Greach 2014 - Metaprogramming with groovy

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IvAn LOpez MartIn @ilopmar
Metaprogramming
with Groovy

Iván Lopez Martín @ilopmar
I work at Kaleidos
Using Groovy/Grails since 4 years
Creator of www.bokzuy.com
Developed some G...

- A dynamic language like Groovy allows to "delay" to runtime some
checks that are usually performed during the compilatio...

What is
Metaprogramming?

From Wikipedia:
Metaprogramming is the writing of computer programs that
write or manipulate other programs (or themselves...

Runtime
Metaprogramming

- Groovy provides this capability through the Meta-Object Protocol
(MOP).
- We can use MOP to invoke methods dynamically a...

Groovy
Groovy
Java
Java
MOP
What is the Meta Object Protocol (MOP)?
8/54

Intercepting methods
using MOP

- Classes compiled by Groovy implements GroovyObject interface.
- We can implement GroovyInterceptable to hook into the ex...

GroovyInterceptable example
11/54

GroovyInterceptable example (II)
12/54

- Groovy maintains a meta class of type MetaClass for each class.
- Maintains a collection of all methods and properties o...

MetaClass example
14/54

MOP
Method Injection

- In Groovy we can “open” a class at any time.
- Method Injection at code-writing time, we know the names of
methods we wa...

- MetaClassImpl: Default meta class, it's used in the vast majority of
case.
- ExpandoMetaClass: allow the addition or rep...

Adding methods using MetaClass
18/54

Adding properties using MetaClass
19/54
* Note: This is only true for Groovy.
In Grails all MetaClass are ExpandoMetaClass...

Adding constructor using MetaClass
20/54

Overriding methods using MetaClass
21/54

Overriding methods using MetaClass
22/54

- Changes made to a MetaClass are “persistent” and hard to revert.
- Categories are useful to change the meta class in a c...

Categories example

Categories example (II)
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Categories example (II)
26/54

Categories example (II)
27/54

- JAR file with classes that provide extra methods to other classes.
- It also has to contain a meta-information file
- Ad...

Extension modules example
29/54

- A mixin allow “bring in” or “mix in” implementations from multiple
classes.
- Groovy first call the mixed-in class.
- Mi...

Mixins example
31/54

MOP
Method Synthesis

- Dynamically figure out the behaviour for methods upon invocation.
- A synthesized method may not exist as a separate met...

Check for methods and properties
34/54

MOP Instances Synthesis
35/54

MethodMissing example
36/54

MethodMissing example
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MethodMissing example
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Compile-time
Metaprogramming

- Advanced feature.
- Analyze and modify a program’sstructure at compile time.
- Cross-cutting features:
- Inspect classes...

- AST: Abstract Syntax Tree
- During compilation the AST is transformed
- Hook into different phases to change the final b...

- Groovy provides out-of-the-box a lot of AST Transformations
- @EqualsAndHashCode, @ToString, @TuppleConstructor,
@Canoni...

Global
AST Transformations

- There's no need to annotate anything.
- Applied to every single source unit during compilation.
- Can be applied to any ...

Local
AST Transformations

- Annotate the code and only applied to that code.
- Easy to debug.
- No need to create metadata file in a jar.
- Steps: D...

Local AST Transformation example
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Local AST Transformation example

Local AST Transformation example

- Most of us are not Groovy Commiters, so how do we create the
necessary nodes for our AST Transformation.
- Check de docu...

- Yes. We can use AstBuilder
- buildFromSpec: Provides a DSL for the ClassNode objects
- buildFromString: Creates the AST ...

Recap:
Why we should use
all of this?

- Groovy provides meta-programming features out-of-the-box, it's a
pity not using them.
- Metaprogramming is easy to use a...

http://lopezivan.blogspot.comhttp://lopezivan.blogspot.com
@ilopmar@ilopmar
https://github.com/lmivanhttps://github.com/lm...

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