An Introduction to Groovy for Java Developers

An Introduction to Groovy for Java
Developers
Kostas Saidis
www.niovity.com
Java Hellenic User Group Meetup
May 17, 2014

Outline
Introduction
to Groovy
1. Meet
Groovy
What is Groovy?
Groovy History
Why Groovy
2. Groovy
Basics
Using Groovy
Language
Overview
Closures
Lists and
Ranges
Maps
3. Real-life
Groovy
Open Public
Data Hackathon
Our Web App
JSON and REST
Groovlets

About Me
Kostas Saidis
saiko@niovity.com
Software & Data
Architect
Twitter:
@saikos
Linkedin:
http://gr.linkedin.com/in/saiko
Short bio:
Academia
BSc @ cs.unipi.gr (2001)
MSc @ di.uoa.gr (2004)
PhD @ di.uoa.gr (2011)
Industry
Freelance consultant, developer &
instructor since 1999
Founder & Managing Director
Java
Early Java enthusiast (1997)
Diving into Groovy since 2011

Target audience
Java developers
with little or no experience in
Groovy or dynamic languages.

So, what brings us here?
The Java Platform
1. The language
2. The development kit
3. The virtual machine
The JVM is the key component!
The language and the JDK are aging, striving to catch up with new
developments.
Yet, Java 8 (with Streams and Lambdas) is a huge step forward!

What really brings us here!
The fun of programming
And Groovy programming is a whole lotta fun!

1. Meet Groovy
What is
Groovy?
Groovy
History
Why Groovy

What is Groovy?
Groovy is
a feature-rich
Java-friendly
dynamic language
for the Java platform

Dynamic language?
What is a dynamic language?
A language that allows the types of variables to be
changed at runtime...
...among other things!
In Groovy
We can also change the behavior of objects and classes at runtime (with
Metaprogramming).

Example
Introduce a new method in Strings
1 String.metaClass.isUpperCase = {−>
2 delegate.toCharArray().every{ Character.isUpperCase(it) }
3 }
4 assert "GROOVY".isUpperCase() == true
5 assert "java".isUpperCase() == false

Keep an open mind
A language that doesn't affect the way
you think about programming, is not
worth knowing.
Alan Perlis (1922 - 1990)
ACM Turing Award, 1966

Languages and Typing
Static vs. Dynamic
Statically typed: resolves the types of variables during
compilation. (e.g. Java)
Dynamically typed: resolves the types of variables at runtime.
(Groovy)
Weak vs. Strong
Strongly typed: you can't coerce to a wrong type --the
language guarantees type conformance. (Java & Groovy)
Weakly typed: you can screw everything up in all possible ways.
(e.g. C)

Java-friendly?
Groovy is:
A super version of Java
Augments Java with additional features → the GDK extends JDK in
so many helpful ways!
Designed as a companion language for Java
seamless integration with Java → an additional jar at runtime!
syntactically aligned with Java → syntactically correct Java will
work in Groovy (with some gotchas)!
compiles into JVM bytecode and preserves Java semantics → call
Groovy from Java == call Java from Java!
the 2nd language targeting the Java platform (JSR-241) →
Java was the first!

Feature-rich?
Groovy features
Fully object-oriented -- e.g. Traits in new version (2.3).
Optional typing --static or dynamic.
Duck typing.
List, map, range, regular expression literals.
Operator overloading.
Closures.
GroovyBeans.
GString and GPath.
Multimethods and metaprogramming.
Easily build custom DSLs (e.g. Gradle, Spock).

Example
Optional Typing
1 def s1 = "saiko"
2 assert s1.class == String.class
3 s1 = 3
4 assert s1.class == Integer.class
5 Integer s2 = 3
6 try {
7 s2 = "saiko"
8 }
9 catch(ex) {
10 assert ex.message == "Cannot cast object 'saiko' with class '
java.lang.String' to class 'java.lang.Integer'"
11 }

Groovy History
2003: Started by James Strachan and Bob McWhirter.
2004: Commissioned into JSR 241 but was almost abandoned.
2005: Brought back to life by Guillaume Laforge and Jeremy
Rayner.
2007: Groovy version 1.0.
2012: Groovy version 2.0.
2014: Groovy version 2.3 (official support for JDK 8).

Going Mainstream
Awesome Groovy-based tools & frameworks
Grails: Web development framework.
Gradle: Build automation tool. (my next one, if you like!)
Spock: Testing and specification framework.
CodeNarc: Static analysis tool.
easyb: Behavior-driven development framework.
Geb: Browser automation tool.
Griffon: Desktop app framework.
GPars: Multi-paradigm concurrency framework.
Groovy is supported by Pivotal.

Why Groovy?
Open Source. → Apache v2.0 License.
Near-zero learning curve. → You' ll become a Groovyist before
you realize it!
Leverage Java investment. → Rock-solid Java foundations.
Focus on your problem/app. → No more boilerplate!
More expressive, powerful and concise code. →
Productivity boost!

A picture is worth some thousand words

2. Groovy Basics
Using
Groovy
Language
Overview
Closures
Lists and
Ranges
Maps

Groovy Tools
1. groovyc: Compliles groovy sources to JVM class files.
2. groovysh: Executes code interactively.
3. groovyConsole: GUI for interactive code execution
the best place to start .
4. groovy: Executes groovy scripts. Use it like bash, perl, python,
etc. (#!/usr/bin/groovy).
5. groovydoc: Generates documentation (like javadoc).

Using the Groovy Console
Use Groovy console to experiment, test and evaluate code.

Using the Groovy Interpreter
What date/time is it?
groovy −e "print new Date()"
List files recursively
groovy −e "new File('.').
eachFileRecurse { println it }"

Groovy scripts
No mandatory class definitions, no main methods, no
boilerplate.
Groovy creates them for you automagically!
Writing a Groovy script
Fibonacci.groovy
1 def fib(n) { // a method: the return type is not mandatory
2 n<2 ? 1 : fib(n−1)+fib(n−2)
3 }
4 if (args) { // command−line args
5 println fib(args[0] as Integer)
6 }

Groovy scripts continued
Running the script
> groovy Fibonacci 8
34
> groovyc Fibonacci.groovy
> java −cp $GROOVY_HOME/embeddable/
groovy−all.2.0.0.jar:. Fibonacci 8
34

The Groovy Syntax
The majority of Java syntax is part of the Groovy syntax:
packages
imports
control structures
exception handling
classes and methods
object instantiation and method calls
Gotchas: Arrays, additional Groovy keywords, equals checks, etc.

Default imports
The following imports are included by default
java.lang.*
java.util.*
java.net.*
groovy.lang.*
groovy.util.*
java.math.BigInteger and java.math.BigDecimal

Groovy truth and equals
Null value is false.
Empty collection or map is false.
Empty string is false.
Zero value is false.
Gotcha
The == operator performs value equality (like Java
equals() ).
Use the method is for identity (available in every Groovy
object).

Groovy truth
Examples
1 def f = null
2 def t = "string"
3 assert f == false
4 assert t == true
5 list = []
6 map = [a:1]
7 assert list == false
8 assert map == false
9 String s = ""
10 assert s == false
11 i = 1
12 z = 0.0
13 assert i == true
14 assert z == false

Optionals
The following are optional:
semicolons: required only when you write multiple statements
in one line.
variable types: you decide what should be static or dynamic.
return statements: the last evaluated expression is the
default return value Gotcha .
parentheses: in method/function invocations Gotcha .

Optionals
Examples
1 //a method
2 def fac(n) { n<=1? 1 : n*fac(n−1)}
3 assert fac(3) == 6
4 //a method with default arg value
5 Integer factorial(Integer n=3) { return (n<=1? 1 : n*factorial(n
−1)) }
6 assert factorial(3) == 6
7 //invoke them without parentheses
8 def x = fac 3
9 def y = factorial 3
10 assert x == y
11 x = fac 3 + 1 //will calculate the factorial of 4.
12 assert x == fac(4) //Not always what you want!
13 x = fac(3 + 1) //use parentheses to disambiguate
14 //where parentheses are required
15 x = factorial() //Groovy will look for a property otherwise
16 assert x == fac(3)

Everything is an object
In Groovy:
all objects are derived from groovy.lang.GroovyObject, which
extends java.lang.Object.
all integral numbers are java.lang.Integers (or java.lang.Longs
using the proper Java notation).
all real numbers are java.math.BigDecimals.
all booleans are java.lang.Booleans.

Example
Everything's an object
1 def x = 1
2 int y = 1
3 assert x.class == Integer.class
4 assert y.class == Integer.class
5 def l = 1L
6 assert l.class == Long.class
7 def z = 0.3
8 assert z.class == BigDecimal.class
9 def flag = false
10 assert flag.class == Boolean.class

Strings
Groovy strings support:
Single quotes: ordinary strings.
Double quotes: ordinary strings with variable expansion
(GStrings).
Triple quotes: multi-line strings with variable expansion
(GStrings).
Slashy strings: strings enclosed in slashes; no need to escape
backslashes (useful for regular expressions and file paths).
operator overloading.

Examples
Strings
1 def mlStr = """ I am a multi−line
2 string!"""
3 def name = 'Angus';def surname = "Young"
4 //GStrings
5 def fullname = "$name $surname"
6 assert fullname == "Angus Young"
7 //operator overloading
8 def s = name * 3
9 assert s == "AngusAngusAngus"
10 s = fullname − name
11 assert s.trim() == surname
12 //slashy strings: preserve backslashes; no escaping is required
13 s = /nr/
14 assert s.size() == 4

Numbers
In Groovy all numbers are objects and BigDecimal arithmetic is used by
default.
Examples
1 def x = 3
2 assert x.plus(4) == x + 4
3 assert x.multiply(4) = x * 4
4 assert x.mod(4) == x % 4
5 //BigDecimal arithmetic
6 assert x/4 == x.div(4)
7 assert x/4 != x.intdiv(4)
8 assert 1/2 == 0.5
9 assert 1/3 == 0.3333333333

Java beans for human beings
In Groovy classes/beans:
Methods and classes are public by default.
Members are private by default.
The @PackageScope annotation is used for package scoped
class members.
Enhanced bean property support.
Dynamic constructor arguments.
Accessors (getters and setters) are generated automatically.
You can use the this keyword inside static methods (which
refers to this class).

Defining a Groovy class
A Person class
1 class Person {
2 String name
3 String surname
4 @Override
5 String toString() { "$surname, $name" }
6 }
Compile the Person.groovy
groovyc Person.groovy
Result: A ready to use Person.class

Use the Person class
Examples
From Groovy
1 p = new Person(name:"Theodoros", surname:"Kolokotronis")
2 assert p.toString() == "Theodoros Kolokotronis"
From Java
1 public class UsingPerson {
2 public static void main(String[] args) {
3 Person p = new Person();
4 p.setSurname("Young");
5 p.setName("Angus");
6 HashMap map = new HashMap();
7 map.put("name", "James");
8 map.put("surname", "Bond");
9 Person p1 = new Person(map);
10 }
11 }

Additional Operators
Safe navigation: x?.method()
Elvis: x = y ?: "no y"
Spread: ["java","groovy"]*.size() →
[4,6]
and more... <=> , =~ , ==~ , .@ , .&

Groovy powerful switch statement
1 switch(val) {
2 case "String":
3 //a string
4 break
5 case 10..100:
6 //a range
7 break
8 case Date:
9 //a date instance
10 break
11 case ~/gw+/:
12 //a reg−ex
13 break
14 case ['A', 'B']:
15 //a list
16 break
17 case { it instanceOf Number && it > Integer.MAX_VALUE }
18 //a closure
19 break
20 default:
21 //the default, treated as an "else" in Groovy (if
22 //all else fails, run the default). It should always
23 //be at the end of the switch statement.
24 }

Dynamic method dispatch
Java dispatches methods according to the type of the
arguments at compile time.
Groovy dispatches methods according to the type of the
arguments at runtime (multimethods).
Example
1 public void foo(String arg) { System.out.println("String"); }
2 public void foo(Object o) { System.out.println("Object"); }
3 Object o = "The type of o at runtime is String";
4 foo(o);
Java output: Object
Groovy output: String

Other differences from Java
There is no distinction between checked and unchecked
(runtime) exceptions. All exceptions are runtime exceptions!
Assertions are enabled by default.
Support for default values in method arguments.
use , is and as are keywords. Don't use them as variable
names.
You cannot declare more than one variables in for loops.
Arrays should be initialized with list-like syntax:
int[] a = [1, 2, 3] . Avoid int a[]
notation. Gotcha .

What is a closure?
Definition
A closure is a function together with a referencing environment.
Closures are anonymous functions that:
may accept parameters or return a value,
can be assigned to variables,
can be passed as arguments,
capture the variables of their surrounding lexical scope.

Groovy closures
Example
1 //a block of code assigned to a variable
2 def num = { int n −> n <=> 0 }
3 //what type is that?
4 assert num instanceof Closure
5 //it: the default closure argument
6 def num2 = { it <=> 0 }
7 assert num(−3) == num2(−3)
8 def x = 3
9 def times = { it * x }
10 assert times(3) == 9
11 x = 4
12 assert times(3) == 12

Functional Programming with Groovy
Currying & Higher-order functions
1 def func = { Closure c, int i, int j −>
2 c.call(i, j)
3 }
4 def add = func.curry { int i, int j −> i + j }
5 assert add(1,2) == 3
Composition
1 def add1 = { it + 1}
2 def sq = { it*it }
3 assert sq(add1(2)) == 9 //composition
4 def trans1 = add1 >> sq //left to right
5 assert trans1(2) == sq(add1(2))
6 def trans2 = add1 << sq //right to left
7 assert trans2(2) == add1(sq(2))

Closure owner and delegate
Example
1 //All closures have 3 implicit variables:
2 //this: as in Java
3 //owner: the enclosing object (either a closure or this)
4 //delegate: defaults to owner but can be changed
5 def test = {
6 ((this == owner) && (owner == delegate))
7 return { (this != owner) && (owner == delegate) }
8 }
9 assert test()()
10 //changing the delegate
11 def collectWithIndex = { Closure c−>
12 int i = 0
13 delegate.collect { c(it, i++) }
14 }
15 def map = ["name":"Angus", "surname":"Young"]
16 collectWithIndex.delegate = map
17 def list = collectWithIndex { entry, i −> [i, entry.key, entry.
value] }
18 assert list.flatten() == [0, "name", "Angus", 1, "surname", "
Young"]

Java 8 Lambda Expressions
Groovy closures ̸= Java lambda expressions.
Lambda expressions
Language facility for implementing functional interfaces
Interfaces with a single abstract method.
Function, Predicate, Supplier, Consumer.
Type inference at compile time.
Free variables must be final or effective final.
Do make your Java life easier.

Lists
Groovy offers:
Special syntax for list literals.
Additional common list methods.
Combine with closures for extremely expressive and powerful code!
Examples
1 //a list
2 def list = [1, "2", 3.0]
4 assert list.class == ArrayList.class
5 //list elements
6 assert list.get(1) == "2"
7 assert list[1] == "2"
8 assert list.getAt(1) == "2"
9 //list.getAt(i) equivalent to list[i]
10 //negative indexes − not for the standard list.get(i)
11 assert list[−1] == 3.0
12 assert list.getAt(−2) == "2"

More list examples
1 //another list
2 def list3 = [5, 3, 1, ] //trailing comma OK
3 list3[0] = 3
4 list3.putAt(0, 3) //list.putAt(i,val) equivalent to list[i]=val
5 list3.set(0, 3)
6 assert list3 == [3, 3, 1]
7 //the in operator
8 assert '2' in list
9 assert list.contains('2')
10 //operator overloading
11 assert list + [1] == [1, "2", 3.0, 1]
12 assert list − [1] == ["2", 3.0]
13 assert list * 2 == [1, "2", 3.0, 1, "2", 3.0]
14 assert list + list == list * 2
15 //append to list
16 assert list << 1 == [1, "2", 3.0, 1]
17 def otherList = [4, 5.0, "6"]
18 assert list << otherList == [1, "2", 3.0, 1, [4, 5.0, "6"]]
19 //methods
20 assert list.flatten() == [1, "2", 3.0, 1, 4, 5.0, "6"]
21 assert list.flatten().unique() == [1, "2", 3.0, 4, 5.0, "6"]

Even more list examples
1 //methods that accept closure arguments
2 otherList = list.flatten().collect { it as Integer }
3 assert otherList == [1, 2, 3, 1, 4, 5, 6]
4 def toInt = { Object o −>
5 try { return o as Integer }
6 catch(e) { return 0 }
7 }
8 assert list.flatten().collect(toInt) == otherList
9 assert [5, "6", "seven"].collect(toInt) == [5, 6, 0]
10 //convert a list to a set
11 def set = otherList as Set
12 assert otherList instanceof List
13 assert set instanceof Set
14 //collect is available to all groovy objects
15 set.collect { it + 1 } == [2, 3, 4, 5, 6, 7]
16 //list to string
17 assert otherList.unique().join(', ') == '1, 2, 3, 4, 5, 6'

Enough with these list examples
1 //some more list methods
2 assert otherList.count(1) == 2
3 assert otherList.sum() == 22
4 assert otherList.intersect([1, 3, 4]) == [1, 3, 4]
5 assert otherList.disjoint([0, 10, 20])
6 assert otherList.min() == 1
7 assert otherList.max() == 6
8 assert otherList.reverse().unique().sort() == [1, 2, 3, 4, 5, 6]
9 //some more closures
10 assert otherList.findAll { it%2 == 0} == [2, 4, 6]
11 assert otherList.every { it > 0 } //checks whether the closure
holds for every list element
12 assert !otherList.every { it < 1 }
13 def listWithNulls = [1, null, 2]
14 assert !listWithNulls.every() //checks whether groovy truth
holds for every element
15 //the spread (*.) operator
16 def strings = ['one', 'the other', 'and another']
17 assert strings*.size() == strings.collect { it.size() }
18 //list*.action() calls list.collect { it.action() }
19 assert strings*.toUpperCase() == ['ONE', 'THE OTHER', 'AND
ANOTHER']

Ranges
Groovy:
treats ranges as first-class objects.
offers special syntax for ranges.
Examples
1 def letters = 'a'..'z'
2 def digits = 0..9
4 assert letters.class == ObjectRange
5 assert digits.class == IntRange
6 //okay, but wait...
7 assert letters instanceof java.util.List
8 assert digits instanceof java.util.List
9 //ranges are lists of sequential values.
10 assert digits == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
11 assert letters[0] == 'a'
12 assert letters.size() == 26

More ranges examples
1 //methods
2 def otherDigits = digits.collect { it*2 } //or, equivalently
3 otherDigits = digits*.multiply(2)
4 assert otherDigits == [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
5 //half−open ranges
6 def digits2 = 0..<9 //0 up to 9 exclusive
7 assert digits2.size() == 9
8 //get the from,to values of ranges
9 assert digits.from == digits2.from
10 assert digits.to == digits2.to + 1
11 //another way to express a range
12 assert 1..4 == (1..4)
13 //looping
14 (1..4).each { print it }
15 //stepping
16 (1..4).step(2) { print it }
17 //slicing
18 def list = ['one', 'two', 'three', 'four', 'five', 'six']
19 assert list[1..3] == ['two', 'three', 'four']
20 assert list[0..<2] == ['one', 'two']
21 assert list[−1] == 'six'
22 assert list[3..−1] == ['four', 'five', 'six']
23 assert list[−1..−3] == ['six', 'five', 'four']

Maps
Groovy:
offers pecial syntax for map literals.
provides dditional common map methods.
uses maps + closures for duck typing: if it walks and swims like
a duck, it is a duck! → rapid implementation of interfaces
Examples
1 //a map
2 def map = ['id' : 12, 'name' : "John", 'surname' : "Doe"]
3 assert map.size() == 3
4 //keys which are valid Java identifiers need no quotes
5 def now = new Date()
6 def map2 = [
7 id : 1,
8 value : 2.0,
9 "peter's birthday": now
10 ]

More maps examples
1 //we can use map.key when key is valid Java identifier
2 assert map2.id == 1
3 //we can quote the key if needed
4 assert map2."peter's birthday" == now
5 //we can also use subscription notation map[key]
6 assert map2["peter's birthday"] == now
7 //and the standard get method
8 assert map2.get('value') == 2
9 assert map.get('foo') == null
10 //default value when key not found
11 assert map.get('foo', 'No such key') == 'No such key'
12 //adds the default value in the map
13 assert map.containsKey('foo')
14 //what type is the map?
15 try {
16 assert map.class == LinkedHashMap.class
17 }
18 catch(Error e) {
19 assert map.class == null //the map is looking for the 'class
' key
20 map.getClass() == LinkedHashMap.class
21 }

Even more maps examples
1 //put/update values
2 map.newKey = "new key's value"
3 map2['newKey'] = map.newKey
4 //use parentheses to add the value of a var in a map (and not
its name)
5 def anotherKey = "address"
6 map[(anotherKey)] = "Petraki 28"
7 assert map.address == "Petraki 28"
8 //iterate with each
9 map.each { println "In each: ${it.class}" }
10 //the closure argument is a Map.Entry instance
11 map.each { println "In each again: ${it.key} = ${it.value}" }
12 //all Java Map methods are here
13 assert !map.isEmpty()
14 assert map.containsKey('id')
15 assert map.containsValue(12)
16 //use a HashMap instead of a LinkedHashMap
17 def map3 = new HashMap()
18 assert map3.getClass() == HashMap.class
19 //empty map
20 def map4 = [:]
21 assert !map4//empty map is false

Some more maps examples
1 //find in map
2 assert ['id':12, 'foo':'No such key'] == map.findAll{ it.key.
size() <= 3}
3 //group by
4 def list = [1, '2', 3, '4', '5', 6]
5 def map5 = list.groupBy { it.getClass().getSimpleName() }
6 assert ['String': ['2', '4', '5'], 'Integer': [1, 3, 6]] == map5
7 //store closures in a map
8 def actions = [
9 login : { println "login" },
10 logout : { println "logout" }
11 ]
12 //execute the closures
13 actions.login.call() //or
14 actions['logout'].call() //or
15 actions.logout()

Duck typing
1 //Duck typing − a comparator
2 def cmp = [
3 compare : { a, b −> a.size() <=> b.size() }
4 ] as Comparator
5 assert cmp instanceof Comparator
6 //a list
7 def list2 = ['a' , 'quick', 'brown', 'fox', 'jumps', 'over', 'a'
, 'lazy', 'dog']
8 assert list2.sort(cmp) == ['a', 'a', 'fox', 'dog', 'over', 'lazy
', 'quick', 'brown', 'jumps']
9 //a runnable
10 def r = [
11 run : {
12 def t = Thread.currentThread()
13 println "Thread ${t.getId()} starting"
14 t.sleep(1000)
15 println "Thread ${t.getId()} finished"
16 }
17 ] as Runnable
18 assert r instanceof Runnable
19 new Thread(r).start()

Duck typing continued
Another Example
1 interface Doer {
2 void doThis();
3 void doThat();
4 }
5 class DoerImpl implements Doer {
6 void doThis(){"Did this"}
7 void doThat(){"Did that"}
8 }
9 def doer1 = new DoerImpl()
10 assert doer1 instanceof Doer
11 def doer2 = [
12 doThis:{"This done"},
13 doThat:{"That done"}
14 ] as Doer
15 assert doer2 instanceof Doer

3. Real-life
Groovy
Open
Public Data
Hackathon
Our Web
App
JSON
and REST
Groovlets

Open Public Data Hackathon
Held between 15 - 30 April 2014.
Organized by the Ministry of Administration Reform and E-Governance
(ydmed.gr), in association with:
1. aspete.gr
2. hua.gr
3. ellak.gr
Requirement: deliver an app that utilizes open public data (from
data.gov.gr or elsewhere).

We won the 2nd prize!
And Groovy helped us do it!
We shall discuss:
1. How to build a RESTful API with Groovy and Restlet.
2. How to build dynamic web pages with Groovlets.

Our Web App
A prototype of a cloud service for
Accessing/retrieving data
Searching/querying data
Visualizing/embedding data
through a powerful RESTful API.

Quick demo here
opendatacloud.gr

Technologies used
Backend
Java 7.x
Apache Tomcat 7.0.x
Groovy 2.2.x
Restlet 2.2.0
Apache POI 3.10
Elastic Search 1.1.1
Frontend
Bootstrap 3
jQuery 1.10
D3.js

The data model
We have
Kept thins as simple as possible.
Data source groups: Sets of
Data sources: Tabular data in excel files arranged
in columns
and rows.
After all, it is a prototype!

The web.xml
Two servlets
1 <servlet>
2 <servlet−name>RestletServlet</servlet−name>
3 <servlet−class>org.restlet.ext.servlet.ServerServlet</servlet−
class>
4 <init−param>
5 <param−name>org.restlet.application</param−name>
6 <param−value>com.niovity.opd.restlet.App</param−value>
7 </init−param>
8 </servlet>
9 <servlet−mapping>
10 <servlet−name>RestletServlet</servlet−name>
11 <url−pattern>/api/v0/*</url−pattern>
12 </servlet−mapping>
13 <servlet>
14 <servlet−name>groovy</servlet−name>
15 <servlet−class>groovy.servlet.GroovyServlet</servlet−class>
16 </servlet>
17 <servlet−mapping>
18 <servlet−name>groovy</servlet−name>
19 <url−pattern>*.groovy</url−pattern>
20 </servlet−mapping>

The web.xml continued
And a listener
1 <listener>
2 <listener−class>com.niovity.opd.Listener</listener−class>
3 </listener>
1 package com.niovity.opd
2 class Listener implements ServletContextListener {
3 @Override
4 void contextInitialized(ServletContextEvent e) {
5 try {
6 //Initialize log4j & elastic
7 ...
8 //Initialize the config
9 String basePath = ctx.getRealPath("/data")
10 Config.instance.init(basePath)
11 } catch(Exception e) {
12 System.err.println("Fatal error: ${e.getMessage()}")
13 e.printStackTrace(System.err)
14 throw new ServletException(e.getMessage(), e)
15 }
16 }
17 }

Groovy @Singleton Annotation
Config
1 package com.niovity.opd
2 @Singleton class Config {
3 String basePath
4 Map<String, DataSourceGroup> sourceGroups = [:]
5 void init(String basePath) throws Exception {
6 this.basePath = basePath
7 def groups = [:]
8 DataSourceGroup nte = new DataSourceGroup(
9 id: "nte",
10 title:"...", ...
11 )
12 nte.addDataSource(new DataSource(title: "...",...))
13 groups.nte = nte
14 ...
15 sourceGroups = Collections.unmodifiableMap(groups)
16 }
17 }

The Restlet App
The REST router
1 package com.niovity.opd.restlet
2 import org.restlet.Application
3 import org.restlet.Restlet
4 import org.restlet.routing.Router
5 class App extends Application {
6 @Override
7 synchronized Restlet createInboundRoot() {
8 Router router = new Router(getContext())
9 router.attach "/groups", SourceGroups
10 router.attach "/{grp}/{src}", Source
11 router.attach "/search", Search
12 return router
13 }
14 }

An example of a Restlet
GET the source groups
2 import ...
3 class SourceGroups extends ServerResource{
4 @Override
5 protected Representation get() throws ResourceException {
6 def groups = Config.instance.sourceGroups
7 def result = [:]
8 result.total = groups.size()
9 result.groups = []
10 groups.each {
11 def group = [id : it.key, title : it.value.title, ...]
12 group.sources = []
13 it.value.dataSources().eachWithIndex { source, index −>
14 def src = [id:"/${it.key}/$index",...]
15 group.sources.push(src)
16 }
17 result.groups.push(group)
18 }
19 return new JsonRespresentation(result)
20 }
21 }

JSON
JsonRepresentation
2 import groovy.json.JsonBuilder
3 import org.restlet.data.MediaType
4 import org.restlet.representation.WriterRepresentation
5 class JsonRespresentation extends WriterRepresentation {
6 private Map result
7 JsonRespresentation(Map result) {
8 super(MediaType.APPLICATION_JSON)
9 this.result = result ?: [:]
10 }
11 @Override
12 void write(Writer writer) throws IOException {
13 new JsonBuilder(result).writeTo(writer)
14 }
15 }

Groovlets
Groovy scripts on the server side
Compiled on the fly
No boilerplate
With some handy implicit variables provided by default
request,response : Http servler request and
response
params : request parameters
context, application : the ServletContext
object
session : the HttpSession object
out : the writer of the response
sout : the output stream of the response
html: a groovy.xml.MarkupBuilder which writes to out

Display a data source in HTML table
source.grovy
1 String groupId = request.getParameter("grp")
2 String srcId = request.getParameter("src")
3 def group = Config.instance.sourceGroups.get(groupId)
4 def sources = group.dataSources()
5 int index = srcId as Integer
6 def source = group.dataSources().get(index)
7 println """
8 ...
9 <table class="table table−bordered table−striped" >
10 <thead>
11 <tr>
12 <th>#</th>
13 ${source.columnNames.collect {"<th>$it</th>"}.join("")}
14 </tr>
15 </thead>
16 <tbody >
17 """

HTML table continued
source.grovy
1 source.rows.eachWithIndex {Map row, int i −>
2 println """
3 <tr>
4 <td>${i+1}</td>
5 ${source.columnNames.collect{ "<td>${row.get(it)}</td>"}.join
("")}
6 </tr>
7 """
8 }
9 println """
10 </tbody>
11 </table>
12 ...
13 """

Groovy is Java on steroids!
Easy to use: Its integration with the Java platform is insanely
brilliant!
Expressive: Do more with less code!
Effective: Focus on your app and stop wasting your time with
the irrelevant!
Agile: Groovy is agile and rapid prototyping is a few Groovy lines
away!
Leverage Java expertise. Build upon your Java skills.
Powerful: The swiss army knife of the Java hacker!

Swiss army knife, indeed
Ain't this groovy or what?
db.groovy
1 @GrabConfig(systemClassLoader=true)
2 @Grab(group='mysql', module='mysql−connector−java', version='
5.1.6')
3 import groovy.sql.Sql
4 def query="select count(*) as c, date(creationDate) as d from
dofields group by d order by c desc limit 10"
5 def url="jdbc:mysql://localhost:3306/test_db?characterEncoding=
utf−8"
6 def (u,p)=["test","test"]
7 try {
8 def sql = Sql.newInstance(url,u,p,"com.mysql.jdbc.Driver")
9 sql.eachRow(query) { println("${it.c}:${it.d}") }
10 }
11 catch(e) { e.printStackTrace() }

Output
Run it
groovy db
Output
1 9427:2004−08−20
2 6615:2004−10−29
3 5498:2004−10−08
4 5103:2004−08−31
5 4864:2004−10−14
6 4675:2004−10−31
7 4583:2004−10−05
8 4570:2004−08−21
9 4339:2004−09−30
10 4235:2004−10−30

We 've only scratched the surface
Groovy topics not covered
Metaprogramming
Grape
Builders
Working with XML
Working with SQL
Testing
And much, much more...
Perhaps in a 2nd part?

What about performance?
Programmers have spent far too much time
worrying about efficiency in the wrong places
at the wrong times; premature optimization is
the root of all evil.
Donald E. Knuth
ACM Turing Award, 1974

Put in other terms
The reason to use Groovy should be because it
improves your performance not the computer's.
matthew@stackoverflow

About Groovy Performance
Sorry, it hasn't bothered me yet.
Groovy 2.x introduced the @CompileStatic annotation.
Most of Groovy method calls are compiled into direct JVM bytecode
method calls.
People report that Groovy reaches to almost identical performance
with Java.
Start with Groovy and (re)write your performance-critical
classes in Java as you grow.
You can find various benchmarks and performance comparisons
on the web.
Any volunteers for benchmarking, should we do a 2nd part?

Resources
1. Programming Groovy 2, V. Subramaniam, The Pragmatic
Bookshelf, 978-1-937785-30-7, 2013
2. Groovy in Action, D. Koenig, A. Glover, P. King, G. Laforge and J.
Skeet, Manning Publications, 1-932394-84-2, 2007
3. Groovy Web Site
http://groovy.codehaus.org
4. Groovy language documentation beta
http://beta.groovy-lang.org/docs/
groovy-2.3.0/html/documentation/

Thank you
Questions?
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