This document discusses best practices for writing idiomatic Gradle plugins, including: 1. Making the plugin DSL readable, consistent, flexible and expressive. 2. Supporting the same Java versions as Gradle for compatibility. 3. Preferring methods over properties and using annotations properly. 4. Handling collections, maps, overriding dependencies, generated code, extensions and more idiomatically.

1. 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

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

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

5. QUALITY ATTRIBUTES OF DSL
Readability
Consistency
Flexibility
Expressiveness

6. 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 2.6

7. FOR BEST 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
}
// gradle/wrapper/gradle-wrapper.properties
distributionUrl=https://..../distributions/gradle-2.0-all.zip

8. 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'
}

9. 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

10. HOWNOT2 : 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') ]
}

11. 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

12. 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 = []
}

13. 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!!

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

15. 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 = []

16. HOWNOT2 : 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' ]
}

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

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

19. 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

20. USER OVERRIDE LIBRARY VERSION
Example DSL from Asciidoctor
asciidoctorj {
version = '1.5.3-SNAPSHOT'
}
Example DSL from JRuby Base
jruby {
execVersion = '1.7.12'
}

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

22. 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
}

23. 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( 'asciidoctorj' )
// Add dependency at the end of configuration phase
project.afterEvaluate {
project.dependencies {
asciidoctorj "org.asciidoctor:asciidoctorj" +
"${project.asciidoctorj.version}"
}
}
}
}

24. 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.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')

25. NEED2KNOW : '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

26. 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

27. GENERATED CODE
May need to generate code from template and add to
current sourceset(s)
Example: jruby-jar-plugin adds a custom class
file to JAR
Useful for separation of concerns in certain generative
programming environments

28. GENERATED CODE
1. Create generator task using Copy task as transformer
2. Configure generator task
3. Update SourceSet
4. Add dependency between generation and compilation

29. GENERATED CODE
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

30. GENERATED CODE
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 */ }
}
}

31. GENERATED CODE
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')
}
}

32. GENERATED CODE
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'
}
}
}
}

33. NEED2KNOW : 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

34. 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

35. 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'
}
}

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

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

38. 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
)
}

39. 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 'jrubyInitScipt' , { -> this.initScript }
}
}

40. NEED2KNOW : 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.

41. TRICK : SELF-REFERENCING PLUGIN
New plugin depends on functionality in the plugin
Apply plugin direct in build.gradle
apply plugin: new GroovyScriptEngine(
[file('src/main/groovy').absolutePath,
file('src/main/resources').absolutePath].
toArray(new String[2]),
this.class.classLoader
).loadScriptByName('src/main/groovy/spath/to/MyPlugin.groovy')

42. 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))
}

43. 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()}

44. CONCLUSION
Keep your DSL extensions beautiful
Don’t spring surprising behaviour on the user
Email:
Twitter / Ello : @ysb33r
ysb33r@gmail.com