Annotation processing allows for metadata to be added to Java programs without altering the program itself. Annotations can be defined and used to provide information for compilers or allow runtime/compile-time processing. Annotation processors provide a hook into the Java compilation process to generate source code and compile errors/warnings. They work by processing annotated elements in rounds. Testing annotation processors is challenging but libraries like Google's compile-testing library can help. Annotation processing also works on Android by setting up the processor as a Java module and using plugins like android-apt.

1. ANNOTATION PROCESSING
IN ANDROIDEmanuele Zattin - Realm
@emanuelez
19-2-2016 - DroidKaigi

2. DroidKaigi

3. WHAT IS A JAVA
ANNOTATION?

4. LET'S START WITH THE OFFICIAL
DEFINITION
Annotations, a form of metadata, provide data about a
program that is not part of the program itself.
Annotations have no direct effect on the operation of the
code they annotate.
— Oracle

5. WHAT CAN ANNOTATIONS DO?
1. Provide information for the compiler
2. Allow runtime processing
3. Allow compile-time processing

6. HOW TO DEFINE AN ANNOTATION
The simplest annotation can be defined as:
public @interface MyAnnotation {}
and it can be used like this:
@MyAnnotation
public class MyClass {}

7. ANNOTATIONS CAN ACCEPT
ARGUMENTS
public @interface MyAnnotation {
String arg1();
int arg2 default 1;
String[] arg3;
}
Which can be used like this:
@MyAnnotation (
arg1 = "value1", // It's a comma, not a semicolon
arg3 = { "value2", "value3" } // Arrays use curly brackets
)
public class MyClass {}

8. ANNOTATIONS CAN BE ANNOTATED
The most important is @Retention which value can
be:
▸ RetentionPolicy.SOURCE
▸ RetentionPolicy.CLASS
▸ RetentionPolicy.RUNTIME

9. ANNOTATIONS CAN BE ANNOTATED
PART 2
The other important annotation is @Target which
value:
ElementType.ANNOTATION_TYPE
ElementType.CONSTRUCTOR
ElementType.FIELD
ElementType.LOCAL_VARIABLE
ElementType.METHOD

10. ANNOTATIONS CAN BE ANNOTATED
PART 3
Other useful annotations:
▸ @Documented
▸ @Inherited
▸ @Repeatable

11. AN EXAMPLE ANNOTATION
@Retention(RetentionPolicy.CLASS) // Available at compile-time
@Target(ElementType.TYPE) // Can only be applied to classes
@interface MyAnnotation {
String arg1();
int arg2 default 1;
String[] arg3;
}

12. WHAT IS AN
ANNOTATION
PROCESSOR?

13. Annotation Processing is a technique that provides
a hook into the Java compile process.
It allows to produce compiler errors and warnings
and to generate source code and byte code.

14. JSR 269

15. HOW DOES IT WORK?
Here's a high-level example:
1. Normal compilation
2. First round of annotation processing
3. Second round of annotation processing
4. ...

16. IMPLEMENTING A PROCESSOR
public abstract class AbstractProcessor implements Processor {
// more methods here!
void init(
ProcessingEnvironment processingEnv
);
abstract boolean process(
Set<? extends TypeElement> annotations,
RoundEnvironment roundEnv
);
}

17. THE PROCESSING ENVIRONMENT
It provides the tools to write new files and access utility
classes.
Here are the most useful methods:
// Use Filer to write new files
Filer getFiler();
// Use Elements to manage fields, methods and classes
Elements getElementUtils();
// Use Types to deal with classes, converting Type to Element, ...
Types getTypeUtils();

18. THE ROUND ENVIRONMENT
It provides tools to deal with the specific round.
The most useful methods are:
// Get the elements annotated with a given annotation
Set<? extends Element> getElementsAnnotatedWith(Class<? extends Annotation> a);
Set<? extends Element> getElementsAnnotatedWith(TypeElement a);

19. THE MOST USELESS ANNOTATION
public AnnoyingProcessor extends AbstractProcessor {
boolean process( Set<> annotations, RoundEnvironment env) {
Messager m = processingEnv.getMessager();
for (TypeElement te : annotations) {
for (Element e : env.getElementsAnnotatedWith(te)) {
m.printMessage(Diagnostic.Kind.NOTE, "Processing " + e.toString());
}
}
return true;
}
}

20. REGISTERING YOUR PROCESSOR
1. Package your processor in a Jar file
2. The Jar file must contain a file called
javax.annotation.processing.Proce
ssor
located in META-INF/services
3. This file must contain the fully
qualified name of your processor

21. GENERATING JAVA CODE IN YOUR AP
Enter JavaPoet

22. MethodSpec main = MethodSpec.methodBuilder("main")
.addModifiers(Modifier.PUBLIC, Modifier.STATIC)
.returns(void.class)
.addParameter(String[].class, "args")
.addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!")
.build();
TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
.addModifiers(Modifier.PUBLIC, Modifier.FINAL)
.addMethod(main)
.build();
JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld)
.build();
javaFile.writeTo(processingEnv.getFiler());

23. TESTING
Testing annotations processors is hard because the
execution happens at compile time

24. SOME MUCH NEEDED HELP!
Google's compile-testing library

25. EXAMPLE 1
assert_() // it uses the Truth testing library
.about(javaSource())
.that(
JavaFileObjects.forSourceString(
"HelloWorld", "final class HelloWorld {}"
)
).compilesWithoutError();

26. EXAMPLE 2
assert_().about(javaSource())
.that(JavaFileObjects.forResource("HelloWorld.java"))
.processedWith(new MyAnnotationProcessor())
.compilesWithoutError()
.and()
.generatesSources(
JavaFileObjects.forResource("GeneratedHelloWorld.java"));

27. HOW ABOUT
ANNOTATION
PROCESSING IN
ANDROID?

28. PROBLEM 1
The Android framework does not include
javax.annotation.processing

29. SOLUTION
Setup your AP to be a Java module
and to inform your users to use
the Gradle android-apt plugin by Hugo Visser

30. PROBLEM 2
Both your library and your AP might depend on the
annotations

31. SOLUTION
Setup your annotations to be another Java module
on which both the library and the AP depend on

32. PROBLEM 3
Now the Javadoc of your library does not include the
annotations

33. SOLUTION
android.libraryVariants.all { variant ->
task("javadoc${variant.name.capitalize()}", type: Javadoc) {
description "Generates Javadoc for $variant.name."
group 'Docs'
source = variant.javaCompile.source
source "../annotations/src/main/java" // <-- THIS!
ext.androidJar = files(project.android.getBootClasspath())
classpath = files(variant.javaCompile.classpath.files)
+ ext.androidJar
exclude '**/BuildConfig.java'
exclude '**/R.java'
}
}

34. ANDROID LIBRARIES THAT USE AP
▸ ButterKnife
▸ Dagger
▸ Parceler
▸ Realm

35. Thank you!

36. Questions?