Groovy
External
TODO
- Tutorial: www.tutorialspoint.com/groovy/index.htm
TODO Groovy basics: https://docs.gradle.org/current/userguide/writing_build_scripts.html#groovy-dsl-basics
Overview
Introducing Groovy to a Java project is a simple as adding the Groovy JAR file to its classpath.
To experiment with syntax, use the IntelliJ Groovy Console:
Data Types
String
Single-Quoted vs. Double-Quoted Strings
Groovy has both double-quoted and single-quoted String literals. The main difference is that double-quoted String literals support String interpolation:
def x = 10
println "result is $x" // prints: result is 10
Rules for String Interpolation
Groovy supports declaring a string with either single quotes or double quotes.
If the string is single-quoted, the variable interpolation behavior is turned off, similarly to how bash handles single-quoted strings.
def user="Elemental"
echo 'Hello ${user}'
will output:
Hello ${user}
Double-quoted string support dollar-sign based interpolation:
def user="Elemental"
echo "Hello ${user}"
will output:
Hello Elemental
Note that the variable passed to ${} can be a function that returns a String:
static indent(String lead, String object) {
return lead + object
}
def a = "something"
print "${indent("----", a)}"
prints:
----something
Multi-Line Strings
Multi-line strings can be represented in Groovy by enclosing them in """ or ''':
print """
this is
a multi-line
string
""".stripIndent()
print '''
this is
a multi-line
string
'''.stripIndent()
The difference between """ or ''' is in how variable interpolation is handled when variables are declared inside the quoted string. The rules describe here apply: Rules for String Interpolation. As such, the first example below will interpolate the variable, while the second will not:
def v="multi-line"
print """
this is
a ${v}
string
""".stripIndent()
will display:
this is
a multi-line
string
while this
def v="multi-line"
print '''
this is
a ${v}
string
'''.stripIndent()
will display:
this is
a ${v}
string
String Comparison
Operators == and != work with strings. Explain why.
Array
String[] arr = ["a", "b", "c"]
def arr = ["a", "b", "c"].toArray()
Boolean
A boolean variable evaluates to true if assigned a different type variable that is not null or not empty, and false otherwise.
The following expressions assign "true" to b:
boolean b
b = "something"
HashSet s = new HashSet()
s.add("something")
b = s
The following expressions assign "false" to b:
boolean b
b = ""
b = new ArrayList()
b = new HashSet()
List
def list = ["A", "B", "C"]
Internally is stored as an java.util.ArrayList.
To access an element:
list.get(0)
list[0]
Map
Empty map:
Map testingBranches = [:]
Initialization:
def something = [a: 10, b:20, c:30]
def artifacts = [
"chart-A": [
"watchFor": ["src/charts/chart-A"],
"chartName": "a",
],
"chart-B": [
"watchFor": ["src/charts/chart-B"],
"chartName": "b",
],
"script": [
"watchFor": ["src/bin/run", "src/ansible", "script/bin/lib/a.shlib"],
]
]
Element insertion:
something.put('a', 10)
something['a'] = 10
something.a = 10
something.'a' = 10
Element access:
something.get('a')
something['a']
something.a
something.'a'
⚠️ Note that when m.n syntax is used, "n" is interpreted as a literal, not a variable, so even if the variable is defined and initialized, it will not be used.
something.put('a', 1)
String b = 'a'
something.b // ⚠️ will attempt to retrieve something.'b', not something.'a'
Adding elements from another map:
something << ['d':40]
Variables
Variables can be defined using either their type (String) or with the keywords def, which is equivalent with var
String a
def b
var c
def/var act as a replacement for the type name when the actual type is not provided (either because you don't care about it or you're relying on type inference). You can think about them as an alias to Object. def is used when we don't want to restrict the type of an object, and we want to change what can be assigned to a variable any time during runtime.
For scripts, undeclared variables are assumed to come from the Script binding.
Working with Closures
Defining a Closure
def myClosure = { e -> println "Clicked on $e.source" }
Implicit Paramenter
When a closure does not explicitly define a parameter using the '->' syntax, the closure 'always defines an implicit parameter named "it".
Passing Closures to Methods
If the closure is the last argument for a method, it can be passed outside the argument list.
Passing a Method as Closure
def withTestSetup(Closure test) {
performTestSetup()
test()
}
Invocation - the test setup is prior performed on the stack and then doTestLogic1() and doTestLogic2() methods are invoked, wrapped in a closure:
withTestSetup({
doTestLogic1()
doTestLogic2()
})
Template Engines
Files
filename = 'example.txt'
File f = new File(filename)
def lines = f.readLines()
for (line in lines) {
// ...
}
filename = 'example.txt'
File f = new File(filename)
f.eachLine({
if (!it.startsWith("#") && !it.trim().isEmpty()) {
...
}
})
Dynamic Keywords
@Field
Variable annotation used for changing the scope of a variable within a script from being within the run method of the script to being at the class level for the script. The annotated variable will become a private field of the script class.
In Jenkins, if Jenkinsfile variables are not declared @Field, they are not available in methods defined in Jenkinsfile.
Script
Groovy is automatically wrapping every program in a class called Script.
Method Parameters with Default Values
Default values can be assigned to a parameter in a method:
def say(msg = 'Hello', name = 'world') {
"$msg $name!"
}
⚠️ Avoid using default method parameters with constructors, some compilers (for example, the Jenkins runtime Groovy compiler) will complain:
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
file:/var/lib/jenkins/groovyStash/jenkins-jenkinsfile-refactoring-86/playground/ClusterTestingInfo.groovy: -1: cannot reference this inside of this ((java.lang.Object) pipeline, (java.lang.String -> java.lang.String) serverRoot, (java.lang.String -> java.lang.String[]) testRoots, (java.lang.String -> java.lang.String[]) org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(this, false, false, CLUSTER_TEST_ROOTS))(....) before supertype constructor has been called
. At [-1:-1] @ line -1, column -1.
Control Flow
if/else
A null condition evaluates to false:
def a = null
if (a) {
println "blue"
}
else {
println "red"
}
will display "red".