JavaOne 2008 - TS-5793 - Groovy and Grails, changing the landscape of Java EE patterns

Groovy and Grails: Changing the Landscape of Java™ Platform, Enterprise Edition (Java EE Platform) Patterns Graeme Rocher, CTO – G2One Inc Guillaume LaForge, VP of Engineering – G2One Inc TS-5793

Learn how Groovy through its powerful meta-programming techniques is enabling frameworks like Grails to change the way we think about patterns on the Java™ platform

Your Speakers

Graeme Rocher

Project Lead of Grails

CTO of G2One Inc – The Groovy/Grails Company

Member of JSR-241 (Java Specification Request) Expert Group

Author of “The Definitive Guide to Grails”

Your Speakers

Guillaume LaForge

Project Lead of Groovy

VP of Technology at G2One Inc – The Groovy/Grails Company

Lead of JSR-241 Expert Group

Co-Author of “Groovy in Action”

Agenda

Introduction and Why Groovy?

What makes a dynamic language?

Groovy’s Meta Object Protocol

Domain Specific Languages

Meta Programming Patterns

Applying Meta Programming techniques to Java EE platform patterns

The DAO

The Service Locator

What is Groovy?

Groovy is a dynamic language for the Java Virtual Machine (JVM)

Takes inspiration from Smalltalk, Python and Ruby

Integrates with the Java platform language and platform at every level

What is Grails?

A Web platform that implements the full stack from build system down to ORM layer

Leverages existing technologies like Spring, Hibernate, Quartz etc. avoiding re-inventing the wheel

Features and extensible plug-in system and an environment for runtime configuration built on Spring

Getting Started

Download from http://grails.org/Download

Extract zip to disk

Set GRAILS_HOME variable to location on disk

Add $GRAILS_HOME/bin to your environment variables

Download from http://groovy. codehaus .org

Extract zip to disk

Set GROOVY_HOME variable to location on disk

Add $GROOVY_HOME/bin to your environment variables

Why is Groovy dynamic?

A lot of confusion exists on what a factors make a language dynamic

Dynamic vs Static Typing

Strong vs. Weak Typing

Meta programming

Let’s clarify these!

Dynamic/Static/Weak/Strong

The Meta Object Protocol (MOP)

There are many dynamic languages

VB, Python, Ruby, Groovy, JavaScript™ technology

But, only a few have a MOP

Groovy

Ruby

LISP

Smalltalk

A MOP makes the semantics of a program extensible

Groovy’s MOP

Every class has a MetaClass, which can be obtained with:

The MetaClass defines the behaviour of the object and can be inspected:

def obj = "Hello World!" def metaClass = obj.metaClass obj. metaClass.methods .each { println it.name } Access the metaClass property The methods collection returns a list of MetaMethod instances

Using respondsTo and hasProperty

Need to find out whether an object implements a method? Use respondsTo :

Need to find out if an object has a property? Use hasProperty :

def foo = "Hello World!" if(foo. respondsTo(foo, "toUpperCase")) println foo.toUpperCase() if(foo.metaClass. hasProperty( "bytes") ) println foo.bytes.encodeAsBase64()

Useful API References

MetaObjectProtocol

http://groovy. codehaus . org/api/groovy/lang/MetaObjectProtocol .html

MetaMethod

http://groovy. codehaus . org/api/groovy/lang/MetaMethod .html

MetaProperty

http://groovy. codehaus . org/api/groovy/lang/MetaProperty .html

Adding Methods at Runtime

Instance methods can be added using the MetaClass:

As can properties using JavaBeans™ architecture conventions:

class Dog {} Dog. metaClass.bark = { "woof!" } println new Dog().bark() class Dog {} Dog. metaClass.getBreed = { "Bulldog" } println new Dog().breed Assigning a block of code to a property of the MetaClass creates a method! Properties need to follow JavaBeans naming coventions

Static Methods and Constructors

Static methods can be added using the static qualifier:

Constructors can be added using a special constructor property:

Dog.metaClass .static.bark = {new Dog() } println Dog.create().bark() Dog. metaClass .constructor = {String s -> new Dog(name:s) } println new Dog( "Fred" ).name Prefix the method name with the static qualifier to make a static method Careful of Stack overflows when overriding constructors!

Meta-Programming Hooks

invokeMethod , methodMissing , get/setProperty , propertyMissing :

Dog.metaClass . methodMissing = { String name, args-> println "Dogs don’t $name!" } def d = new Dog() d.quack() d.fly() "Dogs don’t quack!" "Dogs don’t fly!" No exception is thrown, the call is intercepted and the messages printed to system out Method missing allows you to intercept failed method dispatch

Meta-Programming Patterns

Intercept , Cache , Invoke

Enables code synthesis

Basic Usage:

Intercept method

Dynamically create new method

Cache new method

Invoke new method

First call takes performance hit, next call faster

Dog.metaClass . methodMissing = {String name, args-> def cached = {Object[] a -> println ”Dogs don’t $name!" } Dog.metaClass . "$name" = cached cached. call(args) } Intercept – implement methodMissing Cache – Dynamically register new method Invoke – call new behaviour

Re-inventing Patterns with the MOP The Data Access Object The Service Locator

The Data Access Object (DAO)

Basic Steps

Define an interface

Define one or many implementations

Inject dependencies into implementations

Dependency Injection (IoC)

Factories

http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html

Implications of DAO

Class explosion

Promotes anemic domain model

Configuration, configuration, configuration

Violates DRY

Creating Dynamic SQL Finders Meta Magic with Groovy

Dynamic SQL Finders Example Book.metaClass. static . methodMissing = { String name, args -> if (name.startsWith("findBy") && args) { def prop = name[6..-1] prop= prop[0].toLowerCase() + prop[1..-1] def callable = { Object[] varArgs -> def results = [] def sql = Sql.newInstance( url, user, pass, driver) sql.eachRow( """select * from ${Book.name} where $prop=?""" , [varArgs[0]]) { results << new Book(title:it.title, author:it.author) } return results } Book.metaClass. "$name" = callable return callable. call(args) } } Intercept Cache Invoke

Real Life Example: GORM in Grails class Album { String title String artist Date releaseDate static hasMany = [songs:Song] } class Song { String title Double duration } table - album table - song GORM classes, also known as domain classes, go in the domain directory id title artist release_date id title duration album_id

GORM in Action Querying and Persistence with GORM

Dynamic Finders & Criteria def albums = Album. list () def recentAlbums = Album. findAllByReleaseDateGreaterThan (new Date()-7) def albumsStartingWithA = Album. findAllByTitleLike ( "A%") def albumsWithSongsAboutSummer = Album. withCriteria { songs { like("title", "%Summmer%") } } List all records Form method expressions Use “like” queries Construct criteria on the fly to query associations

GORM Features

Dynamic finder and persistence methods

Criteria with a Groovy builder

Object-relational Mapping DSL

Caching

Legacy mapping

Locking strategy (optimistic/pessimistic)

Built on Hibernate

+

MOP == Goodbye to the DAO

Repetitive, boilerplate DAO logic gone (DRY)

Logic exists where it belongs - in the domain (DDD)

No direct references to third part dependencies!

The Service Locator / IoC

Pattern of many names

Dependency Injection

Service Locator

Inversion of Control

Abstracts dependency look-up code

http://java.sun.com/blueprints/corej2eepatterns/Patterns/ServiceLocator.html

Implications of Service Locator

Requires factory classes

Class explosion

Promotes anemic domain model

Configuration, configuration, configuration

Automatic Dependency Injection Automatic Dependency Injection with Spring & Grails via Meta-programming

Real Life Example: Spring & Grails class Book { PurchasingService purchasingService Transaction buyBook(User u) { purchasingService.buyBook(this, u) } } Book. metaClass.constructor = {-> def obj = BeanUtils.instantiateClass(Book) applicationContext .getAutowireCapableBeanFactory() . autowireBeanProperties( obj, AutowireCapableBeanFactory.AUTOWIRE_BY_NAME, false) return obj } Book domain class requires reference to PurchasingService Overriding default constructor for class Autowire dependencies into object from Spring at construction time

Real Life Example: Spring & Grails