Groovy for Java Devs

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Gettin’ Groovy for Java Devs
Zachary Klein, Senior Software Engineer

© 2019, Object Computing, Inc. (OCI). All rights reserved. objectcomputing.com
Zachary Klein is a Senior Software Engineer at
OCI. He has been practicing web
development since 2010 and frontend
development since 2015. He’s a contributor
to both the Grails and Micronaut
frameworks, a conference speaker and an
instructor in OCI’s training practice.
Zachary’s home base is in St Louis, MO, along
with his wife, Beth, and their three children.
2
Speaker

1. What is Groovy?
2. Why use Groovy?
3. From Java to Groovy
4. Overview/Demos of
Groovy Features
3
Agenda
5. Groovy Ecosystem
6. What’s Next for Groovy?
7. Resources

© 2019, Object Computing, Inc. (OCI). All rights reserved. objectcomputing.com 4
What is Groovy?

• Dynamic Language for the JVM
• Style-Agnostic: OOP, Functional, Scripts, DSLs
• Static and Dynamic Type-checking
• Compiles to Java Bytecode
• Development sponsored by Object Computing, Inc
• Top-level Project of the Apache Software Foundation
5
What is Groovy?

Why Use Groovy?

• Power
• Advanced programming & productivity-enhancing features
• Versatility
• Can be used for full programs to scripts; OOP or functional
style; testing; build scripts; “glue code”
• Simplicity
• Flat learning curve, expressive & readable code
8
Why Use Groovy?

How to get Groovy?
• SDKMAN
• http://sdkman.io
• Manual Setup
• Download Latest Release
• http://groovy-lang.org/
• Extract Archive
• Set $GROOVY_HOME
• Add $GROOVY_HOME/bin to PATH

Demo: SDKMan

• groovysh - Groovy Shell
11
Playing with Groovy

• groovyConsole - Groovy Console
12
Playing with Groovy

• Groovy Web Console - groovyconsole.appspot.com
13
Playing with Groovy

From Java to Groovy

From Java to Groovy
// PrintIndependenceDay.java
 
import java.util.Calendar; 
import java.util.Date; 
 
public class PrintIndependenceDay { 
 
public static void main(String[] args) { 
Calendar calendar = Calendar.getInstance(); 
calendar.clear(); 
calendar.set(Calendar.MONTH, Calendar.JULY); 
calendar.set(Calendar.DATE, 4); 
calendar.set(Calendar.YEAR, 1776); 
 
Date time = calendar.getTime(); 
 
System.out.println(time); 
} 
}

Rename file to .groovy
// PrintIndependenceDay.groovy 
 
 
 
 
 
} 
}

Remove unnecessary imports
 
 
 
 
} 
}

Remove optional semicolons
 
 
Calendar calendar = Calendar.getInstance() 
calendar.clear() 
calendar.set(Calendar.MONTH, Calendar.JULY) 
calendar.set(Calendar.DATE, 4) 
calendar.set(Calendar.YEAR, 1776) 
 
Date time = calendar.getTime() 
 
System.out.println(time) 
} 
}

Remove getter method calls
 
Calendar calendar = Calendar.instance 
calendar.clear() 
 
Date time = calendar.time 
 
} 
}

Use dynamic typing
 
def calendar = Calendar.instance 
calendar.clear() 
 
def time = calendar.time 
 
} 
}

Use Groovy println
 
calendar.clear() 
 
 
println(time) 
} 
}

Remove class
calendar.clear() 
calendar.set(Calendar.YEAR, 1776)
 
def time = calendar.time
 
println(time)

Remove optional parantheses
calendar.clear() 
calendar.set Calendar.MONTH, Calendar.JULY 
calendar.set Calendar.DATE, 4 
calendar.set Calendar.YEAR, 1776 
 
 
println time

Use with method with a closure
 
calendar.with { 
clear() 
set MONTH, JULY 
set DATE, 4 
set YEAR, 1776 
println time 
}

Comparing Java vs Groovy
 
calendar.with { 
clear() 
set MONTH, JULY 
set DATE, 4 
set YEAR, 1776 
println time 
}
// PrintIndependenceDay.java
 
 
 
 
 
} 
}

Groovy Features

• Semicolons are optional
• Implicit return
• Common packages are auto-imported
• java.lang.*
• java.net.*
• java.util.*
• java.io.*
• etc...
• Parentheses are optional (in some situations)
• No checked exceptions 27
Syntactic Sugar

• Signified by double quotes
• Support string interpolation
• Automatically converted to java.lang.String when required
• Additional methods added - e.g: take(), toURL() etc
• Accessible with array-style syntax: message[0..4]
• Multi-line strings denoted by triple double quotes: """
28
Groovy Strings

• Classes with simple properties
• All properties & methods public by default
• Compiler generates getters & setters
• Rarely need constructors (map constructor provided by default)
• Dot access to properties (a la accessing fields)
29
Plain Old Groovy Objects (POGO)

Everything is an Object

Demo: Syntax, Strings
& POGOs

• Groovy overloads operators symbols to call methods
32
Operators
a + b a.add(b)
a - b a.minus(b)
a * b a.multiply(b)
a / b a.div(b)
a++ next()
a-- a.previous()
a % b a.mod(b)
a ** b a.power(b)
a[b] a.getAt(b)
a[b] = c a.putAt(b, c)
a << b a.leftShift(b)
a >> b a.rightShift(b)
a?.b Null-safe reference
a ?: b “Elvis” operator
http://groovy-lang.org/operators.html

• Groovy adds many useful methods to “normal” Java classes
• .times() - can be called on any number, executes a closure
that number of times
• .with() - can be called on any object, accepts a closure,
calls each unresolved method in the closure on the
referenced object
• .getProperties() - can be called on any object, returns a
Map of all properties in the object
• http://groovy-lang.org/gdk.html
33
Groovy Development Kit (GDK)

Demo: Operators

• Anonymous block of code
• Can (optionally) accept arguments & (optionally) return a value
• Can be passed as an argument or returned as a value
• Similar to Java’s lambdas, but even more powerful
• Can be assigned a delegate
• Lazily evaluated
• Can be used with the Java Streams API
35
Closures

Closures
{ item++ }
{ println it }
{ name -> println name }
{ String x, int y ->
println “X: ${x} / Y: ${y}”
}
Defining a closure
http://groovy-lang.org/closures.html
def isOdd = { int i -> i%2 != 0 }
assert isOdd(3) == true
assert isOdd.call(2) == false
Calling a closure
class Person {
String name
}
class Thing {
String name
}
def p = new Person(name: 'Norman')
def t = new Thing(name: ‘Teapot')
//A closure that calls a delegate method
def upperCasedName =
{ delegate.name.toUpperCase() }
//Call closure with different delegates
upperCasedName.delegate = p
assert upperCasedName() == 'NORMAN'
upperCasedName.delegate = t
assert upperCasedName() == 'TEAPOT'
Closure delegates

• Can be defined as literals
• e.g., a List: [1, 2, 3] - a Map: [a:1, b:2, b:3]
• GDK adds many useful methods to Java collections
• each() - accepts a closure, executes the closure for each item in
the collection
• collect() - accepts a closure, maps over each item in the
collection and returns the result as a new collection
• find() - filters collection for matching item
• findAll() - filters collection for all matching items
37
Collections

Collections
http://docs.groovy-lang.org/latest/html/documentation/working-with-collections.html
def list = [5, 6, 7, 8]
assert list.get(2) == 7
assert list[2] == 7
assert list instanceof java.util.List
List Literal Syntax
[1, 2, 3].each {
println "Item: $it”
}
['a', 'b', 'c'].eachWithIndex { it, i ->
println "$i: $it"
}
Iterating with .each
def map = [name: 'DevNexus', id: 1234]
assert map.get('name') == 'DevNexus'
assert map.get('id') == 1234
assert map['id'] == 1234
assert map.id == 1234
assert map instanceof java.util.Map
Map Literal Syntax
def map = [Bob : 42, Alice: 54, Max : 33]
map.each { it ->
println "Name: $it.key Age: $it.value"
}
map.each { key, value ->
println "Name: $key Age: $value"
}
Iterating with .each

Demo: Closures
& Collections

• Compile-time meta-programming technique
• Adds behavior to classes during compilation
• Applied using annotations
• Groovy includes many useful AST Transformations, eg:
• @EqualsAndHashcode - implements default equals() and
hashcode() methods for the annotated class
• @Immutable - marks class as final, generates equals()/hashcode()/
toString() methods, but no getters,
• You can write your own AST Transformations
40
AST Transformations

AST Transformations
import groovy.transform.ToString
@ToString
class Book {
String title
String author
}
@ToString Transformation
def book = new Book(title: ‘Grails: A Quick
Start Guide', author: ‘Dave Klein')
assert book.toString() == ‘Book(Grails: A
Quick Start Guide, Dave Klein)
Calling toString()
import groovy.transform.EqualsAndHashCode
@EqualsAndHashCode
class Book {
String title
String author
}
@EqualsAndHashCode Transformation
def b1 = new Book(title: ‘Groovy Recipes',
author: ‘Scott Davis')
def b2 = new Book(title: ‘Groovy Recipes',
author: ‘Scott Davis')
assert b1==b2
assert b1.hashCode() == b2.hashCode()
Checking equality/hashcode
http://groovy-lang.org/metaprogramming.html#_code_generation_transformations

• Language artifact, similar to interfaces with default methods
• Allow composition of behaviors - multiple inheritance
• Can have state, using public or private properties
• Can declare abstract methods
• Can declare private methods
• Statically compiled & type-checked
• Can be added to an object at runtime
42
Traits

Traits
http://docs.groovy-lang.org/next/html/documentation/core-traits.html
trait Flyer {
String fly() { "I'm flying!" }
}
trait Speaker {
String speak() { "I'm speaking!" }
}
Define traits
class Duck implements Flyer, Speaker {}
def d = new Duck()
assert d.fly() == "I'm flying!"
assert d.speak() == "I'm speaking!"
Class that implements traits
trait Named {
String name
}
trait Polite extends Named {
String introduce() { "Hello, I am
$name" }
}
Trait inheritance
trait Extra {
String extra() { "I'm an extra!” }
}
class Something {}
def s = new Something() as Extra
s.extra()
Assigning Traits at Runtime

MarkupBuilder & XmlSlurper
import groovy.xml.MarkupBuilder
def writer = new StringWriter()
def xml = new MarkupBuilder(writer)
//Create XML
xml.records() {
book() {
title('Making Java Groovy')
author('Ken Kousen')
}
book() {
title('Programming Groovy 2')
author('Venkat Subramaniam')
}
}
//Parse XML
def records = new XmlSlurper().parseText(writer.toString())
assert records.book.first().author.text() == 'Ken Kousen'
assert records.book.last().title.text() == 'Programming Groovy 2'
http://groovy-lang.org/processing-xml.html

JsonOutput & JsonSlurper
class Person {
String name
}
//Create JSON string from object
def json = JsonOutput.toJson(new Person(name: 'John'))
assert json == '{"name":"John"}'
//Parse JSON string into Map
def jsonSlurper = new JsonSlurper()
def object = jsonSlurper.parseText(json)
assert object instanceof Map
assert object.name == 'John'
http://groovy-lang.org/json.html

Writing & Reading Files
//Writing to a file
new File('names.txt').withWriter('utf-8') { writer ->
writer.writeLine 'Ken Kousen'
writer.writeLine 'Venkat Subramaniam'
writer.writeLine 'Paul King'
}
//Reading file
new File('names.txt').eachLine { line ->
println line
}
http://docs.groovy-lang.org/latest/html/documentation/working-with-io.html
//Print out the names of all the files in the current directory
new File(‘.').eachFile { file ->
println file.name
}

Groovy Ecosystem

• Full-stack web framework built on top of
Spring Boot
• Designed to enhance developer-productivity
• Provides powerful Groovy-based persistence
with GORM
• Leverages Groovy’s meta-programming
capabilities and Traits
• https://grails.org
48
Grails

• Full-stack web framework for building Spring-
based web applications
• Offers Groovy as a language option from the
Spring Initializer
• https://start.spring.io
49
Spring Boot

• Light-weight Groovy-based HTTP
framework
• Build for scalability & performance
• Uses static compilation and type-
checking features of Groovy
• https://ratpack.io
50
Ratpack

• Java Desktop Application framework
• Supports Swing, JavaFX, and more
• Leverages Groovy’s SwingBuilder for
creating Swing UIs with concise code
• http://griffon-framework.org
51
Griffon

• Testing framework for JVM applications
• Built on top of JUnit
• Promotes expressive, behavior-focused testing
• Uses a Groovy-based DSL to provide expressive testing
conventions
• http://spockframework.org
52
Spock

• Concurrency & Parallelism library for
Groovy
• Provides many concurrency abstractions,
including Promises, Actors, Parallel
Collections, and more
• Leverages the flexibility of Groovy
• http://www.gpars.org
53
Gpars

• Full-featured build tool and task runner
• First-class support for Groovy projects, including
a dedicated plugin
• Uses a Groovy DSL for build scripting (a Kotlin
DSL is also available)
• https://gradle.org/
54
Gradle

• Full-stack web framework designed for
microservices & serverless
• Uses AST transformations (in Groovy) to compute
Dependency Injection & other metadatpa at
compilation time
• Blazing fast startup and low memory footprint
• Natively cloud-native
• Supports Groovy functions for AWS Lambda
• https://micronaut.io
55
Micronaut
String myFunction() {
"hello world!"
}
Valid AWS Lambda function!

What’s Next for
Groovy?

• Runs on JDK 9+ (but with warnings)
• New AST Transformations
• New GDK methods: tap()
57
What Is New In Groovy 2.5 & 2.6

• Minimum JDK 8 requirement
• Improved Java 9+ support
• New “Parrot” parser
• Better support for modern Java code
• Support for Lambdas
• New operators: elvis assignment, identity, etc
• Safe array-indexing - null instead of “out of bounds”
• http://groovy-lang.org/releasenotes/groovy-3.0.html
58
What Is Coming in Groovy 3

Resources

Books
https://mrhaki.blogspot.com/search/label/Groovy

Groovy Community Links
• Open Collective: https://opencollective.com/friends-of-groovy
• Groovy Slack: https://groovycommunity.com/
• Groovy Events/Conferences: http://groovy-lang.org/events.html
• Groovy User Groups: http://groovy-lang.org/usergroups.html
• Hubert Klein Ikkink (Mr Haki) blog: https://mrhaki.blogspot.com/
• Baeldung blog https://www.baeldung.com/tag/groovy/
• Groovy Twitter: https://twitter.com/ApacheGroovy

Events:
● objectcomputing.com/events
Training:
● objectcomputing.com/training
● grailstraining.com
● micronauttraining.com
Or email info@ocitraining.com to schedule a custom training program for your team online,
on site, or in our state-of-the-art, Midwest training lab.
LEARN MORE ABOUT OCI EVENTS AND TRAINING

Thank you!

Groovy for Java Devs

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