MetaProgrammingWith

Groovy

Presentation by: Ali and Gaurav

Agenda

▣ Groovy Is Dynamic▣ What is MetaProgramming?▣ Runtime MetaProgramming

□ What is MOP?□ Understanding Groovy□ Meta Class

▣ Intercepting Methods Using MOP□ InvokeMethod□ GroovyInterceptable□ Intercepting Methods using MetaClass

▣ MOP Method Injection□ MetaClass□ Categories□ Mixins/Traits

Agenda (continues..)

▣ MOP Method Synthesis▣ MOP Class Synthesis▣ Compile-time MetaProgramming

□ AST and Compilation□ Groovy AST Transformation□ Global AST Transformation□ Local AST Transformation

▣ Why we should use MetaProgramming?▣ References

Groovy Is Dynamic

▣Groovy allows "Delay" to runtime some checks/decisions that are usually performed during the compilation.

▣Add properties/behaviours in 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.

What isMetaProgramming

?

‘’Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data.

- Wikipedia

Overview

The Groovy language supports two flavors of metaprogramming:

▣Runtime metaprogramming, and▣Compile-time metaprogramming.

The first one allows altering the class model and the behavior of a program at runtime, while the second only occurs at compile-time.

RuntimeMetaProgramming

Runtime MetaProgramming

▣Groovy provides this through Meta-Object Protocol (MOP).

▣We can use MOP to:□Invoke methods dynamically□Synthesize classes and methods on the fly.

With runtime metaprogramming we can postpone to runtime the decision to intercept, inject and even synthesize methods of classes and interfaces.

What is the Meta Object Protocol?

Understanding Groovy

For a deep understanding of Groovy MOP we first need to understand Groovy objects and Groovy’s method handling. In Groovy, we work with three kinds of objects:

▣POJO,▣POGO, and▣Groovy Interceptors

So, for each object Groovy allows metaprogramming but in different manner.

continues...

▣ POJO - A regular Java object, whose class can be written in Java or any other language for the JVM.

▣ POGO - A Groovy object, whose class is written in Groovy. It extends java.lang.Object and implements the groovy.lang.GroovyObject interface by default.

▣ Groovy Interceptor - A Groovy object that implements the groovy.lang.GroovyInterceptable interface and has method-interception capability, which we’ll discuss in the GroovyInterceptable section.

For every method call Groovy checks whether the object is a POJO or a POGO. For POJOs, Groovy fetches it’s MetaClass from the groovy.lang.MetaClassRegistry and delegates method invocation to it.

continues...

For POGOs, Groovy takes more steps, as illustrated below:

Meta Class

▣MetaClass registry for each class.▣Collection of methods/properties.▣We can always modify the metaclass.

Intercepting Methods Using

MOP

Groovy Object Interface

GroovyObject has a default implementation in the groovy.lang.GroovyObjectSupport class and it is responsible to transfer invocation to the groovy.lang.MetaClass object. The GroovyObject source looks like this:

package groovy.lang;

public interface GroovyObject {

Object invokeMethod(String name, Object args);

Object getProperty(String propertyName);

void setProperty(String propertyName, Object newValue);

MetaClass getMetaClass();

void setMetaClass(MetaClass metaClass);}

1.invokeMethod

invokeMethod

This invokeMethod is called when the method you called is not present on a Groovy object. Example:

class InvokeMethodDemo { def invokeMethod(String name, Object args) { return "called invokeMethod $name $args" }

def test() { return 'method exists' }}

def invokeMethodDemo = new InvokeMethodDemo()assert invokeMethodDemo.test() == 'method exists'assert invokeMethodDemo.hello() == 'called invokeMethod hello []'

2.GroovyInterceptab

le

GroovyInterceptable

▣ Classes compiled by Groovy implements GroovyObject interface.

▣ We can implement GroovyInterceptable to hook into the execution process.

package groovy.lang;

public interface GroovyInterceptable extends GroovyObject {}

When a Groovy object implements the GroovyInterceptable interface, it’s invokeMethod() is called for any method calls.

GroovyInterceptable Example

class InterceptionDemo implements GroovyInterceptable { def definedMethod() {}

def invokeMethod(String name, Object args) { "$name invokedMethod" }}

def interceptionDemo = new InterceptionDemo()assert interceptionDemo.definedMethod() == 'definedMethod invokedMethod'assert interceptionDemo.someMethod() == 'someMethod invokedMethod'

3.Intercepting

Methods Using MetaClass

Intercepting Methods using MetaClass

If we want to intercept all methods call but do not want to implement the GroovyInterceptable interface we can implement invokeMethod() on an object’s MetaClass.

▣ 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.

MetaClass Example

class InterceptionThroughMetaClassDemo { void sayHello(String name) { println "========> Hello $name" }}

InterceptionThroughMetaClassDemo.metaClass.invokeMethod = { String methodName, Object args ->

println("Invoking method '$methodName' with args '$args'") def method = InterceptionThroughMetaClassDemo.metaClass.getMetaMethod(methodName, args) method?.invoke(delegate, args)}

def demo = new InterceptionThroughMetaClassDemo()demo.sayHello("ALI")demo.sayHI("EVERYONE")demo.anotherMethod()

MOP Method Injection

MOP Method Injection

▣In Groovy we can “open” a class at any time.▣Injecting methods at code-writing time; we

know the names of methods we want to add.▣Different techniques:

□MetaClass□Categories□Extensions□Mixins / Traits

1.MetaClass

Types of MetaClass

▣ 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.

▣ Other meta classes used internally and for testing.

Note: This is only true for Groovy.▣ In Grails all MetaClass are ExpandoMetaClass.

Adding Methods Using MetaClass

class StringUtils { static String truncate(String text, Integer length, Boolean overflow = false) { text.take(length) + (overflow ? '...' : '') }}String chuckIpsum = "If you can see Chuck Norris, he can see you.\If you can not see Chuck Norris you may be only seconds away from death"

println StringUtils.truncate(chuckIpsum, 72)println StringUtils.truncate(chuckIpsum, 72, true)

String.metaClass.truncateDemo = { Integer length, Boolean overflow = false -> println "truncate string upto length $length" delegate.take(length) + (overflow ? '...' : '')}assert chuckIpsum.truncateDemo(20, true) == StringUtils.truncate(chuckIpsum, 20, true)

Adding Properties Using MetaClass

class Utils {}

def utilsInstance = new Utils()

Utils.metaClass.version = "3.0"utilsInstance.metaClass.released = true

assert utilsInstance.version == "3.0"assert utilsInstance.released == trueprintln("utilsInstance.version: $utilsInstance.version")println("utilsInstance.released: $utilsInstance.released")

Continues.. (Example 2)

class ExpandoLite {

protected dynamicPropMap = [:]

void setProperty(String propName, val) { dynamicPropMap[propName] = val }

def getProperty(String propName) { dynamicPropMap[propName] }}

class ExpandoLiteSpec extends Specification {

void 'test property access'() { given: def ex = new ExpandoLite()

when: ex.companyName = 'Nexthoughts'

then: 'Nexthoughts' == ex.companyName }}

Adding Constructors Using MetaClass

try { println(new Integer(Calendar.instance))}catch (e) {}

Integer.metaClass.constructor << { Calendar calendar -> new Integer(calendar.get(Calendar.DATE))}println("Today's Date: ${new Integer(Calendar.instance)}")

Overriding Methods Using MetaClass

// Integerassert '15' == 15.toString()Integer.metaClass.toString = { delegate == 15 ? 'The answer to life, the universe and everything' : String.valueOf(delegate)}

assert 15.toString() == 'The answer to life, the universe and everything'assert 100.toString() == '100'println("15.toString(): ${15.toString()}")println("100.toString(): ${100.toString()}")

// Booleanassert false.toBoolean() == falseBoolean.metaClass.toBoolean = { !delegate }

assert false.toBoolean() == trueprintln("false.toBoolean(): ${false.toBoolean()}")

2.Categories

Categories

▣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, it resets to its old state.▣Category classes are not special.

Continues.. (Example 1)

class StringUtils { static String truncate(String text, Integer length, Boolean overflow = false) { text.take(length) + (overflow ? '...' : '') }}

use(StringUtils) { println "Hello! Everyone.".truncate(5)}try { println "Hi! Ali.".truncate(5)} catch (MissingMethodException mme) { println mme}

Continues.. (Example 2)

class Distance { def number

String toString() { println("==========> ${number}") "${number}" }}

@Category(Number)class NumberCategory { Distance getMeters() { new Distance(number: this + 'm') }

Distance getKiloMeters() { new Distance(number: this + 'km') }}

use(NumberCategory) { assert 10.kiloMeters.toString() == '10km' assert 50.meters.toString() == '50m'}

3.Mixins / Traits

Mixins

▣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.

continues..

class SpidermanPower { String spiderSense() { "Using spider-sense..." }}

class SupermanPower { String fly() { "Flying..." }}

@Mixin([SpidermanPower])class Person {}

def person = new Person()assert person.spiderSense() == "Using spider-sense..."

assert !(person instanceof SpidermanPower)

Person.mixin SupermanPower

assert person.fly() == "Flying..."assert !(person instanceof SupermanPower)

continues..

class SpidermanPower { String spiderSense() { "Using spider-sense..." }}

class SupermanPower { String fly() { "Flying..." }}

@Mixin([SpidermanPower])class Person {}

def person = new Person()assert person.spiderSense() == "Using spider-sense..."

assert !(person instanceof SpidermanPower)

Person.mixin SupermanPower

assert person.fly() == "Flying..."assert !(person instanceof SupermanPower)

‘’When we started fixing mixin bugs we didn't know if they were a bug or a feature, so we removed mixins and add traits.

- Jochen Theodorou

▣Groovy 2.3+▣Similar to Java 8 default methods▣Supported in JDK 6, 7 and 8▣Stateful▣Composition over inheritance▣Documentation

Traits

Note - Link to Groovy Traits PPT

Traits Example

trait SpidermanPower { String spiderSense() { "Using spider-sense..." }}

class Person implements SpidermanPower {}

def person = new Person()assert person.spiderSense() == "Using spider-sense..."println("=====> person.spiderSense(): ${person.spiderSense()}")assert person instanceof SpidermanPowerdef person2 = person.withTraits SupermanPowerassert person2.fly() == "Flying..."println("=====> person2.fly(): ${person2.fly()}")assert person2 instanceof SupermanPower

trait SupermanPower { String fly() { "Flying..." }}

MOP Method Synthesis

MOP Method Synthesis

▣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.

Check for Methods and Properties

class Person { String name Integer age

String sayHi() { "Hi, my name is ${name} and I'm ${age}" }

String sayHiTo(String name) { "Hi ${name}, how are you?" }}

def p = new Person(name: 'Superman', age: 34)

assert p.respondsTo('sayHi')assert p.respondsTo('sayHiTo', String)assert !p.respondsTo('goodbye')

assert p.hasProperty('name')assert !p.hasProperty('country')

Method Delegation

cl = { -> append "Hi!" append " this is closure delegate demo."}

sb = new StringBuffer()cl.delegate = sb

cl()println "SB: ${sb}"

Continues.. (Example 2)

class ClosureDemo { void append(String arg) { println("append called with: ${arg}") }

void doit() { def cl = { append 'MetaProgramming Demo by' append ' Ali and Gaurav' } def sb = new StringBuffer() cl.delegate = sb cl() println("SB: ${sb}") }

static void main(args) { new ClosureDemo().doit() }}

MOP Class Synthesis

Creating Dynamic Classes with Expando

carA = new Expando()carB = new Expando(year: 2012, miles: 0)carA.year = 2012carA.miles = 10println "carA: " + carAprintln "carB: " + carB

In Groovy we can create a class entirely at runtime. The Groovy Expando class gives us the ability to synthesize classes dynamically.It got its name because it is dynamically expandable. We can assign properties and methods to it either at construction time using a Map or at any time dynamically.

car = new Expando(year: 2012, miles: 0, turn: { println 'turning...' })car.drive = { miles += 10 println "$miles miles driven"}car.drive()car.turn()

Compile-timeMetaProgramming

?

Compile-time MetaProgramming

▣Advanced feature.▣Analyze and modify a program’s structure 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.

AST and Compilation

▣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.

Groovy AST Transformations

▣Groovy provides out-of-the-box a lot of AST Transformations

▣@EqualsAndHashCode, @ToString, @TuppleConstructor, @Canonical, @Grab, @Immutable, @Delegate, @Singleton, @Category, @Log4j, @CompileStatic, @TypeChecked, @Synchronized, etc.

Global AST Transformations

▣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.

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: Define an interface, Define the AST

transformation, Enjoy!

Why we should use

MetaProgramming?

Concept Review

▣Metaprogramming Easy and very out-of-the box

▣Easy and very powerful▣Write better code▣Add Behaviour easily▣Take advantage of this power because

Groovy, it’s Groovy

References

▣ http://groovy-lang.org/metaprogramming.html▣ http://www.slideshare.net/ilopmar/metaprogramming-w

ith-groovy▣ http://www.slideshare.net/ilopmar/gr8conf-2016-metapr

ogramming-with-groovy

Thanks!Any questions?

You can find us at -▣ ali.tanwir@nexthoughts.com▣ gaurav.gupta@nexthoughts.com

You can find demo code used with this presentation at -https://github.com/NexThoughts/groovyMetaProgramming