Dependency Injection Made Simple

Presented at DevFest MN 2016.

I'm not entirely sure these slides will make sense without me actually presenting...

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 the hell do we test copy?!
    

    View Slide

23. What if…
    • You want to change readKeyboard() or
    writePrinter()?
    void copy() {

    char c;

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

    writePrinter(c);

    }

    }
    

    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
    • Clean separation of modules
    

    View Slide

27. View Slide

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

    View Slide

29. View Slide

30. @Test

    public void testCopy() {

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

    TestWriter writer = new TestWriter();

    copy(reader, writer);

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

    }
    

    View Slide

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

    View Slide

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

    View Slide

33. View Slide

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

    View Slide

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

    View Slide

36. Dagger
    Directed Acyclic Graph
    

    View Slide

37. Object Graph
    

    View Slide

38. Greeter
    

    View Slide

39. // 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

40. // 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

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
    Text
    

    View Slide

45. // 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

46. // 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

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. @Inject

    public ManyDependencies(

    Dep1 never,

    Dep2 going,

    Dep3 to,

    Dep4 give,

    Dep5 you,
    Dep6 up

    )
    

    View Slide

50. // Greeter.java

    public class Greeter {

    @Inject Text text;

    

    public void sayText() {

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

    }

    }
    

    View Slide

51. // Greeter.java

    public class Greeter {

    @Inject Text text;

    

    public void sayText() {

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

    }

    }
    

    View Slide

52. public class ManyDependencies {

    @Inject Dep1 never;

    @Inject Dep2 going;

    @Inject Dep3 to;

    @Inject Dep4 let;

    @Inject Dep5 you;

    @Inject Dep6 down;

    }
    

    View Slide

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

    View Slide

54. 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

55. 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

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. Providing Interfaces
    

    View Slide

59. 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

60. 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

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. Single Module
    

    View Slide

65. Multiple Modules
    

    View Slide

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

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

    View Slide

67. View Slide

68. Unused!
    Incomplete!
    

    View Slide

69. // 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

70. // 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

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. Module Overrides
    @Module()

    public final class MyModule {

    @Provides Writer provideWriter() {

    return new PrinterWriter();

    }

    }

    @Module(

    includes = MyModule.class,

    overrides = true

    )

    public final class TestModule {

    @Provides Writer provideWriter() {

    return new TestWriter();

    }

    }
    

    View Slide

77. Module Overrides
    @Module()

    public final class MyModule {

    @Provides Writer provideWriter() {

    return new PrinterWriter();

    }

    }

    @Module(

    includes = MyModule.class,

    overrides = true

    )

    public final class TestModule {

    @Provides Writer provideWriter() {

    return new TestWriter();

    }

    }
    

    View Slide

78. Module Overrides
    @Module()

    public final class MyModule {

    @Provides Writer provideWriter() {

    return new PrinterWriter();

    }

    }

    @Module(

    includes = MyModule.class,

    overrides = true

    )

    public final class TestModule {

    @Provides Writer provideWriter() {

    return new TestWriter();

    }

    }
    

    View Slide

79. Module Overrides
    @Module()

    public final class MyModule {

    @Provides Writer provideWriter() {

    return new PrinterWriter();

    }

    }

    @Module(

    includes = MyModule.class,

    overrides = true

    )

    public final class TestModule {

    @Provides Writer provideWriter() {

    return new TestWriter();

    }

    }
    

    View Slide

80. Module Overrides
    @Module()

    public final class MyModule {

    @Provides Writer provideWriter() {

    return new PrinterWriter();

    }

    }

    @Module(

    includes = MyModule.class,

    overrides = true

    )

    public final class TestModule {

    @Provides Writer provideWriter() {

    return new TestWriter();

    }

    }
    

    View Slide

81. Module Overrides
    @Module()

    public final class MyModule {

    @Provides Writer provideWriter() {

    return new PrinterWriter();

    }

    }

    @Module(

    includes = MyModule.class,

    overrides = true

    )

    public final class TestModule {

    @Provides Writer provideWriter() {

    return new TestWriter();

    }

    }
    

    View Slide

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

    View Slide

83. View Slide

84. Dagger + Gradle
    dependencies {

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

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

    }
    

    View Slide

85. 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

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

    View Slide

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

    View Slide

88. 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

89. 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

90. 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

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

    View Slide

92. Thanks!
    @danlew42
    danlew.net
    

    View Slide