This document provides a 3 sentence summary of the presentation "Idiomatic Gradle Plugin Writing" by Schalk W. Cronjé: The presentation discusses best practices for writing Gradle plugins, including using consistent and readable extensions to the Gradle DSL, supporting offline mode, testing plugins against multiple Gradle versions, and extending existing task types when needed rather than forcing users to use standard configurations. It provides examples of idiomatic ways to handle collections, maps, dependencies, and project extensions within Gradle plugins. The presentation aims to promote quality attributes like readability, consistency, flexibility and expressiveness in plugin authoring.

1. Gr8Day Warsaw 2016
IDIOMATIC GRADLE
PLUGIN WRITING
Schalk W. Cronjé

2. ABOUT ME
Email:
Twitter / Ello : @ysb33r
ysb33r@gmail.com
Gradle plugins authored/contributed to: VFS, Asciidoctor,
JRuby family (base, jar, war etc.), GnuMake, Doxygen, Bintray

3. ABOUT THIS PRESENTATION
Written in Asciidoctor (1.5.3.2)
Styled by asciidoctor-revealjs extension
Built using:
Gradle
gradle-asciidoctor-plugin
gradle-vfs-plugin

4. GET YOUR DAILY GRADLE DOSE
@DailyGradle
#gradleTip

5. THE PROBLEM
There is no consistency in the way plugin authors craft extensions
to the Gradle DSL today

6. QUALITY ATTRIBUTES OF DSL
Readability
Consistency
Flexibility
Expressiveness

7. PROJECT LAYOUT
Figure 1. Plugin project file layout

8. BUILD SCRIPT
repositories {
jcenter()
}
apply plugin : 'groovy'
dependencies {
compile localGroovy()
compile gradleApi()
testCompile ("org.spockframework:spock-core:1.0-groovy-2.3") {
exclude module : 'groovy-all'
}
}

9. TRICK : SPOCK VERSION
ext {
spockGrVer = GroovySystem.version.replaceAll(/.d+$/,'')
}
dependencies {
testCompile ("org.spockframework:spock-core:1.0-${spockGrVer}") {
exclude module : 'groovy-all'
}
}

10. CREATE PLUGIN CLASS
package idiomatic.gradle.authoring
import org.gradle.api.Plugin
import org.gradle.api.Project
class MyExamplePlugin implements Plugin<Project> {
void apply(Project project) {
}
}

11. CREATE PROPERTIES FILE
META-INF/gradle-
plugins/idiomatic.authored.example.properties
implementation-class=idiomatic.gradle.authoring.MyExamplePlugin
Name of file must match plugin identifier

12. CREATE TASK CLASS
package idiomatic.gradle.authoring
import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction
class MyExampleTasks extends DefaultTask {
@TaskAction
void exec() {
}
}

13. FOR BEST COMPATIBILITY
Support same JDK range as Gradle
Gradle 1.x - mininum JDK5
Gradle 2.x - minimum JDK6
Build against Gradle 2.0
Only use later versions if specific new functionality is
required.
Suggested baseline at Gradle 2.10 (for new model)

14. JDK COMPATIBILITY
// build.gradle
targetCompatibility = 1.6
sourceCompatibility = 1.6
project.tasks.withType(JavaCompile) { task ->
task.sourceCompatibility = project.sourceCompatibility
task.targetCompatibility = project.targetCompatibility
}
project.tasks.withType(GroovyCompile) { task ->
task.sourceCompatibility = project.sourceCompatibility
task.targetCompatibility = project.targetCompatibility
}

15. GRADLE BUILD VERSION
// gradle/wrapper/gradle-wrapper.properties
distributionUrl=https://..../distributions/gradle-2.0-all.zip

16. HONOUR OFFLINE
gradle --offline
The build should operate without accessing
network resources.

17. HONOUR OFFLINE
Unset the enabled property, if build is offline
task VfsCopy extends DefaultTask {
VfsCopy() {
enabled = !project.gradle.startParameter.isOffline()
}
}

18. PREFER METHODS OVER PROPERTIES
( IOW To assign or not to assign )
Methods provide more flexibility
Tend to provide better readability
Assignment is better suited towards
One-shot attribute setting
Overriding default attributes
Non-lazy evaluation

19. HOW NOT 2 : COLLECTION OF FILES
Typical implementation …​
class MyTask extends DefaultTask {
@InputFiles
List<File> mySources
}
leads to ugly DSL
task myTask( type: MyTask ) {
myTask = [ file('foo/bar.txt'), new File( 'bar/foo.txt') ]
}

20. COLLECTION OF FILES
myTask {
mySources file( 'path/foobar' )
mySources new File( 'path2/foobar' )
mySources 'file3', 'file4'
mySources { "lazy evaluate file name later on" }
}
Allow ability to:
Use strings and other objects convertible to File
Append lists
Evaluate as late as possible
Reset default values

21. COLLECTION OF FILES
Ignore Groovy shortcut; use three methods
class MyTask extends DefaultTask {
@InputFiles
FileCollection getDocuments() {
project.files(this.documents) // magic API method
}
void setDocuments(Object... docs) {
this.documents.clear()
this.documents.addAll(docs as List)
}
void documents(Object... docs) {
this.documents.addAll(docs as List)
}
private List<Object> documents = []
}

22. STYLE : TASKS
Provide a default instantiation of your new task class
Keep in mind that user would want to create additional
tasks of same type
Make it easy for them!!

23. KNOW YOUR ANNOTATIONS
@Input
@InputFile
@InputFiles
@InputDirectory
@OutputFile
@OutputFiles
@OutputDirectory
@OutputDirectories
@Optional

24. COLLECTION OF STRINGS
import org.gradle.util.CollectionUtils
Ignore Groovy shortcut; use three methods
@Input
List<String> getScriptArgs() {
// stringize() is your next magic API method
CollectionUtils.stringize(this.scriptArgs)
}
void setScriptArgs(Object... args) {
this.scriptArgs.clear()
this.scriptArgs.addAll(args as List)
}
void scriptArgs(Object... args) {
this.scriptArgs.addAll(args as List)
}
private List<Object> scriptArgs = []

25. HOW NOT 2 : MAPS
Typical implementation …​
class MyTask extends DefaultTask {
@Input
Map myOptions
}
leads to ugly DSL
task myTask( type: MyTask ) {
myOptions = [ prop1 : 'foo/bar.txt', prop2 : 'bar/foo.txt' ]
}

26. MAPS
task myTask( type: MyTask ) {
myOptions prop1 : 'foo/bar.txt', prop2 : 'bar/foo.txt'
myOptions prop3 : 'add/another'
// Explicit reset
myOptions = [:]
}

27. MAPS
@Input
Map getMyOptions() {
this.attrs
}
void setMyOptions(Map m) {
this.attrs=m
}
void myOptions(Map m) {
this.attrs+=m
}
private Map attrs = [:]

28. COMPATIBILITY TESTING
How can a plugin author test a plugin against multiple Gradle
versions?

29. COMPATIBILITY TESTING
Gradle 2.7 added TestKit
Gradle 2.9 added multi-distribution testing
TestKit still falls short in ease-of-use
(Hopefully to be corrected over future releases)
What to do for Gradle 2.0 - 2.8?

30. COMPATIBILITY TESTING
GradleTest plugin to the rescue
buildscript {
dependencies {
classpath "org.ysb33r.gradle:gradletest:0.5.4"
}
}
apply plugin : 'org.ysb33r.gradletest'
http://bit.ly/1LfUUU4

31. COMPATIBILITY TESTING
Create src/gradleTest/NameOfTest folder.
Add build.gradle
Add task runGradleTest
Add project structure

32. COMPATIBILITY TESTING
Add versions to main build.gradle
gradleTest {
versions '2.0', '2.2', '2.4', '2.5', '2.9'
}
Run it!
./gradlew gradleTest

33. THANK YOU
Keep your DSL
extensions beautiful
Don’t spring surprising
behaviour on the user
Email:
Twitter / Ello : @ysb33r
#idiomaticgradle
ysb33r@gmail.com
http://bit.ly/1iJmdiP

34. ADVANCED CONCEPTS

35. NOMENCLATURE
Property: A public data member (A Groovy property)
Method: A standard Java/Groovy method
Attribute: A value, set or accessed via the Gradle DSL. Can
result in a public method call or property access.
User: Person authoring or executing a Gradle build script
@Input
String aProperty = 'stdValue'
@Input
void aValue(String s) { ... }
myTask {
aProperty = 'newValue'
aValue 'newValue'
}

36. USER OVERRIDE LIBRARY VERSION
Ship with prefered (and tested) version of dependent
library set as default
Allow user flexibility to try a different version of such
library
Dynamically load library when needed
Still use power of Gradle’s dependency resolution

37. USER OVERRIDE LIBRARY VERSION
Example DSL from Asciidoctor
asciidoctorj {
version = '1.6.0-SNAPSHOT'
}
Example DSL from JRuby Base
jruby {
execVersion = '1.7.12'
}

38. USER OVERRIDE LIBRARY VERSION
1. Create Extension
2. Add extension object in plugin apply
3. Create custom classloader

39. USER OVERRIDE LIBRARY VERSION
Step 1: Create project extension
class MyExtension {
// Set the default dependent library version
String version = '1.5.0'
MyExtension(Project proj) {
project= proj
}
@PackageScope
Project project
}

40. USER OVERRIDE LIBRARY VERSION
Step 2: Add extension object in plugin apply
class MyPlugin implements Plugin<Project> {
void apply(Project project) {
// Create the extension & configuration
project.extensions.create('asciidoctorj',MyExtension,project)
project.configuration.maybeCreate( 'int_asciidoctorj' )
// Add dependency at the end of configuration phase
project.afterEvaluate {
project.dependencies {
int_asciidoctorj "org.asciidoctor:asciidoctorj" +
"${project.asciidoctorj.version}"
}
}
}
}

41. USER OVERRIDE LIBRARY VERSION (2.5+)
Step 2: Add extension object Gradle 2.5+
class MyPlugin implements Plugin<Project> {
void apply(Project project) {
// Create the extension & configuration
project.extensions.create('asciidoctorj',MyExtension,project)
def conf = configurations.maybeCreate( 'int_asciidoctorj' )
conf.defaultDependencies { deps ->
deps.add( project.dependencies.create(
"org.asciidoctor:asciidoctorj:${asciidoctorj.version}")
)
}
}
}

42. USER OVERRIDE LIBRARY VERSION
Step 3: Custom classloader (usually loaded from task action)
// Get all of the files in the `asciidoctorj` configuration
def urls = project.configurations.int_asciidoctorj.files.collect {
it.toURI().toURL()
}
// Create the classloader for all those files
def classLoader = new URLClassLoader(urls as URL[],
Thread.currentThread().contextClassLoader)
// Load one or more classes as required
def instance = classLoader.loadClass(
'org.asciidoctor.Asciidoctor$Factory')

43. NEED 2 KNOW : 'AFTEREVALUATE'
afterEvaluate adds to a list of closures to be executed
at end of configuration phase
Execution order is FIFO
Plugin author has no control over the order

44. STYLE : PROJECT EXTENSIONS
Treat project extensions as you would for any kind of
global configuration.
With care!
Do not make the extension configuration block a task
configuration.
Task instantiation may read defaults from extension.
Do not force extension values onto tasks

45. NEED 2 KNOW : PLUGINS
Plugin author has no control over order in which plugins
will be applied
Handle both cases of related plugin applied before or after
yours

46. EXTEND EXISTING TASK
Task type extension by inheritance is not always best
solution
Adding behaviour to existing task type better in certain
contexts
Example: jruby-jar-plugin wants to semantically
describe bootstrap files rather than force user to use
standard Copy syntax

47. EXTEND EXISTING TASK
jruby-jar-plugin without extension
jrubyJavaBootstrap {
// User gets exposed (unnecessarily) to the underlying task type
// Has to craft too much glue code
from( {
// @#$$!!-ugly code goes here
} )
}
jruby-jar-plugin with extension
jrubyJavaBootstrap {
// Expressing intent & context.
jruby {
initScript = 'bin/asciidoctor'
}
}

48. EXTEND EXISTING TASK
1. Create extension class
2. Add extension to task
3. Link extension attributes to task attributes (for caching)

49. EXTEND EXISTING TASK
Create extension class
class MyExtension {
String initScript
MyExtension( Task t ) {
// TODO: Add Gradle caching support
// (See later slide)
}
}

50. EXTEND EXISTING TASK
Add extension class to task
class MyPlugin implements Plugin<Project> {
void apply(Project project) {
Task stubTask = project.tasks.create
( name : 'jrubyJavaBootstrap', type : Copy )
stubTask.extensions.create(
'jruby',
MyExtension,
stubTask
)
}

51. EXTEND EXISTING TASK
Add Gradle caching support
class MyExtension {
String initScript
MyExtension( Task t ) {
// Tell the task the initScript is also a property
t.inputs.property 'jrubyInitScript' , { -> this.initScript }
}
}

52. NEED 2 KNOW : TASK EXTENSIONS
Good way extend existing tasks in composable way
Attributes on extensions are not cached
Changes will not cause a rebuild of the task
Do the extra work to cache and provide the user with a
better experience.

53. ADD GENERATED JVM SOURCE SETS
May need to generate code from template and add to
current sourceset(s)
Example: Older versions of jruby-jar-plugin added
a custom class file to JAR
Useful for separation of concerns in certain generative
programming environments

54. ADD GENERATED JVM SOURCE SETS
1. Create generator task using Copy task as transformer
2. Configure generator task
3. Update SourceSet
4. Add dependency between generation and compilation

55. ADD GENERATED JVM SOURCE SETS
Step1 : Add generator task
class MyPlugin implements Plugin<Project> {
void apply(Project project) {
Task stubTask = project.tasks.create
( name : 'myGenerator', type : Copy )
configureGenerator(stubTask)
addGeneratedToSource(project)
addTaskDependencies(project)
}
void configureGenerator(Task t)
{ /* TODO: <-- See next slides */ }
void addGeneratedToSource(Project p)
{ /* TODO: <-- See next slides */ }
void addTaskDependencies(Project p)
{ /* TODO: <-- See next slides */ }
}
This example uses Java, but can apply to any kind of sourceset
that Gradle supports

56. ADD GENERATED JVM SOURCE SETS
Step 2 : Configure generator task
/* DONE: <-- See previous slide for apply() */
void configureGenerator(Task stubTask) {
project.configure(stubTask) {
group "Add to correct group"
description 'Generates a JRuby Java bootstrap class'
from('src/template/java') {
include '*.java.template'
}
into new File(project.buildDir,'generated/java')
rename '(.+).java.template','$1.java'
filter { String line ->
/* Do something in here to transform the code */ }
}
}

57. ADD GENERATED JVM SOURCE SETS
Step 3 : Add generated code to SourceSet
/* DONE: <-- See earlier slide for apply() */
void addGeneratedToSource(Project project) {
project.sourceSets.matching { it.name == "main" } .all {
it.java.srcDir new File(project.buildDir,'generated/java')
}
}

58. ADD GENERATED JVM SOURCE SETS
Step 4 : Add task dependencies
/* DONE: <-- See earlier slide for apply() */
void addTaskDependencies(Project project) {
try {
Task t = project.tasks.getByName('compileJava')
if( t instanceof JavaCompile) {
t.dependsOn 'myGenerator'
}
} catch(UnknownTaskException) {
project.tasks.whenTaskAdded { Task t ->
if (t.name == 'compileJava' && t instanceof JavaCompile) {
t.dependsOn 'myGenerator'
}
}
}
}

59. TRICK : SAFE FILENAMES
Ability to create safe filenames on all platforms from input
data
Example: Asciidoctor output directories based upon
backend names
// WARNING: Using a very useful internal API
import org.gradle.internal.FileUtils
File outputBackendDir(final File outputDir,
final String backend) {
// FileUtils.toSafeFileName is your magic method
new File(outputDir, FileUtils.toSafeFileName(backend))
}

60. TRICK : SELF-REFERENCING PLUGIN
New plugin depends on functionality in the plugin
Apply plugin direct in build.gradle
apply plugin: new GroovyScriptEngine(
['src/main/groovy','src/main/resources'].
collect{ file(it).absolutePath }
.toArray(new String[2]),
project.class.classLoader
).loadScriptByName('book/SelfReferencingPlugin.groovy')

61. TRICK : OPERATING SYSTEM
Sometimes customised work has to be done on a specific
O/S
Example: jruby-gradle-plugin needs to set TMP in
environment on Windows
// This is the public interface API
import org.gradle.nativeplatform.platform.OperatingSystem
// But to get an instance the internal API is needed instead
import org.gradle.internal.os.OperatingSystem
println "Are we on Windows? ${OperatingSystem.current().isWindows()}

62. THANK YOU
Keep your DSL
extensions beautiful
Don’t spring surprising
behaviour on the user
Email:
Twitter / Ello : @ysb33r
#idiomaticgradle
ysb33r@gmail.com
http://bit.ly/1iJmdiP