accessible through a processor API  In J2SE 5.0 didn’t standardize a processing API - We used apt, a stand-alone tool, along with the Mirror API (com.sun.mirror)
  • 30. At Build Time: JSR 269  In Java SE 6 (December 11, 2006) - JSR 269: Pluggable Annotation Processing API -http://docs.oracle.com/javase/6/docs/technotes/guides/javac/index.htm - Leverages JSR 199 (Java Compiler API) - Javax.tools - Programmatically invoke javac - Implements ServiceLoader interface of SPI (JSR 24 - ServiceProvider API)) - Provides the DiagnosticListener interface to allow listening for warnings and error by the compiler - Extends javac as a plug-in allowing to write custom annotation processors - Seamless integration with javac - Finds if there is an annotation registered to process a particular annotation - Plugs it into the compiler
  • 31. At Build Time: JSR 269  JSR 269 Defines 2 APIs - Writing annotation processors and interacting with the processing environment - Javax.annotation.processing - Modeling the Java Programming Language - Javax.lang.model - Type declarations and types (Accommodates Generics)
  • 32. Code
  • 33. Code  Implement javax.annotation.processing.Processor or extend javax.annotation.processing.AbstractProcessor - Process method - Returns whether or not the annotations processed are claimed by the processor. This determines whether other processors can further process them or not  Configure the processor using annotations - @SupportedAnnotationTypes - Register the annotations to be processed with their full-qualified name. Wildcats are supported as well - @SupportedSourceVersion - The Java version supported - @SupportedOptions - Register supported command-line options
  • 34. Code  Annotation processor registration - Javac command-line options - -processor <processor classes> - -proc:none or -proc:only. It is enabled by default - -processorpath <annotation path> - Leverage JSR 24 (ServiceProvider API) to automatically register processors - Compile the processor with –proc:none and package it in a jar - Include in META-INF/services a file named javax.annotation.processing.Processor containing a text entry of the full- qualified name of the processor class - Optional: Multiple processor can be registered or ordered
  • 35. Metadata Processing  Annotated elements might have nested annotated elements and so forth  Visitor Design Pattern - Separation of annotation processing code from the object structure
  • 37. What is a Metamodel?  Metadata is processed into a model that can be accessed programmatically - Static - Dynamic  A association of the data and its metadata
  • 38. Static Metamodel - A metamodel based on - One all-knowing “god object” encapsulating all possible metadata values that could be associated with the one annotated element - Advantages - Simple - Statically typed - Disadvantages - Nulls all over
  • 40. Dynamic Metamodel  Ravioli Code - Metamodel is structured in small and loosely-coupled components  Decorator Design Pattern - Annotated elements are decorated with annotations that bring in metadata  Advantages - Flexible  Disadvantages - Complex
  • 43. Validation?  Ensuring the validity or correctness of semantics of the metamodel - Verification that a set of constraints are satisfied  Compliance additional consistency constraints outside the Java language specification - Java only has @Target  Example - Assuming that you are writing your own JSR 318 (Enterprise javabeans 3.1) implementation - You should not allow a POJO to be annotated with both @Statefull and @Stateless
  • 44. Constraint Satisfaction  This is anything but simple - A Constraint Satisfaction Problem - Can be resolved by multiple algorithms (Backtracking, Constrain reparation, and local search) - These algorithms are out of the scope of this presentation
  • 45. Validating the Metamodel  The imperative way - In Java - A jungle of conditional statements
  • 46. Validating the Metamodel  The logical way (Logic Programming) - Semantics are represented declaratively as predicates (Facts and Rules) - Procedurally interpreted query resolution - Constraint Logic Programming (An extension of Logic Programming) - Used for digital circuit verification • Prolog is King - tuProlog: Implementation of the Prolog interpreter in Java - http://alice.unibo.it/xwiki/bin/view/Tuprolog/ - Jlog: Implementation of the Prolog interpreter in Java - http://jlogic.sourceforge.net/ - JPL: Java Interface to SWI Prolog - http://www.swi-prolog.org/FAQ/Java.html  Yeah you’re gonna have to learn that
  • 47. Validating the Metamodel  Clojure - Lisp-like language on the JVM - core.logic supports logic and constraint programming  Other - Prova Rule Language - http://www.prova.ws/ - Mercury - http://www.mercury.csse.unimelb.edu.au/
  • 48. Validating the Metamodel  Rules Engine - Drools Expert - A highly performance optimized rules engine - Rules are written in mvel (A powerful expression language) or XML - Integrates with Java collaborator POJOs
  • 50. Transformations  Changing the structure of existing code - AST (Abstract Syntax Tree) rewriting - Adding/removing behavior - Automatic generation of cookie-cutter code - Create complex class hierarchies  Notable byte code manipulation libraries - ASM - GCLIB - Javassist - Etc…
  • 51. Transformations  ASM - Small - Fast - Low-level - Require a good understanding of byte-code and Java Language Spec - Good documentation - http://asm.ow2.org/  GCLIB - Built on top of ASM - High performance - Issues with signed jars - Dropped by Hibernate in favor of Javassist for lack of support and active - http://cglib.sourceforge.net/
  • 52. Transformations  Javassist - Very easy to use - Works with signed jars - Good documentation - Excellent Javadoc - Adopted by hibernate - Slower than GCLIB - http://www.jboss.org/javassist/
  • 53. Weaving in Cross-Cutting Concerns  OOP creates a hierarchical object model by nature  Certain things are not necessary a part of the application logic - Logging - Transactions - Security - Etc…  AOP (Aspect Oriented Programming) - Code weaving - Dynamic Proxies
  • 54. Mapping  Map one object to another based on the metamodel  Example - Dozer - http://dozer.sourceforge.net/ - ORM Frameworks
  • 55. Mapping Metadata Row Class Metamodel POJO Class Mapper Creates Row Instance POJO Instance
  • 56. Other  Inversion of Control - Dependency Injection (JSR 330)  Instrumentation - Monitor and measure performance - Diagnostics - Trace info - Etc…  Etc…
  • 58. Metaprogramming in the Wild  Spring - Lightweight POJO-based frameworks - Non-invasiveness - Stereotyping a class in lieu of inheritance to acquire behavior - Arbitrary methods signature instead of overriding inherited ones - Declarative configuration - Dependency Injection  Rails - Rapid Development - Convention over configuration - Active Record Pattern  Hibernate/JPA (JSR 317) - ORM  Guice  Spring AOP  Etc…
  • 59. Model-Driven Engineering  Philosophy - In an application - Artifacts that are good candidates for auto-generation - Why? - The architecture is defined by - Making abstractions guided by the separation of concerns - Most deal with the same concerns - Avoid rediscovering the best approach every time - Code generation, Scaffolding, etc…
  • 61. Averroes  A metaprogramming framework - Auto-discovery of annotations - @Discoverable - Annotation processing - Metamodel construction - Metamodel validation - Annotations of annotations (meta-metadata) - @AccompaniedWith - @AccompaniedWithAtLeastOne - @ForType - @ForTypeAnnotatedWith - @ForTypeIncluding - @NotAccompaniedWith  A work in progress… - https://github.com/PolymathicCoder/Averroes