This document presents a detailed guide on idiomatic Gradle plugin development, addressing the importance of consistency, readability, and flexibility in Gradle DSL. It covers various aspects including project layout, task creation, plugin compatibility testing, and best practices for authoring plugins. The presentation also offers insights on extending existing tasks and managing library versions, emphasizing user experiences and configuration integrity.

1. #GradleSummit Gradle Summit 2016
IDIOMATIC GRADLE
PLUGIN WRITING
Schalk W. Cronjé

2. 1
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. 2

4. 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

5. 4
GET YOUR DAILY GRADLE DOSE
@DailyGradle
#gradleTip

6. 5
6
THE PROBLEM
There is no consistency in the way plugin authors craft extensions
to the Gradle DSL today

7. QUALITY ATTRIBUTES OF DSL
Readability
Consistency
Flexibility
Expressiveness

8. 7
PROJECT LAYOUT
Figure 1. Plugin project le layout

9. 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'
}
}

10. 9 . 1
9 . 2
TRICK : SPOCK VERSION
ext {
spockGrVer = GroovySystem.version.replaceAll(/.d+$/,'')
}
dependencies {
testCompile ("org.spockframework:spock-core:1.0-${spockGrVer}") {
exclude module : 'groovy-all'
}
}

11. 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) {
}
}

12. 10 . 1
10 . 2
CREATE PROPERTIES FILE
META-INF/gradle-
plugins/idiomatic.authored.example.properties
implementation-class=idiomatic.gradle.authoring.MyExamplePlugin
Name of le must match plugin identi er

13. 11
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

14. FOR BEST COMPATIBILITY
Support same JDK range as Gradle
Gradle 1.x - mininum JDK5
Gradle 2.x - minimum JDK6
Build against Gradle 2.0
… unless proper compatibility testing is in place
Suggested baseline at Gradle 2.12 (for new model)
Only use later versions if speci c new functionality is
required.

15. 12 . 1
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
}
(Fixed in 2.14)

16. 12 . 2
12 . 3
GRADLE BUILD VERSION
gradle/wrapper/gradle-wrapper.properties
distributionUrl=https://..../distributions/gradle-2.0-all.zip

17. 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!!

18. 13 . 1
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() {
}
}

19. 13 . 2
14 . 1
HONOUR OFFLINE
gradle --offline
The build should operate without accessing
network resources.

20. 14 . 2
HONOUR OFFLINE
Unset the enabled property, if build is of ine
task VfsCopy extends DefaultTask {
VfsCopy() {
enabled = !project.gradle.startParameter.isOffline()
}
}

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

22. 15
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') ]
}

23. 16 . 1
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

24. 16 . 2
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 = []
}

25. 16 . 3
KNOW YOUR TASK ANNOTATIONS
@Input
@InputFile
@InputFiles
@InputDirectory
@Nested
@OutputFile
@OutputFiles
@OutputDirectory
@OutputDirectories
@Optional

26. 17
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 = []

27. 18
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' ]
}

28. 19 . 1
19 . 2
MAPS
task myTask( type: MyTask ) {
myOptions prop1 : 'foo/bar.txt', prop2 : 'bar/foo.txt'
myOptions prop3 : 'add/another'
// Explicit reset
myOptions = [:]
}

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

30. 19 . 3
20 . 1
COMPATIBILITY TESTING
How can a plugin author test a plugin against multiple Gradle
versions?

31. COMPATIBILITY TESTING
Gradle 2.7 added TestKit
2.9 added multi-distribution testing
Really became useful in 2.12/2.13
What to do for Gradle 2.0 - 2.8?

32. 20 . 2
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

33. 20 . 3
COMPATIBILITY TESTING
Create src/gradleTest/NameOfTest folder.
Add build.gradle
Add task runGradleTest
Add project structure

34. 20 . 4
COMPATIBILITY TESTING
Add versions to main build.gradle
gradleTest {
versions '2.0', '2.2', '2.4', '2.5', '2.9'
}
Run it!
./gradlew gradleTest

35. 20 . 5
TRICK : SAFE FILENAMES
Ability to create safe lenames 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))
}

36. 21
22 . 1
CONVERTING EXTENSION TO NEW MODEL
Quickly convert an existing extension to be useable within
model
Easy migration path for existing users of a plugin
Little rewrite of code

37. CONVERTING EXTENSION TO NEW MODEL
Existing extension code
class ExternalToolExtension {
String executable = 'make'
List<String> execArgs = []
void execArgs(String... args) {
this.execArgs.addAll(args as List)
}
}
In plugin apply
project.extensions.create('externalTool',ExternalToolExtension)

38. 22 . 2
22 . 3
LINKING EXTENSION TO NEW MODEL
Old build script style
externalTool {
executable 'gmake'
execArgs '-s','-B'
}

39. LINKING EXTENSION TO NEW MODEL
New model style
model {
externalTool {
executable 'gmake'
executable = 'gmake'
execArgs = ['-i']
execArgs '-s','-B'
}
}

40. 22 . 4
22 . 5
LINKING EXTENSION TO NEW MODEL
Create model rule
class ExtensionContainerRules extends RuleSource {
@Model
ExternalToolExtension externalTool(ExtensionContainer ext) {
ext.getByType(ExternalToolExtension)
}
}

41. LINKING EXTENSION TO NEW MODEL
Disadvantages
Changes made in the extension automatically re ects new
model.
Order of new model evaluation and
project.afterEvaluate execution not guaranteed.
Gradle can never guarantee the con guration to be
immutable.

42. 22 . 6
MIGRATING EXTENSION TO UNMANAGED MODEL
Eliminate some of the issues of linking.
Similar minimal code changes as for linking.
Need to take care of decoration yourself.
Remove creation of extension when plugin is applied.

43. 23 . 1
MIGRATING EXTENSION TO UNMANAGED MODEL
Modify extension class
class ExternalToolExtension {
String executable = 'make'
List<String> execArgs = []
void execArgs(String... args) {
this.execArgs.addAll(args as List)
}
void executable(String exe) { // <-- Add this
this.executable = exe
}
}

44. 23 . 2
23 . 3
MIGRATING EXTENSION TO UNMANAGED MODEL
Model rule remains unchanged
class ExtensionContainerRules extends RuleSource {
@Model
ExternalToolExtension externalTool(ExtensionContainer ext) {
ext.getByType(ExternalToolExtension)
}
}

45. 23 . 4
MIGRATING EXTENSION TO UNMANAGED MODEL
Disadvantages
Gradle can never guarantee the con guration to be
immutable.
Gradle will not decorate the extension with any other
methods.

46. 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')

47. 24
GET THE BOOKS
https://leanpub.com/b/idiomaticgradle

48. 25
THANK YOU
Keep your DSL extensions beautiful
Don’t spring surprising behaviour on the user
Email:
Twitter / Ello : @ysb33r
#idiomaticgradle
ysb33r@gmail.com

49. 26
ADVANCED CONCEPTS
User override library version
Extend (decorate) existing task
Add generated JVM source sets
Operating system

50. 27
USER OVERRIDE LIBRARY VERSION
Ship with prefered (and tested) version of dependent
library set as default
Allow user exibility to try a different version of such
library
Dynamically load library when needed
Still use power of Gradle’s dependency resolution

51. 28 . 1
USER OVERRIDE LIBRARY VERSION
Example DSL from Asciidoctor
asciidoctorj {
version = '1.6.0-SNAPSHOT'
}
Example DSL from JRuby Base
jruby {
execVersion = '1.7.12'
}

52. 28 . 2
28 . 3
USER OVERRIDE LIBRARY VERSION
1. Create Extension
2. Add extension object in plugin apply
3. Create custom classloader

53. 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
}

54. 28 . 4
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}"
}
}
}
}

55. 28 . 5
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}")
)
}
}
}

56. 28 . 6
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')

57. 28 . 7
NEED 2 KNOW : 'AFTEREVALUATE'
afterEvaluate adds to a list of closures to be executed
at end of con guration phase
Execution order is FIFO
Plugin author has no control over the order

58. 28 . 8
STYLE : PROJECT EXTENSIONS
Treat project extensions as you would for any kind of global
con guration.
With care!
Do not make the extension con guration block a task
con guration.
Task instantiation may read defaults from extension.
Do not force extension values onto tasks

59. 28 . 9
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 les rather than force user to use
standard Copy syntax

60. 29 . 1
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'
}
}

61. 29 . 2
29 . 3
EXTEND EXISTING TASK
1. Create extension class
2. Add extension to task
3. Link extension attributes to task attributes (for caching)

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

63. 29 . 4
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
)
}

64. 29 . 5
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 }
}
}

65. 29 . 6
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.

66. 29 . 7
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 le to JAR
Useful for separation of concerns in certain generative
programming environments

67. 30 . 1
ADD GENERATED JVM SOURCE SETS
1. Create generator task using Copy task as transformer
2. Con gure generator task
3. Update SourceSet
4. Add dependency between generation and compilation

68. 30 . 2
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

69. 30 . 3
ADD GENERATED JVM SOURCE SETS
Step 2 : Con gure 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 */ }
}
}

70. 30 . 4
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')
}
}

71. 30 . 5
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'
}
}
}
}

72. 30 . 6
TRICK : OPERATING SYSTEM
Sometimes customised work has to be done on a speci c
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()}

73. 31
GET THE BOOKS
https://leanpub.com/b/idiomaticgradle

74. 32
THANK YOU
Keep your DSL extensions beautiful
Don’t spring surprising behaviour on the user
Email:
Twitter / Ello : @ysb33r
#idiomaticgradle
ysb33r@gmail.com