Dependency Injection Made Simple (360|AnDev)

Version of talk given at 360|AnDev.

Transcript

1. Dependency injection made simple
   Dan Lew
   

   View Slide

2. What is a
   dependency?
   

   View Slide

3. A depends on B
   

   View Slide

4. • Data provider depends on database
   • Image Loader depends on HTTP
   • REST depends on HTTP
   • REST depends on deserializer
   • Login screen depends on all the above
   

   View Slide

5. What is dependency
   injection?
   

   View Slide

6. "Dependency Injection" is
   a 25-dollar term for a
   5-cent concept.
   http://www.jamesshore.com/Blog/Dependency-Injection-Demystified.html
   ~James Shore
   

   View Slide

7. public class Example {

   private final Dependency dependency;

   

   public Example() {

   dependency = new Dependency();

   }

   }
   VS
   public class Example {

   private final Dependency dependency;

   

   public Example(Dependency dependency) {

   this.dependency = dependency;

   }

   }
   

   View Slide

8. public class Example {

   private final Dependency dependency;

   

   public Example() {

   dependency = new Dependency();

   }

   }
   VS
   public class Example {

   private final Dependency dependency;

   

   public Example(Dependency dependency) {

   this.dependency = dependency;

   }

   }
   

   View Slide

9. • Constructor injection
   public class Example {

   private final Dependency dependency;

   

   public Example(Dependency dependency) {

   this.dependency = dependency;

   }

   }
   • Method injection
   public class Example {

   private Dependency dependency;

   

   public Example() { }

   

   public void setDependency(Dependency dependency) {

   this.dependency = dependency;

   }

   }
   

   View Slide

10. View(Context context)
    

    View Slide

11. View Slide

12. But Why?
    • Share dependencies
    • Configure dependencies externally
    

    View Slide

13. Context wrappedContext =
    new ContextThemeWrapper(context, R.style.MyTheme);
    

    View view = new View(wrappedContext);
    

    View Slide

14. Dependency inversion
    principle
    • High-level modules should not depend on low-level
    modules. Both should depend on abstractions.
    • Abstractions should not depend on details. Details
    should depend on abstractions.
    

    View Slide

15. A depends on B
    

    View Slide

16. A is coupled to B
    

    View Slide

17. View Slide

18. View Slide

19. void copy() {

    char c;

    while ((c = readKeyboard()) != -1) {

    writePrinter(c);

    }

    }
    

    View Slide

20. What if…
    • …You want to read from disk?
    enum InputDevice {

    KEYBOARD, DISK

    }

    

    void copy(InputDevice input) {

    char c;

    while (true) {

    if (input == InputDevice.KEYBOARD) {

    c = readKeyboard();

    } else if (input == InputDevice.DISK) {

    c = readDisk();

    } else {

    throw new IllegalArgumentException("Whoops!");

    }

    

    if (c == -1) {

    return;

    }

    

    writePrinter(c);

    }

    }
    

    View Slide

21. What if…
    • …You want to write to the monitor?
    enum OutputDevice {

    PRINTER, MONITOR

    }

    

    void copy(OutputDevice outputDevice) {

    char c;

    while ((c = readKeyboard()) != -1) {

    if (outputDevice == OutputDevice.PRINTER) {

    writePrinter(c);

    } else if (outputDevice == OutputDevice.MONITOR) {

    writeMonitor(c);

    } else {

    throw new IllegalArgumentException("Whoops again!");

    }

    }

    }
    

    View Slide

22. • What do we test in the read / write modules?
    • How do we test copy?
    

    View Slide

23. Dependency inversion
    to the rescue
    

    View Slide

24. View Slide

25. interface Reader {

    char read();

    }

    

    interface Writer {

    void write(char c);

    }

    

    void copy(Reader reader, Writer writer) {

    char c;

    while ((c = reader.read()) != -1) {

    writer.write(c);

    }

    }
    

    View Slide

26. But Why?
    • Share dependencies
    • Configure dependencies externally
    

    View Slide

27. View Slide

28. But Why?
    • Share dependencies
    • Configure dependencies externally
    • Clean separation of modules
    

    View Slide

29. View Slide

30. But Why?
    • Share dependencies
    • Configure dependencies externally
    • Clean separation of modules
    • Easy testing
    

    View Slide

31. View Slide

32. @Test

    public void testCopy() {

    TestReader reader = new TestReader("Hello, world!");

    TestWriter writer = new TestWriter();

    copy(reader, writer);

    assertEquals("Hello, world!", writer.getWritten());

    }
    

    View Slide

33. But Why?
    • Share dependencies
    • Configure dependencies externally
    • Clean separation of modules
    • Easy testing
    • Easy debugging
    

    View Slide

34. …Why not?
    • More verbose
    • More complex
    

    View Slide

35. View Slide

36. Framework Advantages
    • Handles injection busywork
    • Handles logic (e.g. singletons, lazy loading)
    • Unified system for dependencies
    

    View Slide

37. Frameworks
    • Dagger 1
    • Dagger 2
    • Guice
    • Spring
    • PicoContainer
    • …And on and on and on…
    

    View Slide

38. Dagger
    Directed Acyclic Graph
    

    View Slide

39. Object Graph
    

    View Slide

40. Greeter
    

    View Slide

41. // Greeter.java
    public class Greeter {

    @Inject public Greeter() { }

    

    public void helloWorld() {

    System.out.println("Hello, world!");

    }

    }
    // MyModule.java
    @Module(injects = Greeter.class)

    public final class MyModule { }
    // Retrieving Greeter
    ObjectGraph objectGraph = ObjectGraph.create(new MyModule());
    Greeter greeter = objectGraph.get(Greeter.class);
    greeter.helloWorld();
    

    View Slide

42. // Greeter.java
    public class Greeter {

    @Inject public Greeter() { }

    

    public void helloWorld() {

    System.out.println("Hello, world!");

    }

    }
    // MyModule.java
    @Module(injects = Greeter.class)

    public final class MyModule { }
    // Retrieving Greeter
    ObjectGraph objectGraph = ObjectGraph.create(new MyModule());
    Greeter greeter = objectGraph.get(Greeter.class);
    greeter.helloWorld();
    

    View Slide

43. // Greeter.java
    public class Greeter {

    @Inject public Greeter() { }

    

    public void helloWorld() {

    System.out.println("Hello, world!");

    }

    }
    // MyModule.java
    @Module(injects = Greeter.class)

    public final class MyModule { }
    // Retrieving Greeter
    ObjectGraph objectGraph = ObjectGraph.create(new MyModule());
    Greeter greeter = objectGraph.get(Greeter.class);
    greeter.helloWorld();
    

    View Slide

44. // Greeter.java
    public class Greeter {

    @Inject public Greeter() { }

    

    public void helloWorld() {

    System.out.println("Hello, world!");

    }

    }
    // MyModule.java
    @Module(injects = Greeter.class)

    public final class MyModule { }
    // Retrieving Greeter
    ObjectGraph objectGraph = ObjectGraph.create(new MyModule());
    Greeter greeter = objectGraph.get(Greeter.class);
    greeter.helloWorld();
    

    View Slide

45. // Greeter.java
    public class Greeter {

    @Inject public Greeter() { }

    

    public void helloWorld() {

    System.out.println("Hello, world!");

    }

    }
    // MyModule.java
    @Module(injects = Greeter.class)

    public final class MyModule { }
    // Retrieving Greeter
    ObjectGraph objectGraph = ObjectGraph.create(new MyModule());
    Greeter greeter = objectGraph.get(Greeter.class);
    greeter.helloWorld();
    

    View Slide

46. Greeter
    Text
    

    View Slide

47. // Text.java

    public class Text {

    @Inject public Text() { }

    

    public String getText() {

    return "Hello, world!";

    }

    }

    // Greeter.java

    public class Greeter {

    private final Text text;

    

    @Inject public Greeter(Text text) {

    this.text = text;

    }

    

    public void sayText() {

    System.out.println(text.getText());

    }

    }
    

    View Slide

48. // Text.java

    public class Text {

    @Inject public Text() { }

    

    public String getText() {

    return "Hello, world!";

    }

    }

    // Greeter.java

    public class Greeter {

    private final Text text;

    

    @Inject public Greeter(Text text) {

    this.text = text;

    }

    

    public void sayText() {

    System.out.println(text.getText());

    }

    }
    

    View Slide

49. // Text.java

    public class Text {

    @Inject public Text() { }

    

    public String getText() {

    return "Hello, world!";

    }

    }

    // Greeter.java

    public class Greeter {

    private final Text text;

    

    @Inject public Greeter(Text text) {

    this.text = text;

    }

    

    public void sayText() {

    System.out.println(text.getText());

    }

    }
    

    View Slide

50. // Text.java

    public class Text {

    @Inject public Text() { }

    

    public String getText() {

    return "Hello, world!";

    }

    }

    // Greeter.java

    public class Greeter {

    private final Text text;

    

    @Inject public Greeter(Text text) {

    this.text = text;

    }

    

    public void sayText() {

    System.out.println(text.getText());

    }

    }
    

    View Slide

51. @Inject

    public ManyDependencies(

    Dep1 bulbasaur,

    Dep2 charmander,

    Dep3 squirtle,

    Dep4 caterpie,

    Dep5 pikachu,
    Dep6 slowpoke

    )
    

    View Slide

52. // Greeter.java

    public class Greeter {

    @Inject Text text;

    

    public void sayText() {

    System.out.println(text.getText());

    }

    }
    

    View Slide

53. // Greeter.java

    public class Greeter {

    @Inject Text text;

    

    public void sayText() {

    System.out.println(text.getText());

    }

    }
    

    View Slide

54. public class ManyDependencies {

    @Inject Dep1 eevee;

    @Inject Dep2 ditto;

    @Inject Dep3 snorlax;

    @Inject Dep4 articuno;

    @Inject Dep5 dragonite;

    @Inject Dep6 mew;

    }
    

    View Slide

55. Providers
    • Explicitly provide dependencies
    • Greater control over dependency
    • Can’t annotate dependency
    • Dependency inversion
    

    View Slide

56. Providers
    @Module(injects = Greeter.class)

    public final class MyModule {

    @Provides Text provideText() {

    return new Text("Hello, World!");

    }

    

    @Provides Greeter provideGreeter(Text text) {

    return new Greeter(text);

    }

    }
    

    View Slide

57. Providers
    @Module(injects = Greeter.class)

    public final class MyModule {

    @Provides Text provideText() {

    return new Text("Hello, World!");

    }

    

    @Provides Greeter provideGreeter(Text text) {

    return new Greeter(text);

    }

    }
    

    View Slide

58. Providers
    @Module(injects = Greeter.class)

    public final class MyModule {

    @Provides Text provideText() {

    return new Text("Hello, World!");

    }

    

    @Provides Greeter provideGreeter(Text text) {

    return new Greeter(text);

    }

    }
    

    View Slide

59. Providers
    @Module(injects = Greeter.class)

    public final class MyModule {

    @Provides Text provideText() {

    return new Text("Hello, World!");

    }

    

    @Provides Greeter provideGreeter(Text text) {

    return new Greeter(text);

    }

    }
    

    View Slide

60. Providing Interfaces
    

    View Slide

61. Providing Interfaces
    public interface Reader {

    char read();

    }

    public class KeyboardReader implements Reader {

    @Override

    public char read() { /* implementation */ }

    }

    @Module(injects = Reader.class)

    public final class MyModule {

    @Provides Reader provideReader() {

    return new KeyboardReader();

    }

    }
    

    View Slide

62. Providing Interfaces
    public interface Reader {

    char read();

    }

    public class KeyboardReader implements Reader {

    @Override

    public char read() { /* implementation */ }

    }

    @Module(injects = Reader.class)

    public final class MyModule {

    @Provides Reader provideReader() {

    return new KeyboardReader();

    }

    }
    

    View Slide

63. Providing Interfaces
    public interface Reader {

    char read();

    }

    public class KeyboardReader implements Reader {

    @Override

    public char read() { /* implementation */ }

    }

    @Module(injects = Reader.class)

    public final class MyModule {

    @Provides Reader provideReader() {

    return new KeyboardReader();

    }

    }
    

    View Slide

64. Providing Interfaces
    public interface Reader {

    char read();

    }

    public class KeyboardReader implements Reader {

    @Override

    public char read() { /* implementation */ }

    }

    @Module(injects = Reader.class)

    public final class MyModule {

    @Provides Reader provideReader() {

    return new KeyboardReader();

    }

    }
    

    View Slide

65. Providing Interfaces
    public interface Reader {

    char read();

    }

    public class KeyboardReader implements Reader {

    @Override

    public char read() { /* implementation */ }

    }

    @Module(injects = Reader.class)

    public final class MyModule {

    @Provides Reader provideReader() {

    return new KeyboardReader();

    }

    }
    

    View Slide

66. Single Module
    

    View Slide

67. Multiple Modules
    

    View Slide

68. Multiple Modules
    • Compile time
    @Module(includes = HttpModule.class)

    public final class RestModule { }
    • Runtime
    ObjectGraph.create(new RestModule(), new HttpModule());
    Warning!
    

    View Slide

69. View Slide

70. Unused!
    Incomplete!
    

    View Slide

71. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

72. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

73. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

74. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

75. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

76. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

77. // HttpModule.java
    @Module(library = true)

    public final class HttpModule {

    @Provides OkHttp provideOkHttp() {

    return new OkHttp();

    }

    }

    // RestModule.java
    @Module(complete = false)

    public final class RestModule {

    @Provides Retrofit provideRetrofit(OkHttp okHttp) {

    return new Retrofit(okHttp);

    }

    }
    

    View Slide

78. Other Dagger Features
    • Singletons
    @Provides @Singleton Greeter provideGreeter()
    • Lazy injecting
    @Inject Lazy text;
    • Qualifiers
    @Provides @Named("Greeter1") Greeter provideGreeter()
    

    View Slide

79. View Slide

80. Dagger + Gradle
    dependencies {

    provided 'com.squareup.dagger:dagger-compiler:1.2.2'

    compile 'com.squareup.dagger:dagger:1.2.2'

    }
    

    View Slide

81. Dagger + Gradle
    apply plugin: 'com.neenbedankt.android-apt'

    

    dependencies {

    apt 'com.squareup.dagger:dagger-compiler:1.2.2'

    compile 'com.squareup.dagger:dagger:1.2.2'

    }
    https://bitbucket.org/hvisser/android-apt
    

    View Slide

82. Dagger + ProGuard
    • Google ProGuard config
    • Use Dagger 2
    

    View Slide

83. Injecting Android
    • Activity, Service, Fragment, etc…
    • Problem: We don’t control construction!
    • Solution: ObjectGraph.inject()
    

    View Slide

84. Injecting Activities
    public class MyActivity extends Activity {

    

    @Inject Dependency dependency;

    

    @Override

    protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    ObjectGraph objectGraph =
    ((MyApplication) getApplication()).getObjectGraph();

    objectGraph.inject(this);

    

    setContentView(R.layout.main);

    }

    }
    

    View Slide

85. Injecting Activities
    public class MyActivity extends Activity {

    

    @Inject Dependency dependency;

    

    @Override

    protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    ObjectGraph objectGraph =
    ((MyApplication) getApplication()).getObjectGraph();

    objectGraph.inject(this);

    

    setContentView(R.layout.main);

    }

    }
    

    View Slide

86. Injecting Activities
    public class MyActivity extends Activity {

    

    @Inject Dependency dependency;

    

    @Override

    protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    ObjectGraph objectGraph =
    ((MyApplication) getApplication()).getObjectGraph();

    objectGraph.inject(this);

    

    setContentView(R.layout.main);

    }

    }
    

    View Slide

87. u2020
    • Unicode character for “dagger”: †
    • Injector pattern
    • https://github.com/JakeWharton/u2020
    

    View Slide

88. Thanks!
    @danlew42
    danlew.net
    

    View Slide