Annotation processing shared on Android Taipei 2016-12-19

1. Annotation Processing
Benjamin Cheng

2. Examples

3. AutoValue github.com/google/auto/

4. AutoValue github.com/google/auto/

5. Butterknife jakewharton.github.io/butterknife/

6. Butterknife jakewharton.github.io/butterknife/

8. Annotation Processing

10. Annotation Processing (Why?)
● Reduce writing boilerplate code
● May beyond our imagination

11. Annotation Processing (What?)
● Part of javac
● @Annotation based
● Generate .java source files (new files only)

12. Components
● Annotations
● Processor
● APT (Annotation Processing Tool)
● Annotated Sources

13. Annotations

14. Annotation Processor

15. Annotation Processor (conti.)

16. Annotation Processor (conti.)

17. Elements

18. ServiceLoader Discovery File
- com/example/ExampleProcessor.class
- META-INF/services/javax.annotation.processing.Processor
com.example.ExampleProcessor

19. Processing Rounds
javac
BuilderProcesso
r
OtherProcessor
out/
User.class
UserBuilder.class
@Other
public class GeneratedClass {
// …
}

20. Processing Rounds (conti.)
javac
BuilderProcesso
r
OtherProcessor
out/
User.class
UserBuilder.class
public class OtherGenClass {
// …
}
GeneratedClass.class

21. Processing Rounds (conti.)
javac
BuilderProcesso
r
OtherProcessor
out/
User.class
UserBuilder.class
public class OtherGenClass {
// …
}
GeneratedClass.class

22. Processing Rounds (conti.)
javac
BuilderProcesso
r
OtherProcessor
out/
User.class
UserBuilder.class
GeneratedClass.class
OtherGenClass.class

23. Add to APT Classpath

24. Code Generation

25. Code Generation
● StringBuilder
● JavaPoet

26. JavaPoet
● Use Fluent API with builders
● Based on Specs
○ TypeSpec
○ MethodSpec
○ ParameterSpec
○ FiledSpec
github.com/square/javapoet/

27. JavaPoet (conti.) github.com/square/javapoet/

28. JavaPoet (conti.) github.com/square/javapoet/
public final class UserBuilder { … }

29. JavaPoet (conti.) github.com/square/javapoet/
public final class UserBuilder {
// fields
private String username;
}

30. JavaPoet (conti.) github.com/square/javapoet/

31. JavaPoet (conti.) github.com/square/javapoet/

32. JavaPoet (conti.) github.com/square/javapoet/

33. Testing and Debugging

34. Testing and Debugging
● Guava
● compile-testing
● Truth

35. References
● Droidcon NYC 2016 - @Eliminate("Boilerplate") https://youtu.be/NBkl_SIHUr8
● #droidconDE 2015: Hannes Dorfmann – Annotation Processing 101
https://youtu.be/43FFfTyDYEg http://hannesdorfmann.com/annotation-
processing/annotationprocessing101
● Android Tech Talk: Annotation Processing Boilerplate Destruction by Jake
Wharton https://youtu.be/dOcs-NKK-RA
https://speakerdeck.com/jakewharton/annotation-processing-boilerplate-
destruction-square-waterloo-2014
● The 10-step guide to annotation processing in Android studio by Chris Logan
http://blog.stablekernel.com/the-10-step-guide-to-annotation-processing-in-

36. Q&A