This document provides an introduction to Groovy for Java developers. It discusses Groovy's features such as closures, optional typing and syntax, and how it compiles to Java bytecode. It then provides examples of writing a simple Groovy script to generate XML, using closures, mocking objects in tests, and building projects with Ant.

1. Practical Groovy
for Java Developers
Gabriel Dogaru
gabi@gabrieldogaru.com
March 10 2012

2. Agenda
Groovy introduction
Closures
First Script
Testing with Groovy
Building with groovy

3. What is groovy?
“Groovy is what Java would look like had it been
written in the 21st century.”
An agile and dynamic language for the JVM
Inspired by languages like Python, Ruby and
Smalltalk
Compiles straight to Java bytecode
Compiled or interpreted
Supports Domain-Specific Languages and other
compact syntax

4. A programming language is for thinking
about programs, not for expressing
programs you've already thought of. It
should be a pencil, not a pen.
Paul Graham

5. I think I know java!
//... Person.java //... Person.groovy
//... Person.groovy
public class Person { public class Person {
private String name; class Person { name;
private String
private int age; String name
private int age;
int age
public Person() { public Person() {
}
} }
public Person(String name, int age) { public Person(String name, int age) {
this.name = name; //... Person.groovyname;
this.name =
this.age = age; class Person { age;
this.age =
} }
def name
public String getName() { def ageString getName() {
public
return name; } return name;
} }
public void setName(String name) { person = new Person(name: "Ion", age: 66)
public void setName(String name) {
this.name = name; this.name = name;
} //person.setAge(61)
}
person.age = 61
public int getAge() { public int getAge() {
return age; return age;
//System.out.println(person.getName())
} }
println person.name
public void setAge(int age) { public void setAge(int age) {
this.age = age; //> Ionthis.age = age;
} }
} }

6. Gotchas
 Automatic Imports
 Everything is an object
 Optional data type declaration (Duck
Typing)
 Optional exception handling
 Optional semicolons and parentheses
 Optional return statements
 Operator overloading

7. Groovy Closures
 Like a "code block" or a method pointer
 May be passed as arguments
 May accept arguments
 May return values
 Makes anonymous inner classes obsolete

8. Closures
//... ClosureExample.java
//... ClosureExample.java //... ClosureExample.groovy
import com.google.common.base.Predicate;
import java.util.ArrayList; import com.google.common.base.Predicate;
import com.google.common.collect.Iterables;
import java.util.Arrays; import com.google.common.collect.Iterables;
import java.util.ArrayList; import java.util.ArrayList;
def numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]
import java.util.List;
import java.util.Arrays; import java.util.Arrays;
def oddNumbers = numbers.findAll {it % 2!=0}
import java.util.List; import java.util.List;
public class ClosureExample { oddNumbers.each {println it}
public static void main(String[] args) {
public class ClosureExample { public class ClosureExample {
List<Integer> numbers = Arrays.asList(
public static void main(String[] args) { public static void main(String[] args) {
List<Integer> numbers = 2, 3, 4, 5, 6, 7, 8, 9);
1, Arrays.asList( List<Integer> numbers = Arrays.asList(
1, 2, 3, 4, 5, 6, 7, 8, 9); 1, 2, 3, 4, 5, 6, 7, 8, 9);
List<> oddNumbers = new ArrayList<..>();
Iterable<> number : numbers) {
for (int oddNumbers=Iterables.filter(numbers, Iterable<> oddNumbers=Iterables.filter(numbers,
new Predicate<Integer>() {
if (number % 2 != 0) { new Predicate<Integer>() {
def odd = {Integer n->
public boolean apply(Integer number) {
oddNumbers.add(number); public boolean apply(Integer number) {
return return number % 2 != 0;
n%2 != 0
} return number % 2 != 0;
} } } }
}); });
for (int number : oddNumbers) { def numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]
for (int number : oddNumbers) { for (int number : oddNumbers) {
System.out.println(number);
System.out.println(number); def oddNumbers = numbers.findAll odd
System.out.println(number);
}} oddNumbers.each {println it}
}
}
} }
}
} }

9. Cooo...ol
How do I start ?

10. Write your first script
//… WriteSomeXML .java
@GrabResolver(name = 'maven-central', root = 'repo1.maven.org')
import java.io.File;
@Grapes(
import java.io.FileOutputStream;
import javax.xml.parsers.*;
@Grab(group = 'xerces', module = 'xercesImpl', version = '2.10.0')
)//… write xml
import org.w3c.dom.*;
import org.apache.xml.serialize.*;
public class WriteSomeXML {
new File("persons.xml").withWriter {writer ->
import javax.xml.parsers.* {
public static void main(String[] args)
def xml = new MarkupBuilder(writer)
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
import org.apache.xml.serialize.*
xml.persons db = null;
DocumentBuilder {
dbf Document doc = null;
= DocumentBuilderFactory.newInstance()
db = {person(id: 99) {
trydbf.newDocumentBuilder()
db = dbf.newDocumentBuilder();
doc = firstname("John")
db.newDocument()
doc = db.newDocument(); <?xml version="1.0" encoding="UTF-8"?>
root = lastname("Smith")
doc.createElement"persons“
Element root = doc.createElement("persons");
<persons>
doc.appendChild root
}}} doc.appendChild(root);
Element el = doc.createElement("person"); <person id="99">
el = doc.createElement"person“
root.appendChild(el);
root.appendChild "99"); <firstname>John</firstname>
el.setAttribute("id", el
el.setAttribute "id", doc.createElement("firstname");
Element firstname = "99“
<lastname>Smith</lastname>
//… read xml
el.appendChild(firstname);
firstname = doc.createElement("firstname") </person>
firstname.appendChild(doc.createTextNode("John")); </persons>
el.appendChild firstname
def parser lastname =XmlParser()
Element = new doc.createElement("lastname");
firstname.appendChild doc.createTextNode("John")
el.appendChild(lastname);
def persons = parser.parse("persons.xml")
lastname.appendChild(doc.createTextNode("Smith"));
lastname = doc.createElement "lastname“
println "${persons.person[0].@id}"
OutputFormat out = new OutputFormat(doc);
el.appendChild lastnamelastname.appendChild
out.setIndenting(true);
persons.person.eachnew XMLSerializer(new FileOutputStream(new File("persons.xml")), out);
doc.createTextNode="Smith" ->
XMLSerializer xmlSer { person
out println person.firstname[0].text()
=xmlSer.serialize(doc);
new OutputFormat doc
} catch (Exception e) {
out.indenting = true
} e.printStackTrace();
xmlSer = new XMLSerializer(new FileOutputStream(new File("persons.xml")),
} out)
xmlSer.serialize doc
}
}

11. Write your tests with groovy
<dependency>
<groupId>org.codehaus.groovy</groupId>
<artifactId>groovy</artifactId>
<scope>test</scope>
</dependency>
....
<build>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<compilerId>groovy-eclipse-compiler</compilerId>
</configuration>
<dependencies>
<dependency>
<groupId>org.codehaus.groovy</groupId>
<artifactId>groovy-eclipse-compiler</artifactId>
</dependency>
</dependencies>
</plugin>
....

12. Classes under test
//… ExchangeRateDAO.java
public interface ExchangeRateDAO {
BigDecimal load(String currency);
void save(String currency, BigDecimal value);
}
//… ExchangeRateService.java
public class ExchangeRateService {
private final ExchangeRateDAO rateDAO;
public ExchangeRateService(ExchangeRateDAO rateDAO) {
this.rateDAO = rateDAO;
}
public BigDecimal getRate(String currency) {
return rateDAO.load(currency);
}
public void updateRate(String currency, BigDecimal value) {
rateDAO.save(currency, value);
}
}

13. Mocking – map coercion
//… DynamicTest.groovy
class DynamicTest {
@Test
public void test1() {
ExchangeRateDAO rateDAO = {1.0} as ExchangeRateDAO
ExchangeRateService service = new ExchangeRateService(rateDAO)
assertEquals 1.0, service.getRate("USD")
}
@Test(expected = IllegalAccessError.class)
public void test_named_methods() {
ExchangeRateDAO rateDAO = [
load: {String v -> 1.0},
save: {String k, BigDecimal v -> throw new IllegalAccessError()}
] as ExchangeRateDAO
ExchangeRateService service = new ExchangeRateService(rateDAO)
assertEquals 1.0, service.getRate("USD")
service.updateRate("1", 2)
}
}

14. Mocking - MockFor
//… MockForTest.groovy
class MockForTest {
@Test
public void test() {
def mockContext1 = new MockFor(ExchangeRateDAO)
mockContext1.demand.load("USD") { 1.0 }
def dummyDAO = mockContext1.proxyDelegateInstance()
def service = new ExchangeRateService(dummyDAO)
assertEquals 1.0, service.getRate("USD")
mockContext1.verify dummyDAO
}
}

15. Build with groovy
<?xml version="1.0" encoding="UTF-8"?>
<project name=“Ant Example" default="main" basedir=".">
<property environment="env"/>
<taskdef name="groovy" classname="org.codehaus.groovy.ant.Groovy">
<classpath>
<fileset dir="${env.GROOVY_HOME}" includes="embeddable/groovy-all-
*.jar,lib/ivy*.jar"/>
</classpath>
</taskdef>
<target name=“print-xml">
<groovy>
xmlfiles = new File('.').listFiles().findAll{ it =~ '.xml$' }
xmlfiles.sort().each{ println it.toString() }
</groovy>
</target>
</project>

16. Build with Ant
def ant = new AntBuilder()
ant.echo(file:'Temp.java',
''' class Temp { public static void main(String[] args) {System.out.println("Hello"); }}
''')
ant.javac(srcdir:'.', includes:'Temp.java', fork:'true')
ant.java(classpath:'.', classname:'Temp', fork:'true')
ant.echo('Done')
// =>
// [javac] Compiling 1 source file
// [java] Hello
//[echo] Done

18. 's
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