Annotation Processing Boilerplate Destruction (Droidcon NYC 2014)

The Java programming language has a knack for requiring a lot of boilerplate code. Annotation processing is a feature of the Java compiler which provides hooks that allow automatic code generation based on annotations. This greatly simplifies your code by pushing the burden of the boilerplate on automated tooling.

This talk will briefly showcase existing annotation processors for Android development. We will then focus on how to get started writing your own annotation processor.

1. Annotation Processing Jake Wharton Boilerplate Destruction!
2. Annotation Processing Jake Wharton Boilerplate Destruction!
3. Dagger
4. Dagger class CoffeeMaker {
 @Inject Lazy<Heater> heater;
 @Inject Pump pump;
 
 public void brew() {
 heater.get().on();
 pump.pump();
 System.out.println(" [_]P coffee! [_]P ");
 heater.get().off();
 }
 }
 square.github.io/dagger/
5. Butter Knife
6. Butter Knife class SimpleActivity extends Activity {
 @InjectView(R.id.title) TextView title;
 @InjectView(R.id.subtitle) TextView subtitle;
 @InjectView(R.id.hello) Button hello;
 @InjectView(R.id.list_of_things) ListView listOfThings;
 @InjectView(R.id.footer) TextView footer;
 
 // ...
 } jakewharton.github.io/butterknife/
7. Butter Knife class SimpleActivity extends Activity {
 @InjectView(R.id.title) TextView title;
 @InjectView(R.id.subtitle) TextView subtitle;
 @InjectView(R.id.hello) Button hello;
 @InjectView(R.id.list_of_things) ListView listOfThings;
 @InjectView(R.id.footer) TextView footer;
 
 // ...
 } jakewharton.github.io/butterknife/ ButterKnife.inject(this);
8. AutoValue
9. AutoValue @AutoValue
 abstract static class Animal {
 static Animal create(String name, int numberOfLegs) {
 return new AutoValue_Animal(name, numberOfLegs);
 } 
 abstract String name();
 abstract int numberOfLegs();
 }
 github.com/google/auto/
10. AutoParcel
11. AutoParcel @AutoParcel
 abstract class SomeModel implements Parcelable {
 abstract String name();
 abstract List<SomeSubModel> subModels();
 abstract Map<String, OtherSubModel> modelsMap();
 
 static SomeModel create(String name, List<SomeSubModel> subModels, Map<String, OtherSubModel> modelsMap) {
 return new AutoParcel_SomeModel(name, subModels, modelsMap);
 }
 } github.com/frankiesardo/auto-parcel/
12. Schematic
13. Schematic interface NoteColumns {
 
 @DataType(INTEGER) @PrimaryKey @AutoIncrement
 String _ID = "_id";
 
 @DataType(INTEGER) @References(table = NotesDatabase.LISTS,
 column = ListColumns._ID)
 String LIST_ID = "listId";
 
 @DataType(TEXT) String NOTE = "note";
 }
14. Schematic @Database(version = NotesDatabase.VERSION)
 public final class NotesDatabase {
 
 public static final int VERSION = 1;
 
 @Table(ListColumns.class)
 public static final String LISTS = "lists";
 
 @Table(NoteColumns.class)
 public static final String NOTES = "notes";
 
 }
15. Schematic @ContentProvider(authority = NotesProvider.AUTHORITY,
 database = NotesDatabase.class)
 public final class NotesProvider {
 // ...
 } github.com/SimonVT/schematic
16. Doclet http://docs.oracle.com/javase/8/docs/technotes/guides/javadoc/doclet/overview.html
17. Doclet import com.sun.javadoc.*; ! public class ExampleDoclet {
 public static boolean start(RootDoc root) {
 ! ! ! 
 }
 } http://docs.oracle.com/javase/8/docs/technotes/guides/javadoc/doclet/overview.html
18. Doclet import com.sun.javadoc.*; ! public class ExampleDoclet {
 public static boolean start(RootDoc root) {
 ! ! ! 
 }
 } import com.sun.javadoc.*; ! public class ExampleDoclet {
 public static boolean start(RootDoc root) {
 ClassDoc[] classes = root.classes();
 for (int i = 0; i < classes.length; i++) {
 System.out.println(classes[i]);
 }
 return true; 
 }
 } http://docs.oracle.com/javase/8/docs/technotes/guides/javadoc/doclet/overview.html
19. Doclet import com.sun.javadoc.*; ! public class ExampleDoclet {
 public static boolean start(RootDoc root) {
 ! ! ! 
 }
 } import com.sun.javadoc.*; ! public class ExampleDoclet {
 public static boolean start(RootDoc root) {
 ClassDoc[] classes = root.classes();
 for (int i = 0; i < classes.length; i++) {
 System.out.println(classes[i]);
 }
 return true; 
 }
 } javadoc -doclet ExampleDoclet -docletpath . RandomClass.java \
 FooBarClass.java HelloClass.java WorldClass.java import com.sun.javadoc.*; ! public class ExampleDoclet {
 public static boolean start(RootDoc root) {
 ClassDoc[] classes = root.classes();
 for (int i = 0; i < classes.length; i++) {
 System.out.println(classes[i]);
 }
 return true; 
 }
 } http://docs.oracle.com/javase/8/docs/technotes/guides/javadoc/doclet/overview.html
20. Java 1.5
21. Java 1.5
22. Java 1.5 • JSR 14: Generics
23. Java 1.5 • JSR 14: Generics • JSR 175: Annotations
24. Java 1.5 • JSR 14: Generics • JSR 175: Annotations • Also a whole bunch of other really awesome things!
25. Java 1.5 • JSR 14: Generics • JSR 175: Annotations • Also a whole bunch of other really awesome things! • apt command-line tool
26. Annotation Processing Tool: apt apt
27. apt
28. @Target(ElementType.TYPE)
 public @interface Example {
 } apt
29. @Target(ElementType.TYPE)
 public @interface Example {
 } @Example
 public class RandomClass {
 // ...
 } @Example
 public class FooBarClass {
 // ...
 } @Example
 public class HelloClass {
 // ...
 } @Example
 public class WorldClass {
 // ...
 } @Target(ElementType.TYPE)
 public @interface Example {
 } apt
30. interface AnnotationProcessor {
 void process();
 } apt
31. interface AnnotationProcessorFactory { 
 AnnotationProcessor getProcessorFor( Set<AnnotationTypeDeclaration> atds, AnnotationProcessorEnvironment env); 
 Collection<String> supportedAnnotationTypes(); 
 Collection<String> supportedOptions(); 
 } interface AnnotationProcessor {
 void process();
 } apt
32. apt public class ExampleAnnotationProcessorFactory implements AnnotationProcessorFactory { 
 
 
 
 
 
 
 
 
 
 
 
 
 }
33. apt public class ExampleAnnotationProcessorFactory implements AnnotationProcessorFactory { 
 
 
 
 
 
 
 
 
 
 @Override public Collection<String> supportedOptions() {
 return Collections.emptyList();
 }
 }
34. apt public class ExampleAnnotationProcessorFactory implements AnnotationProcessorFactory { 
 
 
 
 
 
 @Override public Collection<String> supportedAnnotationTypes() {
 return Collections.singletonList(Example.class.getName());
 }
 
 @Override public Collection<String> supportedOptions() {
 return Collections.emptyList();
 }
 }
35. apt public class ExampleAnnotationProcessorFactory implements AnnotationProcessorFactory { 
 @Override public AnnotationProcessor getProcessorFor( Set<AnnotationTypeDeclaration> atds,
 AnnotationProcessorEnvironment env) {
 return new ExampleAnnotationProcessor(atds, env);
 }
 
 @Override public Collection<String> supportedAnnotationTypes() {
 return Collections.singletonList(Example.class.getName());
 }
 
 @Override public Collection<String> supportedOptions() {
 return Collections.emptyList();
 }
 }
36. apt public class ExampleAnnotationProcessorFactory implements AnnotationProcessorFactory { 
 @Override public AnnotationProcessor getProcessorFor( Set<AnnotationTypeDeclaration> atds,
 AnnotationProcessorEnvironment env) {
 return new ExampleAnnotationProcessor(atds, env);
 }
 
 @Override public Collection<String> supportedAnnotationTypes() {
 return Collections.singletonList(Example.class.getName());
 }
 
 @Override public Collection<String> supportedOptions() {
 return Collections.emptyList();
 }
 }
37. apt public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 
 
 
 
 
 
 
 
 }
38. apt public class 
 
 
 
 
 
 
 
 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 private final Set<AnnotationTypeDeclaration> atds;
 private final AnnotationProcessorEnvironment env;
 
 public ExampleAnnotationProcessor( Set<AnnotationTypeDeclaration> atds,
 AnnotationProcessorEnvironment env) {
 this.atds = atds;
 this.env = env;
 } 
 }
39. apt public class 
 
 
 
 
 
 
 
 
 } public class 
 
 Set<AnnotationTypeDeclaration> atds 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 
 
 
 
 
 
 
 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 // ...
 
 
 
 
 
 
 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 // ...
 
 @Override public void process() {
 ! ! 
 } ! 
 }
40. apt public class 
 
 
 
 
 
 
 
 
 } public class 
 
 Set<AnnotationTypeDeclaration> atds 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 
 
 
 
 
 
 
 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 // ...
 
 
 
 
 
 
 
 } public class ExampleAnnotationProcessor implements AnnotationProcessor { 
 // ...
 
 @Override public void process() { for (AnnotationTypeDeclaration atd : atds) {
 MemberDeclaration member = atd.getDeclaringType();
 System.out.println(member);
 }
 } ! ! }
41. apt apt -classpath out/ src/RandomClass.java \
 src/FooBarClass.java src/HelloClass.java \
 src/WorldClass.java http://docs.oracle.com/javase/1.5.0/docs/tooldocs/share/apt.html
42. JSR 269
43. JSR 269 • 6 months after Java 5 released
44. JSR 269 • 6 months after Java 5 released • “Pluggable Annotation Processing API”
45. JSR 269 • 6 months after Java 5 released • “Pluggable Annotation Processing API” • Google, Oracle, Sun, BEA Systems sponsored
46. JSR 269 • 6 months after Java 5 released • “Pluggable Annotation Processing API” • Google, Oracle, Sun, BEA Systems sponsored • Simplify and standardize processing annotations
47. JSR 269 • 6 months after Java 5 released • “Pluggable Annotation Processing API” • Google, Oracle, Sun, BEA Systems sponsored • Simplify and standardize processing annotations • No standalone tool, part of javac
48. interface AnnotationProcessor {
 void process();
 }
49. interface Processor {
 Set<String> getSupportedOptions();
 
 Set<String> getSupportedAnnotationTypes();
 
 SourceVersion getSupportedSourceVersion();
 
 void init(ProcessingEnvironment processingEnv);
 
 boolean process(Set<? extends TypeElement> annotations,
 RoundEnvironment roundEnv);
 
 Iterable<? extends Completion> getCompletions(Element element,
 AnnotationMirror annotation,
 ExecutableElement member,
 String userText);
 }
50. public class ExampleProcessor extends AbstractProcessor {
 
 
 
 
 
 
 
 
 
 
 
 
 
 }
51. public class ExampleProcessor extends AbstractProcessor {
 @Override public SourceVersion getSupportedSourceVersion() {
 
 }
 
 
 
 
 
 
 
 
 
 
 }
52. public class ExampleProcessor extends AbstractProcessor {
 @Override public SourceVersion getSupportedSourceVersion() {
 return SourceVersion.latestSupported();
 }
 
 
 
 
 
 
 
 
 
 
 }
53. public class ExampleProcessor extends AbstractProcessor {
 @Override public SourceVersion getSupportedSourceVersion() {
 return SourceVersion.latestSupported();
 }
 
 @Override public Set<String> getSupportedAnnotationTypes() {
 
 }
 
 
 
 
 
 
 }
54. public class ExampleProcessor extends AbstractProcessor {
 @Override public SourceVersion getSupportedSourceVersion() {
 return SourceVersion.latestSupported();
 }
 
 @Override public Set<String> getSupportedAnnotationTypes() {
 return Collections.singleton(Example.class.getName());
 }
 
 
 
 
 
 
 }
55. public class ExampleProcessor extends AbstractProcessor {
 @Override public SourceVersion getSupportedSourceVersion() {
 return SourceVersion.latestSupported();
 }
 
 @Override public Set<String> getSupportedAnnotationTypes() {
 return Collections.singleton(Example.class.getName());
 }
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 
 }
 }
56. public class ExampleProcessor extends AbstractProcessor {
 // ...
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 ! ! ! ! ! 
 }
 }
57. public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 
 
 
 
 
 }
 }
58. public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element);
 }
 
 
 }
 }
59. public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element);
 }
 
 return false;
 }
 }
60. public class ExampleProcessor extends AbstractProcessor {
 @Override public SourceVersion getSupportedSourceVersion() {
 return SourceVersion.latestSupported();
 }
 
 @Override public Set<String> getSupportedAnnotationTypes() {
 return Collections.singleton(Example.class.getName());
 }
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element);
 }
 return false;
 }
 }
61. Execution
62. Execution • Happens automatically when javac runs.
63. Execution • Happens automatically when javac runs. • ServiceLoader finds and creates Processor instances.
64. Service Loader
65. Service Loader • META-­‐INF/services/
66. Service Loader • META-­‐INF/services/ • File name of implemented fully-qualified class name.
67. Service Loader • META-­‐INF/services/ • File name of implemented fully-qualified class name. • javax.annotation.processing.Processor
68. Service Loader • META-­‐INF/services/ • File name of implemented fully-qualified class name. • javax.annotation.processing.Processor • Each line implementation fully-qualified class name.
69. Service Loader • META-­‐INF/services/ • File name of implemented fully-qualified class name. • javax.annotation.processing.Processor • Each line implementation fully-qualified class name. • com.example.ExampleProcessor
70. Execution • Happens automatically when javac runs. • ServiceLoader finds and creates Processor instances.
71. Execution • Happens automatically when javac runs. • ServiceLoader finds and creates Processor instances. • Inside a full JVM.
72. Execution • Happens automatically when javac runs. • ServiceLoader finds and creates Processor instances. • Inside a full JVM. • Compiled sources and its dependencies not available.
73. Execution • Happens automatically when javac runs. • ServiceLoader finds and creates Processor instances. • Inside a full JVM. • Compiled sources and its dependencies not available. • Bring your own dependencies.
74. Environment
75. Environment interface Processor {
 Set<String> getSupportedOptions();
 
 Set<String> getSupportedAnnotationTypes();
 
 SourceVersion getSupportedSourceVersion();
 
 void init(ProcessingEnvironment processingEnv);
 
 boolean process(Set<? extends TypeElement> annotations,
 RoundEnvironment roundEnv);
 
 Iterable<? extends Completion> getCompletions(Element element,
 AnnotationMirror annotation,
 ExecutableElement member,
 String userText);
 }
76. Environment interface Processor {
 Set<String> getSupportedOptions();
 
 Set<String> getSupportedAnnotationTypes();
 
 SourceVersion getSupportedSourceVersion();
 
 void init(ProcessingEnvironment processingEnv);
 
 boolean process(Set<? extends TypeElement> annotations,
 RoundEnvironment roundEnv);
 
 Iterable<? extends Completion> getCompletions(Element element,
 AnnotationMirror annotation,
 ExecutableElement member,
 String userText);
 }
77. Environment interface Processor {
 Set<String> getSupportedOptions();
 
 Set<String> getSupportedAnnotationTypes();
 
 SourceVersion getSupportedSourceVersion();
 
 void init(ProcessingEnvironment processingEnv);
 
 boolean process(Set<? extends TypeElement> annotations,
 RoundEnvironment roundEnv);
 
 Iterable<? extends Completion> getCompletions(Element element,
 AnnotationMirror annotation,
 ExecutableElement member,
 String userText);
 }
78. Environment interface Processor {
 Set<String> getSupportedOptions();
 
 Set<String> getSupportedAnnotationTypes();
 
 SourceVersion getSupportedSourceVersion();
 
 void init(ProcessingEnvironment processingEnv);
 
 boolean process(Set<? extends TypeElement> annotations,
 RoundEnvironment roundEnv);
 
 Iterable<? extends Completion> getCompletions(Element element,
 AnnotationMirror annotation,
 ExecutableElement member,
 String userText);
 }
79. Environment interface Processor {
 Set<String> getSupportedOptions();
 
 Set<String> getSupportedAnnotationTypes();
 
 SourceVersion getSupportedSourceVersion();
 
 void init(ProcessingEnvironment processingEnv);
 
 boolean process(Set<? extends TypeElement> annotations,
 RoundEnvironment roundEnv);
 
 Iterable<? extends Completion> getCompletions(Element element,
 AnnotationMirror annotation,
 ExecutableElement member,
 String userText);
 }
80. Processing Environment
81. Processing Environment interface ProcessingEnvironment {
 Map<String, String> getOptions(); 
 SourceVersion getSourceVersion();
 
 Locale getLocale();
 
 Messager getMessager();
 
 Filer getFiler();
 
 Elements getElementUtils();
 
 Types getTypeUtils();
 }
82. Processing Environment interface ProcessingEnvironment {
 Map<String, String> getOptions(); 
 SourceVersion getSourceVersion();
 
 Locale getLocale();
 
 Messager getMessager();
 
 Filer getFiler();
 
 Elements getElementUtils();
 
 Types getTypeUtils();
 }
83. Processing Environment interface ProcessingEnvironment {
 Map<String, String> getOptions(); 
 SourceVersion getSourceVersion();
 
 Locale getLocale();
 
 Messager getMessager();
 
 Filer getFiler();
 
 Elements getElementUtils();
 
 Types getTypeUtils();
 }
84. Processing Environment interface ProcessingEnvironment {
 Map<String, String> getOptions(); 
 SourceVersion getSourceVersion();
 
 Locale getLocale();
 
 Messager getMessager();
 
 Filer getFiler();
 
 Elements getElementUtils();
 
 Types getTypeUtils();
 }
85. Processing Environment interface ProcessingEnvironment {
 Map<String, String> getOptions(); 
 SourceVersion getSourceVersion();
 
 Locale getLocale();
 
 Messager getMessager();
 
 Filer getFiler();
 
 Elements getElementUtils();
 
 Types getTypeUtils();
 }
86. Elements
87. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
88. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 } TypeElement
89. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 } TypeElement VariableElement
90. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 } TypeElement VariableElement ExecutableElement
91. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 } TypeElement VariableElement ExecutableElement ExecutableElement
92. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 } TypeElement VariableElement ExecutableElement ExecutableElement
93. Elements @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 } TypeElement VariableElement ExecutableElement ExecutableElement
94. Types @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
95. Types @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
96. Types @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
97. Types @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
98. Types @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
99. Types @Example
 public class ExampleClass implements Runnable { 
 private final String name;
 
 public ExampleClass(String name) {
 this.name = name;
 }
 
 @Override public void run() {
 System.out.println("Hello, " + name);
 }
 }
100. Environment
101. Environment public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element
 }
 return false;
 }
 } 
 
 
 
 
 
 
 
 
 );
 
 
 

102. Environment public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element
 }
 return false;
 }
 } 
 
 
 
 
 
 
 
 
 );
 
 
 

103. Environment public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println( );
 }
 return false;
 }
 } 
 
 
 
 
 
 
 
 
 element.asType()
 
 
 

104. Environment public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element.asType());
 }
 return false;
 }
 }
105. Environment public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element.asType());
 }
 return false;
 }
 }
106. Environment public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element.asType());
 }
 return false;
 }
 }
107. Round Environment
108. Round Environment public interface RoundEnvironment {
 boolean processingOver();
 
 boolean errorRaised();
 
 Set<? extends Element> getRootElements();
 
 Set<? extends Element> getElementsAnnotatedWith(
 TypeElement a);
 
 Set<? extends Element> getElementsAnnotatedWith(
 Class<? extends Annotation> a);
 }
109. Round Environment public interface RoundEnvironment {
 boolean processingOver();
 
 boolean errorRaised();
 
 Set<? extends Element> getRootElements();
 
 Set<? extends Element> getElementsAnnotatedWith(
 TypeElement a);
 
 Set<? extends Element> getElementsAnnotatedWith(
 Class<? extends Annotation> a);
 }
110. Processing Rounds
111. Processing Rounds @Example
 public class RandomClass {
 // ...
 }
112. Processing Rounds @Example
 public class RandomClass {
 // ...
 } javac
113. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac
114. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac javac
115. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Example
 public class RandomClass {
 // ...
 } javac
116. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Example
 public class RandomClass {
 // ...
 } javac ExampleProcessor
117. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 } javac ExampleProcessor
118. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 } javac OtherProcessor
119. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 } javac
120. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 } javac
121. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 }
122. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 } javac
123. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } @Example
 public class RandomClass {
 // ...
 } javac
124. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac
125. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac ExampleProcessor
126. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac OtherProcessor
127. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac OtherProcessor public class OtherGenClass {
 // ...
 }
128. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 }
129. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 }
130. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 }
131. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 } OtherGenClass.class public class OtherGenClass {
 // ...
 }
132. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 } ExampleProcessor OtherGenClass.class public class OtherGenClass {
 // ...
 }
133. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 } OtherProcessor OtherGenClass.class public class OtherGenClass {
 // ...
 }
134. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 } OtherGenClass.class public class OtherGenClass {
 // ...
 }
135. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } javac public class OtherGenClass {
 // ...
 } OtherGenClass.class public class OtherGenClass {
 // ...
 }
136. Processing Rounds @Example
 public class RandomClass {
 // ...
 } ExampleProcessor OtherProcessor javac RandomClass.class out/ @Other
 public class GeneratedClass {
 // ...
 } GeneratedClass.class @Example
 public class RandomClass {
 // ...
 } @Other
 public class GeneratedClass {
 // ...
 } public class OtherGenClass {
 // ...
 } OtherGenClass.class public class OtherGenClass {
 // ...
 }
138. JavaCompiler
139. ExampleProcessor JavaCompiler OtherProcessor
140. ProcessingEnvironment ExampleProcessor JavaCompiler Filer OtherProcessor
141. ProcessingEnvironment ExampleProcessor JavaCompiler Filer OtherProcessor init() init()
142. ProcessingEnvironment ExampleProcessor JavaCompiler Filer OtherProcessor init() init()
143. ProcessingEnvironment ExampleProcessor JavaCompiler Filer OtherProcessor init() init()
144. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() init()
145. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init()
146. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init()
147. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init()
148. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init()
149. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init()
150. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init()
151. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init() process()
152. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init() process()
153. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init() process()
154. ProcessingEnvironment ExampleProcessor JavaCompiler RoundEnvironment Filer OtherProcessor init() process() init() process()
155. Parsing and Generation
156. Parsing and Generation public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 
 for (Element element : elements) {
 
 
 }
 return false;
 }
 }
157. Parsing and Generation public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 
 for (Element element : elements) {
 
 
 }
 return false;
 }
 } 
 
 
 
 
 
 
 
 
 // Look at attributes of 'element'.
 
 
 
 

158. Parsing and Generation public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 
 for (Element element : elements) {
 
 
 }
 return false;
 }
 } 
 
 
 
 
 
 
 
 
 // Look at attributes of 'element'.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 // Generate some code based on attributes.
 
 
 

159. Parsing and Generation
160. Parsing and Generation @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 
 
 
 
 return false;
 }

161. Parsing and Generation @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 
 
 
 
 return false;
 }
 
 
 
 List<ExampleModel> models = parseExampleAnnotations(env);
162. Parsing and Generation @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 
 
 
 
 return false;
 }
 
 
 
 List<ExampleModel> models = parseExampleAnnotations(env); 
 
 
 
 
 
 
 
 
 
 List<ExampleModel> parseExampleAnnotations(RoundEnvironment env) {
 // ...
 }
163. Parsing and Generation @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 
 
 
 
 return false;
 }
 
 
 
 List<ExampleModel> models = parseExampleAnnotations(env); 
 
 
 
 for (ExampleModel model : models) {
 emitExampleCode(model);
 } 
 
 
 
 
 
 
 
 
 
 List<ExampleModel> parseExampleAnnotations(RoundEnvironment env) {
 // ...
 }
164. Parsing and Generation @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 
 
 
 
 return false;
 }
 
 
 
 List<ExampleModel> models = parseExampleAnnotations(env); 
 
 
 
 for (ExampleModel model : models) {
 emitExampleCode(model);
 } 
 
 
 
 
 
 
 
 
 
 List<ExampleModel> parseExampleAnnotations(RoundEnvironment env) {
 // ...
 } 
 
 
 
 
 
 
 
 
 ! ! 
 
 void emitExampleCode(ExampleModel model) {
 // ...
 }
165. Parsing and Generation
166. Parsing and Generation Element
167. Parsing and Generation Element
168. Parsing and Generation Element ExampleModel ! name: “Foo” package: “com.example” public: true
169. Parsing and Generation Element ExampleModel ! name: “Foo” package: “com.example” public: true
170. Parsing and Generation Element ExampleModel ! name: “Foo” package: “com.example” public: true ExampleModel ! name: “Foo” package: “com.example” public: true element:
171. Parsing and Generation Element ExampleModel ! name: “Foo” package: “com.example” public: true ExampleModel ! name: “Foo” package: “com.example” public: true element: GeneratedClass
172. Parsing and Generation Element ExampleModel ! name: “Foo” package: “com.example” public: true ExampleModel ! name: “Foo” package: “com.example” public: true element: GeneratedClass
173. Bring Your Own Dependencies
174. Bring Your Own Dependencies • Guava, Guava, Guava!
175. Bring Your Own Dependencies • Guava, Guava, Guava! • JavaWriter
176. Bring Your Own Dependencies • Guava, Guava, Guava! • JavaWriter • google/auto ‘common’
177. Bring Your Own Dependencies • Guava, Guava, Guava! • JavaWriter • google/auto ‘common’ • AutoService (google/auto)
178. JavaWriter
179. JavaWriter JavaFileObject sourceFile = filer.createSourceFile(name, element);
180. JavaWriter JavaFileObject sourceFile = filer.createSourceFile(name, element); ! ! ! JavaWriter writer = new JavaWriter(sourceFile.openWriter());
 writer.emitPackage("com.example")
 .emitAnnotation(Other.class)
 .beginType("GeneratedClass", "class")
 .emitSingleLineComment("...")
 .endType();
181. JavaWriter JavaFileObject sourceFile = filer.createSourceFile(name, element); ! ! @com.example.Other
 public class GeneratedClass {
 // ...
 } ! ! ! JavaWriter writer = new JavaWriter(sourceFile.openWriter());
 writer.emitPackage("com.example")
 .emitAnnotation(Other.class)
 .beginType("GeneratedClass", "class")
 .emitSingleLineComment("...")
 .endType();
182. JavaWriter JavaFileObject sourceFile = filer.createSourceFile(name, element); import com.example.Other; ! @Other
 public class GeneratedClass {
 // ...
 } ! ! ! JavaWriter writer = new JavaWriter(sourceFile.openWriter());
 writer.emitPackage("com.example")
 .emitImports(Other.class)
 .emitAnnotation(Other.class)
 .beginType("GeneratedClass", "class")
 .emitSingleLineComment("...")
 .endType(); JavaFileObject sourceFile = filer.createSourceFile(name, element);
183. AutoService
184. AutoService • Remember?
 META-­‐INF/services/javax.annotation.processing.Processor
185. AutoService • Remember?
 META-­‐INF/services/javax.annotation.processing.Processor • AutoService generates service files from annotations
186. AutoService • Remember?
 META-­‐INF/services/javax.annotation.processing.Processor • AutoService generates service files from annotations ! public class ExampleProcessor extends AbstractProcessor { // ... }
187. AutoService • Remember?
 META-­‐INF/services/javax.annotation.processing.Processor • AutoService generates service files from annotations ! public class ExampleProcessor extends AbstractProcessor { // ... } @AutoService(Processor.class)
188. Testing and Debugging
189. Testing and Debugging • Google to the rescue!
190. Testing and Debugging • Google to the rescue! • Guava
191. Testing and Debugging • Google to the rescue! • Guava • Guava
192. Testing and Debugging • Google to the rescue! • Guava • Guava • compile-testing
193. Testing and Debugging • Google to the rescue! • Guava • Guava • compile-testing • Truth
194. final class TestProcessors {
 static Iterable<? extends Processor> exampleProcessors() {
 return Collections.singletonList(
 new ExampleProcessor()
 );
 }
 }
195. String input = Joiner.on('\n').join(
 "package test;",
 "import com.example.Example;",
 "@Example",
 "public class ExampleClass {",
 "}"
 );
196. String input = Joiner.on('\n').join(
 "package test;",
 "import com.example.Example;",
 "@Example",
 "public class ExampleClass {",
 "}"
 ); JavaFileObject inputFile =
 JavaFileObjects.forSourceString("test.Test", input);
197. JavaFileObject inputFile =
 JavaFileObjects.forSourceString("test.Test", input); String input = """\
 package test; 
 import com.example.Example; 
 @Example
 public class ExampleClass {
 } """ .stripIndent()
198. String expected = Joiner.on('\n').join(
 "package test;",
 "import com.example.Other;",
 "@Other",
 "public class GeneratedClass {",
 "}"
 );
199. String expected = Joiner.on('\n').join(
 "package test;",
 "import com.example.Other;",
 "@Other",
 "public class GeneratedClass {",
 "}"
 ); JavaFileObject expectedFile =
 JavaFileObjects.forSourceString("test.GeneratedClass", expected);
200. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile);
201. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile);
202. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile);
203. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile); String input = Joiner.on('\n').join(
 "package test;",
 "import com.example.Example;",
 "@Example",
 "public class ExampleClass {",
 "}"
 );
204. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile); String input = Joiner.on('\n').join(
 "package test;",
 "import com.example.Example;",
 "@Example",
 "public class ExampleClass {",
 "}"
 ); final class TestProcessors {
 static Iterable<? extends Processor> exampleProcessors() {
 return Collections.singletonList(
 new ExampleProcessor()
 );
 }
 }
205. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile); String input = Joiner.on('\n').join(
 "package test;",
 "import com.example.Example;",
 "@Example",
 "public class ExampleClass {",
 "}"
 ); final class TestProcessors {
 static Iterable<? extends Processor> exampleProcessors() {
 return Collections.singletonList(
 new ExampleProcessor()
 );
 }
 } String expected = Joiner.on('\n').join(
 "package test;",
 "import com.example.Other;",
 "@Other",
 "public class GeneratedClass {",
 "}"
 );
206. ASSERT.about(javaSource())
 .that(inputFile)
 .processedWith(exampleProcessors())
 .compilesWithoutError()
 .and()
 .generatesSources(expectedFile); String input = Joiner.on('\n').join(
 "package test;",
 "import com.example.Example;",
 "@Example",
 "public class ExampleClass {",
 "}"
 ); final class TestProcessors {
 static Iterable<? extends Processor> exampleProcessors() {
 return Collections.singletonList(
 new ExampleProcessor()
 );
 }
 } String expected = Joiner.on('\n').join(
 "package test;",
 "import com.example.Other;",
 "@Other",
 "public class GeneratedClass {",
 "}"
 );
207. Debugging
208. public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element
 }
 return false;
 }
 } Debugging
209. public class ExampleProcessor extends AbstractProcessor {
 // ...
 
 @Override public boolean process(
 Set<? extends TypeElement> annotations,
 RoundEnvironment env) {
 Set<? extends Element> elements
 = env.getElementsAnnotatedWith(Example.class);
 for (Element element : elements) {
 System.out.println(element
 }
 return false;
 }
 } Debugging
210. Now What?
212. Questions?