Annotation Processing 101

Writing Java application can be annoying because the Java programming language requires to write boilerplate code. Writing Android apps makes no difference. Moreover, on Android you have to write a lot of code for doing simple things like setting up a Fragment with arguments, implement the Parcelable interface, define ViewHolder classes for RecyclerViews and so on.

Fortunately, the writing of boilerplate code can be reduced by using Annotation processing, a feature of the Java compiler that allows to setup hooks to generate code based on annotations.

The aim of the talk is to understand the annotation processing fundamentals and to be able to write an annotation processor. Furthermore, existing annotation processors for Android will be showcased.

Video Link: https://www.youtube.com/watch?v=43FFfTyDYEg

Transcript

1. Annotation Processing 101
   Hannes Dorfmann
   @sockeqwe
   +HannesDorfmann
   hannesdorfmann.com
   

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2. Annotation Processing
   ● Java 5 (Sep. 2004)
   ● JSR 269: 6 months after Java 5 released
   ● Plugin system for Annotation Processors
   

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3. Annotation Processing
   ● Part of javac
   ● @Annotation based
   ● Generates java source code files
   

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4. Reduces writing
   boilerplate code
   

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5. public class MyActivity extends Activity {
   @InjectView(R.id.title) TextView title;
   @InjectView(R.id.text) TextView text;
   @InjectView(R.id.submit) Button submit;
   @InjectView(R.id.imageView) ImageView image;
   }
   Butter Knife
   http://jakewharton.github.io/butterknife/
   

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6. public class MyActivity extends Activity {
   @Icicle String username;
   @Icicle int counter;
   }
   Icepick
   https://github.com/frankiesardo/icepick
   

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7. public class ItemFragment extends Fragment {
   @Arg long itemId;
   @Arg int categoryId;
   }
   FragmentArgs
   https://github.com/sockeqwe/fragmentargs
   

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8. @Parcel
   public class Person {
   String firstname;
   String lastname;
   int age;
   }
   Parceler
   http://parceler.org
   

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9. public class AnimalsAdapter extends SupportAnnotatedAdapter
   implements AnimalsAdapterBinder {
   @ViewType(layout = R.layout.list_dog)
   public final int dog = 0;
   @ViewType(layout = R.layout.list_cat)
   public final int cat = 1;
   AnnotatedAdapter
   

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10. public class AnimalsAdapter extends SupportAnnotatedAdapter
    implements AnimalsAdapterBinder {
    @Override
    public void bindViewHolder(DogViewHolder vh, int position) {
    Dog dog = (Dog) animals.get(position);
    vh.name.setText(dog.getName());
    }
    AnnotatedAdapter
    

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11. public class AnimalsAdapter extends SupportAnnotatedAdapter
    implements AnimalsAdapterBinder {
    @Override
    public void bindViewHolder(CatViewHolder vh, int position) {
    Cat cat = (Cat) animals.get(position);
    vh.image.setImageBitmap(cat.getImage());
    }
    }
    AnnotatedAdapter
    https://github.com/sockeqwe/AnnotatedAdapter
    

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12. Performance
    

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13. Performance
    ● Compile time code generation
    ● Full JVM
    ● “Native” java code (as hand-written)
    ● No reflections!
    

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14. LoganSquare
    https://github.com/bluelinelabs/LoganSquare
    Parsing 60 items
    68 ms
    Gson
    Jackson
    LoganSquare
    76 ms
    Serializing 60 items
    74 ms
    60 ms
    

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15. LoganSquare
    https://github.com/bluelinelabs/LoganSquare
    Parsing 60 items
    68 ms
    Gson
    Jackson
    LoganSquare
    76 ms
    17
    Serializing 60 items
    74 ms
    60 ms
    10
    

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16. Dagger 2
    ● Google Scale
    ● Replacing Dagger 1 with Dagger 2
    ● Saving 13 % CPU by just moving DI
    https://www.youtube.com/watch?v=oK_XtfXPkqw (36:00)
    

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17. Otto 1.3.7
    https://github.com/greenrobot/EventBus
    EventBus 2.4
    2000
    4000
    18000
    16000
    8000
    12000
    EventBus 3.0
    

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18. Otto 1.3.7
    https://github.com/greenrobot/EventBus
    EventBus 2.4 EventBus 3.0
    2000
    4000
    18000
    16000
    8000
    12000
    EventBus 3.0
    

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19. Limitation
    ● Can generate only new files
    ● Can’t manipulate already existing files
    ○ Bytecode manipulation (ASM, Afterburn)
    ○ AST manipulation (Project Lombok)
    

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20. Not debuggable!
    

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21. Example
    Pizza Store
    

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22. public interface Meal {
    public float getPrice();
    }
    public class MargheritaPizza implements Meal {
    @Override public float getPrice() {
    return 6f;
    }
    }
    

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23. public class CalzonePizza implements Meal {
    @Override public float getPrice() {
    return 8.5f;
    }
    }
    public class Tiramisu implements Meal {
    @Override public float getPrice() {
    return 4.5f;
    }
    }
    

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24. public class PizzaStore {
    private MealFactory factory = new MealFactory();
    public static void main(String[] args) throws IOException {
    PizzaStore pizzaStore = new PizzaStore();
    Meal meal = pizzaStore.order( readConsole() );
    System.out.println("Bill: $" + meal.getPrice());
    }
    public Meal order(String mealName) {
    return factory.create(mealName);
    }
    }
    

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25. public class MealFactory {
    public Meal create(String id) {
    if ("Margherita".equals(id)) return new MargheritaPizza();
    if ("Calzone".equals(id)) return new CalzonePizza();
    if ("Tiramisu".equals(id)) return new Tiramisu();
    throw new IllegalArgumentException("Unknown meal”);
    }
    }
    

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26. @Target(ElementType.TYPE)
    public @interface Factory {
    String id();
    Class type();
    }
    

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27. @Factory(
    id = "Margherita",
    type = Meal.class
    )
    public class MargheritaPizza implements Meal {
    @Override
    public float getPrice() {
    return 6f;
    }
    }
    

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28. @Factory(
    id = "Calzone",
    type = Meal.class
    )
    public class CalzonePizza implements Meal {
    @Override
    public float getPrice() {
    return 8.5f;
    }
    }
    

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29. Pizza Store Factory
    1. Only classes can be @Factory
    

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30. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    

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31. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    

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32. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    4. id is String and must be unique in factory group
    

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33. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    4. id is String and must be unique in factory group
    5. @Factory classes must inherit from specified type
    

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34. @Factory(
    id = "Margherita",
    type = Meal.class
    )
    public class MargheritaPizza implements Meal {
    @Override
    public float getPrice() {
    return 6f;
    }
    }
    

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35. @Factory(
    id = "Margherita",
    type = Meal.class
    )
    public class MargheritaPizza implements Meal {
    @Override
    public float getPrice() {
    return 6f;
    }
    }
    

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36. public class FactoryProcessor extends AbstractProcessor {
    private Types typeUtils;
    private Elements elementUtils;
    private Filer filer;
    private Messager messager;
    private Map factoryClasses;
    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
    typeUtils = processingEnv.getTypeUtils();
    elementUtils = processingEnv.getElementUtils();
    filer = processingEnv.getFiler();
    messager = processingEnv.getMessager();
    }
    

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37. public class FactoryProcessor extends AbstractProcessor {
    @Override
    public Set getSupportedAnnotationTypes() {
    Set annotations = new LinkedHashSet();
    annotations.add(Factory.class.getCanonicalName());
    return annotations;
    }
    

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38. public class FactoryProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set extends TypeElement> annotations,
    RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    }
    }
    }
    

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39. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    

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40. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    

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41. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    

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42. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    VariableElement
    

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43. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    VariableElement
    ExecuteableElement
    

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44. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    VariableElement
    ExecuteableElement
    ExecuteableElement
    

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45. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    VariableElement
    ExecuteableElement
    ExecuteableElement
    TypeElement
    

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46. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    VariableElement
    ExecuteableElement
    ExecuteableElement
    TypeElement
    

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47. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeElement
    VariableElement
    VariableElement
    ExecuteableElement
    ExecuteableElement
    TypeElement
    

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48. @Example
    public class Foo {
    private int a;
    private Other other;
    public Foo () { ... }
    public void setA (
    int newA
    ) { ... }
    }
    TypeMirror
    

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49. public class FactoryProcessor extends AbstractProcessor {
    @Override
    public synchronized void init(ProcessingEnvironment processingEnv);
    @Override
    public Set getSupportedAnnotationTypes();
    @Override
    public boolean process(Set extends TypeElement> annotations,
    RoundEnvironment rv);
    }
    

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50. FactoryProcessor.jar
    - com
    - example
    - FactoryProcessor.class
    - META-INF
    - services
    - javax.annotation.processing.Processor
    

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51. @AutoService(Processor.class)
    public class FactoryProcessor extends AbstractProcessor {
    @Override
    public synchronized void init(ProcessingEnvironment processingEnv);
    @Override
    public Set getSupportedAnnotationTypes();
    @Override
    public boolean process(Set extends TypeElement> annotations,
    RoundEnvironment rv);
    }
    

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52. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    4. id is String and must be unique in factory group
    5. @Factory classes must inherit from specified type
    

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53. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (! (annotatedElement instanceof TypeElement) ){
    ...
    }
    }
    }
    

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54. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (! (annotatedElement instanceof TypeElement) ){
    ...
    }
    }
    }
    

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55. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (annotatedElement.getKind() != ElementKind.CLASS) {
    messager.printMessage(
    Diagnostic.Kind.ERROR,
    "Only classes can be annotated with @Factory",
    annotatedElement );
    }
    }
    }
    

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56. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (annotatedElement.getKind() != ElementKind.CLASS) {
    error( annotatedElement, "Only classes can be annotated with @Factory");
    }
    }
    }
    public void error(Element e, String msg) {
    messager.printMessage(Diagnostic.Kind.ERROR, "Error", e);
    }
    

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57. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (annotatedElement.getKind() != ElementKind.CLASS) {
    error( annotatedElement, "Only classes can be annotated with @Factory");
    annotatedElement = null;
    }
    annotatedElement.getSimpleName();
    }
    }
    

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58. Caused by: java.lang.NullPointerException
    at com.example.processor.FactoryProcessor.process(FactoryProcessor.java:161)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor
    (JavacProcessingEnvironment.java:793)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.
    discoverAndRunProcs(JavacProcessingEnvironment.java:722)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.access$1700
    (JavacProcessingEnvironment.java:97)
    ... 32 more
    

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59. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (annotatedElement.getKind() != ElementKind.CLASS) {
    error( annotatedElement, "Only classes can be annotated with @Factory");
    annotatedElement = null;
    }
    annotatedElement.getSimpleName();
    }
    }
    

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60. public boolean process(Set<> annotations, RoundEnvironment rv) {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (annotatedElement.getKind() != ElementKind.CLASS) {
    error( annotatedElement, "Only classes can be annotated with @Factory");
    }
    checkOtherThings( annotatedElement );
    }
    }
    public void checkOtherThings(Element element) { ... }
    

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61. public boolean process(Set<> annotations, RoundEnvironment rv) {
    try {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    if (annotatedElement.getKind() != ElementKind.CLASS) {
    error( annotatedElement, "Only classes can be annotated with @Factory");
    }
    checkOtherThings( annotatedElement );
    }
    } catch (ProcessingException e) {
    error(e.getElement(), e.getMessage());
    }
    }
    

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62. public void checkOtherThings(Element element) throws ProcessingException
    

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63. public void checkOtherThings(Element element) throws ProcessingException
    public class ProcessingException extends Exception {
    Element element;
    public ProcessingException(Element element, String msg, Object... args) {
    super(String.format(msg, args));
    this.element = element;
    }
    public Element getElement() {
    return element;
    }
    }
    

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64. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    4. id is String and must be unique in factory group
    5. @Factory classes must inherit from specified type
    

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65. public void checkConstructor(TypeElement e) throws ProcessingException {
    for (Element enclosed : e.getEnclosedElements()) {
    if (enclosed.getKind() == ElementKind.CONSTRUCTOR) {
    ExecutableElement constructorElement = (ExecutableElement) enclosed;
    if (constructorElement.getParameters().size() == 0 &&
    constructorElement.getModifiers().contains(Modifier.PUBLIC)) {
    return;
    }
    }
    throw new ProcessingException(e, "No public empty constructor");
    }
    

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66. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    4. id is String and must be unique in factory group
    5. @Factory classes must inherit from specified type
    

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67. public class FactoryAnnotatedClass {
    TypeElement annotatedClassElement;
    String id;
    String qualifiedGroupClassName;
    public FactoryAnnotatedClass(TypeElement element) throws ProcessingException {
    this.annotatedClassElement = classElement;
    }
    }
    

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68. public FactoryAnnotatedClass(TypeElement element) throws ProcessingException {
    Factory annotation = classElement.getAnnotation(Factory.class);
    this.id = annotation.id();
    if (StringUtils.isEmpty(id)) {
    throw new ProcessingException(classElement,
    "id() is null or empty!);
    }
    }
    

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69. public FactoryAnnotatedClass(TypeElement element) throws ProcessingException {
    Factory annotation = classElement.getAnnotation(Factory.class);
    try {
    Class> clazz = annotation.type();
    qualifiedGroupClassName = clazz.getCanonicalName();
    } catch (MirroredTypeException mte) {
    }
    }
    

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70. public FactoryAnnotatedClass(TypeElement element) throws ProcessingException {
    Factory annotation = classElement.getAnnotation(Factory.class);
    try {
    Class> clazz = annotation.type();
    qualifiedGroupClassName = clazz.getCanonicalName();
    } catch (MirroredTypeException mte) {
    DeclaredType classTypeMirror = (DeclaredType) mte.getTypeMirror();
    TypeElement classTypeElement = (TypeElement) classTypeMirror.asElement();
    qualifiedGroupClassName = classTypeElement.getQualifiedName().toString();
    }
    }
    

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71. public class FactoryGroupedClasses {
    String qualifiedClassName;
    Map itemsMap =
    new LinkedHashMap();
    public FactoryGroupedClasses(String qualifiedClassName) {
    this.qualifiedClassName = qualifiedClassName;
    }
    }
    

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72. public class FactoryGroupedClasses {
    public void add(FactoryAnnotatedClass toInsert) throws ProcessingException {
    }
    }
    

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73. public class FactoryGroupedClasses {
    public void add(FactoryAnnotatedClass toInsert) throws ProcessingException {
    FactoryAnnotatedClass existing = itemsMap.get(toInsert.getId());
    }
    }
    

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74. public class FactoryGroupedClasses {
    public void add(FactoryAnnotatedClass toInsert) throws ProcessingException {
    FactoryAnnotatedClass existing = itemsMap.get(toInsert.getId());
    if (existing != null) {
    throw new ProcessingException(toInsert.getTypeElement(),
    "Conflict: Class with same id found“ );
    }
    }
    }
    

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75. public class FactoryGroupedClasses {
    public void add(FactoryAnnotatedClass toInsert) throws ProcessingException {
    FactoryAnnotatedClass existing = itemsMap.get(toInsert.getId());
    if (existing != null) {
    throw new ProcessingException(toInsert.getTypeElement(),
    "Conflict: Class with same id found“ );
    }
    itemsMap.put(toInsert.getId(), toInsert);
    }
    }
    

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76. FactoryProcessor
    Map
    FactoryGroupedClasses
    Map
    FactoryAnnotatedClass
    String id
    

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77. public boolean process(Set<> annotations, RoundEnvironment rv) {
    try {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    }
    } catch (ProcessingException e) { ... }
    }
    

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78. public boolean process(Set<> annotations, RoundEnvironment rv) {
    try {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    FactoryAnnotatedClass annotatedClass =
    new FactoryAnnotatedClass( annotatedElement );
    }
    } catch (ProcessingException e) { ... }
    }
    

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79. public boolean process(Set<> annotations, RoundEnvironment rv) {
    try {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    FactoryAnnotatedClass annotatedClass =
    new FactoryAnnotatedClass( annotatedElement );
    FactoryGroupedClasses factoryClass =
    getOrCreateFactoryClass( annotatedClass.getQualifiedGroupName() );
    }
    } catch (ProcessingException e) { ... }
    }
    

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80. public boolean process(Set<> annotations, RoundEnvironment rv) {
    try {
    for (Element annotatedElement : rv.getElementsAnnotatedWith(Factory.class)) {
    FactoryAnnotatedClass annotatedClass =
    new FactoryAnnotatedClass( annotatedElement );
    FactoryGroupedClasses factoryClass =
    getOrCreateFactoryClass( annotatedClass.getQualifiedGroupName());
    factoryClass.add(annotatedClass);
    }
    } catch (ProcessingException e) { ... }
    }
    

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81. Pizza Store Factory
    1. Only classes can be @Factory
    2. @Factory classes must have empty constructor
    3. Same types are grouped to one factory
    4. id is String and must be unique in factory group
    5. @Factory classes must inherit from specified type
    

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82. public void checkInheritance(FactoryAnnotatedClass ac) throws ProcessingException {
    TypeElement currentClass = ac.getTypeElement();
    }
    

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83. public void checkInheritance(FactoryAnnotatedClass ac) throws ProcessingException {
    TypeElement currentClass = ac.getTypeElement();
    while (true) {
    TypeMirror superClassType = currentClass.getSuperclass();
    currentClass = (TypeElement) typeUtils.asElement(superClassType);
    }
    }
    

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84. public void checkInheritance(FactoryAnnotatedClass ac) throws ProcessingException {
    TypeElement currentClass = ac.getTypeElement();
    while (true) {
    TypeMirror superClassType = currentClass.getSuperclass();
    if (superClassType.getKind() == TypeKind.NONE)
    throw new ProcessingException( classElement, "Class doesn’t inherit Meal” );
    currentClass = (TypeElement) typeUtils.asElement(superClassType);
    }
    }
    

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85. public void checkInheritance(FactoryAnnotatedClass ac) throws ProcessingException {
    TypeElement currentClass = ac.getTypeElement();
    while (true) {
    TypeMirror superClassType = currentClass.getSuperclass();
    if (superClassType.getKind() == TypeKind.NONE)
    throw new ProcessingException( classElement, "Class doesn’t inherit Meal” );
    if (superClassType.toString().equals( ac.getQualifiedFactoryGroupName() ) )
    return;
    currentClass = (TypeElement) typeUtils.asElement(superClassType);
    }
    }
    

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86. Code generation
    

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87. private Map factoryClasses;
    public boolean process(Set<> annotations, RoundEnvironment rv) {
    try {
    ...
    } catch (ProcessingException e) { ... }
    for (FactoryGroupedClasses factoryClass : factoryClasses.values()) {
    factoryClass.generateCode(elementUtils, filer);
    }
    }
    

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88. public void generateCode(Elements elementUtils, Filer filer) throws IOException {
    MethodSpec.Builder method = MethodSpec.methodBuilder("create")
    .addModifiers(Modifier.PUBLIC)
    .addParameter(String.class, "id")
    .returns(TypeName.get(superClassName.asType()));
    }
    

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89. public Meal create(String id) {
    }
    

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90. public void generateCode(Elements elementUtils, Filer filer) throws IOException {
    MethodSpec.Builder method = MethodSpec.methodBuilder("create")
    ...
    for (FactoryAnnotatedClass item : itemsMap.values()) {
    method
    .beginControlFlow("if ($S.equals(id))", item.getId())
    .addStatement("return new $L()",
    item.getTypeElement().getQualifiedName().toString())
    .endControlFlow();
    }
    }
    

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91. public Meal create(String id) {
    if ("Margherita".equals(id))
    return new MargheritaPizza();
    if ("Calzone".equals(id))
    return new CalzonePizza();
    if ("Tiramisu".equals(id))
    return new Tiramisu();
    }
    

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92. public void generateCode(Elements elementUtils, Filer filer) throws IOException {
    MethodSpec.Builder method = MethodSpec.methodBuilder("create")
    ...
    method.addStatement(
    "throw new IllegalArgumentException($S + id)",
    "Unknown id = "
    );
    }
    

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93. public Meal create(String id) {
    if ("Margherita".equals(id))
    return new MargheritaPizza();
    if ("Calzone".equals(id))
    return new CalzonePizza();
    if ("Tiramisu".equals(id))
    return new Tiramisu();
    throw new IllegalArgumentException( "Unknown id = ” +id );
    }
    

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94. public void generateCode(Elements elementUtils, Filer filer) throws IOException {
    MethodSpec.Builder method = MethodSpec.methodBuilder("create")
    ...
    TypeSpec typeSpec =
    TypeSpec.classBuilder( factoryClassName )
    .addMethod( method.build() ).build();
    // Write MealFactory.java
    JavaFile.builder(packageName, typeSpec).build().writeTo(filer);
    }
    

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95. public class MealFactory {
    public Meal create(String id) {
    if ("Margherita".equals(id))
    return new MargheritaPizza();
    if ("Calzone".equals(id))
    return new CalzonePizza();
    if ("Tiramisu".equals(id))
    return new Tiramisu();
    throw new IllegalArgumentException( "Unknown id = ” +id );
    }
    }
    

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96. Attempt to recreate a file for type
    com.example.MealFactory
    

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97. Processing Rounds
    Round Input Output
    1. PizzaStore.java
    Meal.java
    CalzonePizza.java
    Tiramisu.java
    Magerita.java
    MealFactory.java
    

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98. Processing Rounds
    Round Input Output
    1. PizzaStore.java
    Meal.java
    CalzonePizza.java
    Tiramisu.java
    Magerita.java
    MealFactory.java
    2. MealFactory.java -- none --
    

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99. Processing Rounds
    Round Input Output
    1. PizzaStore.java
    Meal.java
    CalzonePizza.java
    Tiramisu.java
    Magerita.java
    MealFactory.java
    2. MealFactory.java -- none --
    3. -- none -- -- none --
    

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100. public class FactoryProcessor extends AbstractProcessor {
     private Map factoryClasses;
     public boolean process(Set<> annotations, RoundEnvironment rv) {
     try {
     // scan annotations
     } catch (ProcessingException e) { ... }
     for (FactoryGroupedClasses factoryClass : factoryClasses.values()) {
     factoryClass.generateCode(elementUtils, filer);
     }
     }
     }
     

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101. public class FactoryProcessor extends AbstractProcessor {
     private Map factoryClasses;
     public boolean process(Set<> annotations, RoundEnvironment rv) {
     try {
     // scan annotations
     } catch (ProcessingException e) { ... }
     for (FactoryGroupedClasses factoryClass : factoryClasses.values()) {
     factoryClass.generateCode(elementUtils, filer);
     }
     }
     }
     

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102. public class FactoryProcessor extends AbstractProcessor {
     private Map factoryClasses;
     public boolean process(Set<> annotations, RoundEnvironment rv) {
     try {
     // scan annotations
     } catch (ProcessingException e) { ... }
     for (FactoryGroupedClasses factoryClass : factoryClasses.values()) {
     factoryClass.generateCode(elementUtils, filer);
     }
     }
     }
     

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103. public class FactoryProcessor extends AbstractProcessor {
     private Map factoryClasses;
     public boolean process(Set<> annotations, RoundEnvironment rv) {
     try {
     // scan annotations
     } catch (ProcessingException e) { ... }
     for (FactoryGroupedClasses factoryClass : factoryClasses.values()) {
     factoryClass.generateCode(elementUtils, filer);
     }
     factoryClasses.clear();
     }
     }
     

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104. Instantiation
     

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105. public class PizzaStore {
     private MealFactory factory = new MealFactory();
     public static void main(String[] args) throws IOException {
     PizzaStore pizzaStore = new PizzaStore();
     Meal meal = pizzaStore.order( readConsole() );
     System.out.println("Bill: $" + meal.getPrice());
     }
     public Meal order(String mealName) {
     return factory.create(mealName);
     }
     }
     

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107. Instantiation using Reflections
     ButterKnife.inject(Activity);
     

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108. Instantiation using Reflections
     ButterKnife.inject(Activity);
     try {
     Class> injector = Class.forName(activityClassName + "$$ViewInjector");
     } catch (ClassNotFoundException e) { ... }
     

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109. We are hiring ;-)
     http://hannesdorfmann.com
     https://github.com/sockeqwe/annotationprocessing101
     

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