Groovy Introduction at Javaforum 2010

for Developers
by Leonard Axelsson (@xlson)

1

Thursday, June 17, 2010

SweGUG

Leonard Axelsson
• Groovy user since 2006
• Co-Founder of SweGUG
–Swedish Groovy User Group
• Speaker at GTUG, Agical Geeknight, JFokus,
SweGUG
• Developer/Consultant at Qbranch Stockholm

2


SweGUG

Agenda
• Overview and obligatory Hello World
• Syntax Overview
• Tips and Trix

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SweGUG

Groovy Overview
• Originated in 2003
• Under the Apache License
• Grammar derived from Java 1.5

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SweGUG

Groovy Overview
• Dynamic language
–Inspired by Python, Ruby and Smalltalk
• Object Oriented
• Easy to learn for Java devs
–Supports Java style code out of the box
• Scriptable
• Embeddable

5


SweGUG

Dynamic Language
• No compile-time checking
– int i = “Hello” throws exception at runtime
• Ducktyping
– def keyword allows you to care about what the object
does, not what it is
• Supports custom DSLs
new DateDSL().last.day.in.december( 2009 )

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SweGUG

Gotchas
• == uses equals()
–not object identity
–is() used for identity comparission
• return keyword is optional
• Methods and classes are public by default
• All exceptions are unchecked exceptions
• There are no primitives

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SweGUG

The obligatory Hello World

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SweGUG

How many in here know Groovy?

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SweGUG

Groovy can be coded the “Java”
way. So basically all of you!

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SweGUG

HelloWorld.java

class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}

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SweGUG

HelloWorld.groovy

class HelloWorld {
}
}

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SweGUG

Removing ...

class HelloWorld {
}
}

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SweGUG

Removing ...

class HelloWorld {
System.out.println("Hello World!")
}
}

14


SweGUG

Removing ...

class HelloWorld {
println("Hello World!")
}
}

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SweGUG

Removing ...

println("Hello World!")

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SweGUG

This is it

println "Hello World!"

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SweGUG

Feature Overview
• Properties
–dynamic getters and setters
• Closures
–reusable blocks of code
• Meta Object Protocol
–rewrite behaviour at runtime
• Many additions to the JDK

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SweGUG

Default imports
• java.io.*
• java.lang.*
• java.math.BigDecimal
• java.math.BigInteger
• java.net.*
• java.util.*
• groovy.lang.*
• groovy.util.*

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SweGUG

Strings

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SweGUG

Remember this?

println "Hello World!"

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SweGUG

Let’s add something ...
• Macros supported in double-quoted strings
String whome = "World!"

println "Hello $whome"

Output:
Hello World!

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SweGUG

Plain Old Groovy Objects

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SweGUG

POGO’s
• Properties
–getters and setters are created automatically
• Named Parameters
–clariﬁes intent
class Person {
String name
String lastname
}

def anders = new Person(name: 'Anders', lastname: 'Andersson')
assert anders.getName() == 'Anders'

// Anders gets married and changes his lastname
anders.setLastname("Sundstedt")
assert anders.lastname == "Sundstedt"

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SweGUG

POGO’s
• Getters and setters can be overridden
class Person {
def name
def lastname

String setLastname(String lastname) {
this.lastname = lastname.reverse()
}
}

def anders = new Person(name: 'Anders', lastname: 'Andersson')

// Anders does a strange change of his lastname
anders.lastname = "Sundstedt"
assert anders.lastname == "tdetsdnuS"

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SweGUG

Built in syntax for lists and maps

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SweGUG

Lists
List syntax:
def names = ['Leonard', 'Anna', 'Anders']
assert names instanceof List
assert names.size() == 3
assert names[0] == 'Leonard'

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SweGUG

Maps

def map = [name: 'Leonard',
lastname: 'Axelsson']
assert map.name == 'Leonard'
assert map['lastname'] == 'Axelsson'

map.each{ key, value ->
println "Key: $key, Value: $value"
}

Output:
Key: name, Value: Leonard
Key: lastname, Value: Axelsson

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SweGUG

each, ﬁnd and ﬁndAll

class Person {
String name
String lastname
boolean male
}

def ages = [new Person(name: 'Bo', lastname: 'Olsson', male: true),
new Person(name: 'Gunn', lastname: 'Bertilsson', male: false),
new Person(name: 'Britt', lastname: 'Olsson', male: false)]
// Print all names
ages.each { println "$it.name $it.lastname" }

// Find one male
assert ages.find{ person -> person.male }.name == 'Bo'

// or find all females
assert ages.findAll{ Person p -> !p.male }.size() == 2

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SweGUG

Power Asserts
• New in Groovy 1.7
// Will throw an assertion error
def getNum() { 5 }
assert (1 + 100) == num + 77

Output:
Assertion failed:

assert (1 + 100) == num + 77
          |      | |   |
          101    | 5   82
                 false

at org.codehaus.groovy.runtime.InvokerHelper.assertFailed(InvokerHelper.java:378)
at ...

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SweGUG

Method not found?
• Better feedback in Groovy 1.7
// Will throw a MissingMethodException (no beginsWith method)
def carType = "BMW M3"
if(carType.beginsWith('Volvo')) {
println "It's a Volvo!"
}

Output:
groovy.lang.MissingMethodException: No signature of method: java.lang.String.beginsWith() is applicable for argument types:
(java.lang.String) values: [Volvo]
Possible solutions: endsWith(java.lang.String), startsWith(java.lang.String)
at ...

31


SweGUG

@Grab and grape
• Dependency management using Apache Ivy
–Uses Maven Central
• grape commandline tool
–Adds dependencies to Groovys global classpath
• @Grab annotation
–Great for scripting

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SweGUG

@Grab and GPars
@Grab(group='org.codehaus.gpars', module='gpars', version='0.10')
import groovyx.gpars.GParsExecutorsPool

def importProcessingData = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 ,18, 19, 20]

time('Parallel execution') {
GParsExecutorsPool.withPool {
importProcessingData.eachParallel { data ->
// Insert arbitrary heavy operation here
sleep 200
}

}
}

time('Linear execution') {
importProcessingData.each {
// Insert arbitrary heavy operation here
sleep 200
}
}
Output (executed on dual-‐core machine):
Parallel execution desc, 1449 ms.{
def time(String took: task)
Linear execution took: 4006 Date().time
def startTime = new ms.
def result = task()
def executionTime = new Date().time - startTime
println "$desc took: $executionTime ms."
result
}

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SweGUG

Links
• Groovy
–http://groovy.codehaus.org/
• Groovy Goodness (great tips and trix)
–http://mrhaki.blogspot.com/
• SweGUG
–http://groups.google.com/group/swegug

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Groovy Introduction at Javaforum 2010

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