Dependency Injection Made Simple

Presented at DevFest MN 2016.

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

1. Dependency injection made simple Dan Lew
2. What is a dependency?
3. A depends on B
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
5. What is dependency injection?
6. "Dependency Injection" is a 25-dollar term for a 5-cent concept. http://www.jamesshore.com/Blog/Dependency-Injection-Demystified.html ~James Shore
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;
 }
 }
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;
 }
 }
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;
 }
 }
10. View(Context context)
12. But Why? • Share dependencies • Configure dependencies externally
13. Context wrappedContext = new ContextThemeWrapper(context, R.style.MyTheme); 
 View view = new View(wrappedContext);
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.
15. A depends on B
16. A is coupled to B
19. void copy() {
 char c;
 while ((c = readKeyboard()) != -1) {
 writePrinter(c);
 }
 }
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);
 }
 }
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!");
 }
 }
 }
22. • What do we test in the read / write modules? • How the hell do we test copy?!
23. What if… • You want to change readKeyboard() or writePrinter()? void copy() {
 char c;
 while ((c = readKeyboard()) != -1) {
 writePrinter(c);
 }
 }
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);
 }
 }
26. But Why? • Share dependencies • Configure dependencies externally • Clean separation of modules
28. But Why? • Share dependencies • Configure dependencies externally • Clean separation of modules • Easy testing
30. @Test
 public void testCopy() {
 TestReader reader = new TestReader("Hello, world!");
 TestWriter writer = new TestWriter();
 copy(reader, writer);
 assertEquals("Hello, world!", writer.getWritten());
 }
31. But Why? • Share dependencies • Configure dependencies externally • Clean separation of modules • Easy testing • Easy debugging
32. …Why not? • More verbose • More complex
34. Framework Advantages • Handles injection busywork • Handles logic (e.g. singletons, lazy loading) • Unified system for dependencies
35. Frameworks • Dagger 1 • Dagger 2 • Guice • Spring • PicoContainer • …And on and on and on…
36. Dagger Directed Acyclic Graph
37. Object Graph
38. Greeter
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();
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();
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();
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();
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();
44. Greeter Text
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());
 }
 }
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());
 }
 }
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());
 }
 }
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());
 }
 }
49. @Inject
 public ManyDependencies(
 Dep1 never,
 Dep2 going,
 Dep3 to,
 Dep4 give,
 Dep5 you, Dep6 up
 )
50. // Greeter.java
 public class Greeter {
 @Inject Text text;
 
 public void sayText() {
 System.out.println(text.getText());
 }
 }
51. // Greeter.java
 public class Greeter {
 @Inject Text text;
 
 public void sayText() {
 System.out.println(text.getText());
 }
 }
52. public class ManyDependencies {
 @Inject Dep1 never;
 @Inject Dep2 going;
 @Inject Dep3 to;
 @Inject Dep4 let;
 @Inject Dep5 you;
 @Inject Dep6 down;
 }
53. Providers • Explicitly provide dependencies • Greater control over dependency • Can’t annotate dependency • Dependency inversion
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);
 }
 }
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);
 }
 }
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);
 }
 }
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);
 }
 }
58. Providing Interfaces
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();
 }
 }
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();
 }
 }
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();
 }
 }
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();
 }
 }
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();
 }
 }
64. Single Module
65. Multiple Modules
66. Multiple Modules • Compile time @Module(includes = HttpModule.class)
 public final class RestModule { } • Runtime ObjectGraph.create(new RestModule(), new HttpModule()); Warning!
68. Unused! Incomplete!
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);
 }
 }
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);
 }
 }
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);
 }
 }
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);
 }
 }
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);
 }
 }
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);
 }
 }
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);
 }
 }
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();
 }
 }
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();
 }
 }
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();
 }
 }
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();
 }
 }
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();
 }
 }
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();
 }
 }
82. Other Dagger Features • Singletons @Provides @Singleton Greeter provideGreeter() • Lazy injecting @Inject Lazy<Text> text; • Qualifiers @Provides @Named("Greeter1") Greeter provideGreeter()
84. Dagger + Gradle dependencies {
 provided 'com.squareup.dagger:dagger-compiler:1.2.2'
 compile 'com.squareup.dagger:dagger:1.2.2'
 }
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
86. Dagger + ProGuard • Google ProGuard config • Use Dagger 2
87. Injecting Android • Activity, Service, Fragment, etc… • Problem: We don’t control construction! • Solution: ObjectGraph.inject()
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);
 }
 }
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);
 }
 }
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);
 }
 }
91. u2020 • Unicode character for “dagger”: † • Injector pattern • https://github.com/JakeWharton/u2020
92. Thanks! @danlew42 danlew.net