The document discusses Dagger 2, a dependency injection framework in Java, highlighting its features like compile-time validation and ease of debugging. It explains the concept of dependency injection with examples of classes and how to implement it using Dagger 2's syntax and structure. Additionally, it covers various aspects such as modules, components, scoping, qualifiers, and debugging techniques within Dagger 2.

1. Dagger 2. Right way to do
Dependency Injections
by Anton Minashkin

2. Usual JAVA code
public class Payroll {
...
public long getWithholding(long payInDollars) {
...
return withholding;
}
public long getAfterTaxPay(Employee employee) {
long basePay = EmployeeDatabase.getInstance()
.getBasePay(employee);
long withholding = getWithholding(basePay);
return basePay - withholding;
}
}

3. Usual JAVA code
public class Payroll {
...
public long getWithholding(long payInDollars) {
...
return withholding;
}
public long getAfterTaxPay(Employee employee) {
long basePay = EmployeeDatabase.getInstance()
.getBasePay(employee);
long withholding = getWithholding(basePay);
return basePay - withholding;
}
}

4. Usual JAVA code
public class Payroll {
...
public long getWithholding(long payInDollars) {
...
return withholding;
}
public long getAfterTaxPay(Employee employee) {
long basePay = new EmployeeDatabase()
.getBasePay(employee);
long withholding = getWithholding(basePay);
return basePay - withholding;
}
}

5. What is DI?
In software engineering, dependency
injection is a software design pattern that
implements inversion of control for software
libraries.

6. What is DI?

7. Say “Hi!” to DI
public class Payroll {
...
EmployeeDatabase mEmployeeDatabase;
public Payroll(EmployeeDatabase employeeDatabase) {
mEmployeeDatabase = employeeDatabase;
}
public long getWithholding(long payInDollars) {
...
return withholding;
}
public long getAfterTaxPay(Employee employee) {
long basePay = mEmployeeDatabase.getBasePay(employee);
long withholding = getWithholding(basePay);
return basePay - withholding;
}
}

8. Say “Hi!” to DI
public class Payroll {
...
EmployeeDatabase mEmployeeDatabase;
public Payroll(EmployeeDatabase employeeDatabase) {
mEmployeeDatabase = employeeDatabase;
}
public long getWithholding(long payInDollars) {
...
return withholding;
}
public long getAfterTaxPay(Employee employee) {
long basePay = mEmployeeDatabase.getBasePay(employee);
long withholding = getWithholding(basePay);
return basePay - withholding;
}
}

9. ...but!
Was:
new Payroll().getAfterTaxPay(employee);
Now:
new Payroll(EmployeeDatabase.getInstance())
.getAfterTaxPay(employee);

10. ...but!
Was:
new Payroll().getAfterTaxPay(employee);
Now:
new Payroll(EmployeeDatabase.getInstance())
.getAfterTaxPay(employee);

11. Java! To the rescue!
JSR-330

12. JSR-330
public class Payroll {
...
@Inject
public Payroll(EmployeeDatabase employeeDatabase) {
mEmployeeDatabase = employeeDatabase;
}
...
}
OR
public class Payroll {
...
@Inject
EmployeeDatabase mEmployeeDatabase;
...
}

13. DI frameworks
● Google Guice
● Spring DI
● Java EE6 CDI
● Dagger
● etc.

14. DI frameworks
● Google Guice
● Spring DI
● Java EE6 CDI
● Dagger
● etc.

15. Dagger 2. History
Developed by Square
Adopted by Google (Dagger 2)

16. Dagger 2. Main features
● Android friendly
● JSR-330
● Compile-time DI validation
● Full stack code generation
● User mimic code
● Easy to debug & understand

17. Dagger 2. API
class Thermosiphon implements Pump {
private final Heater heater;
@Inject
Thermosiphon(Heater heater) {
this.heater = heater;
}
...
}

18. Dagger 2. API
class CoffeeMaker {
@Inject Heater heater;
@Inject Pump pump;
...
}

19. Dagger 2. API
@Module
class DripCoffeeModule {
@Provides Heater provideHeater() {
return new ElectricHeater();
}
@Provides Pump providePump(Thermosiphon pump) {
return pump;
}
}

20. Dagger 2. API
@Component(modules = DripCoffeeModule.class)
interface CoffeeShop {
CoffeeMaker maker();
}

21. Dagger 2. API
CoffeeShop coffeeShop = DaggerCoffeeShop.builder()
.dripCoffeeModule(new DripCoffeeModule())
.build();
...
CoffeeShop coffeeShop = DaggerCoffeeShop.create();

22. Dagger 2. API
public class CoffeeApp {
public static void main(String[] args) {
CoffeeShop coffeeShop = DaggerCoffeeShop.create();
coffeeShop.maker().brew();
}
}

23. Dagger 2. Scope
@Provides @Singleton Heater provideHeater() {
return new ElectricHeater();
}

24. Dagger 2. Lazy
class GridingCoffeeMaker {
@Inject Lazy<Grinder> lazyGrinder;
public void brew() {
while (needsGrinding()) {
// Grinder created once on first call to .get() and cached.
lazyGrinder.get().grind();
}
}
}

25. Dagger 2. Provider Injection
class BigCoffeeMaker {
@Inject Provider<Filter> filterProvider;
public void brew(int numberOfPots) {
...
for (int p = 0; p < numberOfPots; p++) {
maker.addFilter(filterProvider.get()); //new filter every time.
maker.addCoffee(...);
maker.percolate();
...
}
}
}

26. Dagger 2. Qualifiers
@Qualifier
@Documented
@Retention(RUNTIME)
public @interface Named {
String value() default "";
}

27. Dagger 2. Qualifiers
class ExpensiveCoffeeMaker {
@Inject @Named("water") Heater waterHeater;
@Inject @Named("hot plate") Heater hotPlateHeater;
...
}

28. Dagger 2. Qualifiers
@Provides @Named("hot plate") Heater provideHotPlateHeater() {
return new ElectricHeater(70);
}
@Provides @Named("water") Heater provideWaterHeater() {
return new ElectricHeater(93);
}

29. Dagger 2. Compile-time validation
@Module
class DripCoffeeModule {
@Provides Heater provideHeater(Executor executor) {
return new CpuHeater(executor);
}
}
...
[ERROR] COMPILATION ERROR :
[ERROR] error: java.util.concurrent.Executor cannot be provided without an @Provides-annotated
method.

30. Dagger 2. Generated code
@Override
public DataManager get() {
DataManager provided = module.provideDataManager(authApiProvider.get(), articleApiProvider.get(),
commentsApiProvider.get());
if (provided == null) {
throw new NullPointerException("Cannot return null from a non-@Nullable @Provides method");
}
return provided;
}
public static Factory<DataManager> create(DataManagerModule module, Provider<AuthApi> authApiProvider,
Provider<ArticleApi> articleApiProvider, Provider<CommentApi> commentApiProvider) {
return new DataManagerModule_ProvideDataManagerFactory(module, authApiProvider, articleApiProvider,
commentApiProvider);
}

31. Dagger 2. Debugging
Here should be example of Guice stacktrace:
VERY-VERY-BAD-STACKTRACE
And here is Dagger stacktrace:
Mmmm… What a lovely stacktrace!

32. Dagger 2. What should I inject?
● Anything that has constructor parameters
● Anything that is out of local scope
● Infrastructure
● Anything that is shared between >1 objects
● Different obj-graphs for different flavors

33. Dagger 2. Where should I inject?
public class MyApp extends Application {
@Override
public void onCreate() {
registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
...
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
inject(activity);
}
...
});
}

34. Dagger 2
Example

35. Dagger 2
Questions?

36. RTFM!
http://google.github.io/dagger/
https://youtu.be/oK_XtfXPkqw
anton.minashkin@outlook.com