Slides from my talk at Greach 2014: "Groovy is a dynamic language that provides different types of metaprogramming techniques. In this talk we’ll mainly see runtime metaprogramming. I’ll explain Groovy Meta-Object-Protocol (MOP), the metaclass, how to intercept method calls, how to deal with method missing and property missing, the use of mixins and categories. All of these topics will be explained with examples in order to understand them. Also, I’ll talk a little bit about compile-time metaprogramming with AST Transformations. AST Transformations provide a wonderful way of manipulating code at compile time via modifications of the Abstract Syntax Tree. We’ll see a basic but powerful example of what we can do with AST transformations." The code is available at: https://github.com/lmivan/greach2014

1. IvAn LOpez MartIn @ilopmar
Metaprogramming
with Groovy

2. Iván Lopez Martín @ilopmar
I work at Kaleidos
Using Groovy/Grails since 4 years
Creator of www.bokzuy.com
Developed some Grails plugins: Postgreslq-Extensions,
Slug-Generator, Ducksboard-API,...
Usual Suspect of Madrid-GUG (Groovy User Group)
Who am I?
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3. - A dynamic language like Groovy allows to "delay" to runtime some
checks that are usually performed during the compilation.
- Add new properties, methods and behaviour during runtime.
- Wide range of applicability:
- DSLs
- Builders
- Advanced logging, tracing, debugging & profiling
- Allow organize the codebase better
- That's why we can talk about Metaprogramming in Groovy.
Groovy is Dynamic
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4. What is
Metaprogramming?

5. From Wikipedia:
Metaprogramming is the writing of computer programs that
write or manipulate other programs (or themselves) as their
data, or that do part of the work at compile time that would
otherwise be done at runtime.
“Writing code that writes code”
What is Metaprogramming?
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6. Runtime
Metaprogramming

7. - Groovy provides this capability through the Meta-Object Protocol
(MOP).
- We can use MOP to invoke methods dynamically and also synthesize
classes and methods on the fly.
Runtime Metaprogramming
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8. Groovy
Groovy
Java
Java
MOP
What is the Meta Object Protocol (MOP)?
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9. Intercepting methods
using MOP

10. - Classes compiled by Groovy implements GroovyObject interface.
- We can implement GroovyInterceptable to hook into the execution
process.
- When implementing invokeMethod(), it is called for every method
(existing and nonexisting).
Groovy Interceptable
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11. GroovyInterceptable example
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12. GroovyInterceptable example (II)
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13. - Groovy maintains a meta class of type MetaClass for each class.
- Maintains a collection of all methods and properties of A.
- If we can't modify the class source code or if it's a Java class we can
modify the meta-class.
- We can intercept methods by implementing the invokeMethod()
method on the MetaClass.
Intercepting methods using MetaClass
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14. MetaClass example
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15. MOP
Method Injection

16. - In Groovy we can “open” a class at any time.
- Method Injection at code-writing time, we know the names of
methods we want to add.
- Different techniques:
- MetaClass
- Categories
- Extensions
- Mixins
MOP Method Injection
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17. - MetaClassImpl: Default meta class, it's used in the vast majority of
case.
- ExpandoMetaClass: allow the addition or replacement of methods,
properties and constructors on the fly.
- ProxyMetaClass: Can decorate a meta class with interception
capabilities.
- Other meta classes used internally and for testing.
MetaClass
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18. Adding methods using MetaClass
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19. Adding properties using MetaClass
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* Note: This is only true for Groovy.
In Grails all MetaClass are ExpandoMetaClass.
Thank you Burt Beckwith for the clarification

20. Adding constructor using MetaClass
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21. Overriding methods using MetaClass
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22. Overriding methods using MetaClass
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23. - Changes made to a MetaClass are “persistent” and hard to revert.
- Categories are useful to change the meta class in a confined small
piece of code.
- A category can alter a class’ MetaClass.
- The MOP is modified in the closure and after the closure execution
is reset to its old state.
- Category classes are no special.
Categories
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24. Categories example

25. Categories example (II)
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26. Categories example (II)
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27. Categories example (II)
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28. - JAR file with classes that provide extra methods to other classes.
- It also has to contain a meta-information file
- Add the JAR to the classpath to enhance the classes.
- The extension methods needs to be public and static.
Extension modules
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29. Extension modules example
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30. - A mixin allow “bring in” or “mix in” implementations from multiple
classes.
- Groovy first call the mixed-in class.
- Mix multiple classes. The last added mixin takes precedence.
- Override a method of a previous Mixin but not methods in the meta
class.
- Mixins cannot easily be un-done.
Mixins
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31. Mixins example
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32. MOP
Method Synthesis

33. - Dynamically figure out the behaviour for methods upon invocation.
- A synthesized method may not exist as a separate method until we
call it.
- invokeMethod, methodMissing and propertyMissing.
- “Intercept, Cache, Invoke” pattern.
MOP Method Synthesis
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34. Check for methods and properties
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35. MOP Instances Synthesis
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36. MethodMissing example
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37. MethodMissing example
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38. MethodMissing example
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39. Compile-time
Metaprogramming

40. - Advanced feature.
- Analyze and modify a program’sstructure at compile time.
- Cross-cutting features:
- Inspect classes for thread safety
- Log messages
- Perform pre- and postcheck operations
all without explicitly modifying the source code.
- We write code that generates bytecode or gets involved during the
bytecode generation.
Compile-time Metaprogramming
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41. - AST: Abstract Syntax Tree
- During compilation the AST is transformed
- Hook into different phases to change the final byte-code.
- Initialization, Parsing, Conversion, Semantic analysis,
Canonicalization, Instruction selection, Class generation, Output,
Finalization.
AST and Compilation
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42. - Groovy provides out-of-the-box a lot of AST Transformations
- @EqualsAndHashCode, @ToString, @TuppleConstructor,
@Canonical, @Grab, @Immutable, @Delegate, @Singleton,
@Category, @Log4j, @CompileStatic, @TypeChecked,
@Synchronized...
Groovy AST Transformations
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43. Global
AST Transformations

44. - There's no need to annotate anything.
- Applied to every single source unit during compilation.
- Can be applied to any phase in the compilation.
- Need a metadata file into the JAR file
(META-INF/services/org.codehaus.groovy.transform.
ASTTransformation)
- Grails uses Global Transformations intensively for example in
GORM.
Global AST Transformations
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45. Local
AST Transformations

46. - Annotate the code and only applied to that code.
- Easy to debug.
- No need to create metadata file in a jar.
- Steps: Define an interface, Define the AST transformation, Enjoy!
Local AST Transformations
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47. Local AST Transformation example
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48. Local AST Transformation example

49. Local AST Transformation example

50. - Most of us are not Groovy Commiters, so how do we create the
necessary nodes for our AST Transformation.
- Check de documentation:
http://groovy.codehaus.org/api/org/codehaus/groovy/ast/ASTNode
.html
- Use an IDE
- Use GroovyConsole and open Groovy AST Browser (CTRL + T)
Creating AST nodes. What?
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51. - Yes. We can use AstBuilder
- buildFromSpec: Provides a DSL for the ClassNode objects
- buildFromString: Creates the AST nodes from a String with the
code.
- buildFromCode: We write directly the source code and the builder
returns the AST.
Is there an easy way?
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52. Recap:
Why we should use
all of this?

53. - Groovy provides meta-programming features out-of-the-box, it's a
pity not using them.
- Metaprogramming is easy to use and it's very powerful.
- We can write better code.
- We can add a lot of behaviour to our code in an easy way.
- We're Groovy developers and we need to take advantage of all its
power.
- Because, Groovy it's groovy.
Why we should use all of this?
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54. http://lopezivan.blogspot.comhttp://lopezivan.blogspot.com
@ilopmar@ilopmar
https://github.com/lmivanhttps://github.com/lmivan
Iván López MartínIván López Martín
lopez.ivan@gmail.comlopez.ivan@gmail.com
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Thank you!
http://kaleidos.net/4B0082http://kaleidos.net/4B0082