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Dependency injection made simple Dan Lew

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What is a dependency?

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A depends on B

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• 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

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What is dependency injection?

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

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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;
 }
 }

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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;
 }
 }

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• 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;
 }
 }

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View(Context context)

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But Why? • Share dependencies • Configure dependencies externally

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

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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.

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A depends on B

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A is coupled to B

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void copy() {
 char c;
 while ((c = readKeyboard()) != -1) {
 writePrinter(c);
 }
 }

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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);
 }
 }

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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!");
 }
 }
 }

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

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Dependency inversion to the rescue

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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);
 }
 }

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But Why? • Share dependencies • Configure dependencies externally

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

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

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@Test
 public void testCopy() {
 TestReader reader = new TestReader("Hello, world!");
 TestWriter writer = new TestWriter();
 copy(reader, writer);
 assertEquals("Hello, world!", writer.getWritten());
 }

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

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…Why not? • More verbose • More complex

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

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

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Dagger Directed Acyclic Graph

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Object Graph

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Greeter

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// 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();

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// 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();

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// 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();

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// 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();

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// 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();

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Greeter Text

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// 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());
 }
 }

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// 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());
 }
 }

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// 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());
 }
 }

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// 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());
 }
 }

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@Inject
 public ManyDependencies(
 Dep1 bulbasaur,
 Dep2 charmander,
 Dep3 squirtle,
 Dep4 caterpie,
 Dep5 pikachu, Dep6 slowpoke
 )

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// Greeter.java
 public class Greeter {
 @Inject Text text;
 
 public void sayText() {
 System.out.println(text.getText());
 }
 }

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// Greeter.java
 public class Greeter {
 @Inject Text text;
 
 public void sayText() {
 System.out.println(text.getText());
 }
 }

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public class ManyDependencies {
 @Inject Dep1 eevee;
 @Inject Dep2 ditto;
 @Inject Dep3 snorlax;
 @Inject Dep4 articuno;
 @Inject Dep5 dragonite;
 @Inject Dep6 mew;
 }

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

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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);
 }
 }

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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);
 }
 }

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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);
 }
 }

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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);
 }
 }

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Providing Interfaces

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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();
 }
 }

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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();
 }
 }

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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();
 }
 }

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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();
 }
 }

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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();
 }
 }

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Single Module

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Multiple Modules

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Multiple Modules • Compile time @Module(includes = HttpModule.class)
 public final class RestModule { } • Runtime ObjectGraph.create(new RestModule(), new HttpModule()); Warning!

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Unused! Incomplete!

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// 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);
 }
 }

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// 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);
 }
 }

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// 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);
 }
 }

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// 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);
 }
 }

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// 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);
 }
 }

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// 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);
 }
 }

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// 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);
 }
 }

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

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Dagger + Gradle dependencies {
 provided 'com.squareup.dagger:dagger-compiler:1.2.2'
 compile 'com.squareup.dagger:dagger:1.2.2'
 }

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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

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Dagger + ProGuard • Google ProGuard config • Use Dagger 2

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

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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);
 }
 }

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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);
 }
 }

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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);
 }
 }

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

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Thanks! @danlew42 danlew.net