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Dagger 2. Right way to do Dependency Injections
by Anton Minashkin
Usual JAVA codepublic 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; }}
Usual JAVA codepublic 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; }}
Usual JAVA codepublic 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; }}
What is DI?In software engineering, dependency injection is a software design pattern that implements inversion of control for software libraries.
What is DI?
Say “Hi!” to DIpublic 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; }}
Say “Hi!” to DIpublic 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; }}
...but!Was:new Payroll().getAfterTaxPay(employee);
Now:new Payroll(EmployeeDatabase.getInstance()) .getAfterTaxPay(employee);
...but!Was:new Payroll().getAfterTaxPay(employee);
Now:new Payroll(EmployeeDatabase.getInstance()) .getAfterTaxPay(employee);
Java! To the rescue!
JSR-330
JSR-330public class Payroll { ... @Inject public Payroll(EmployeeDatabase employeeDatabase) { mEmployeeDatabase = employeeDatabase; }...}
ORpublic class Payroll { ... @Inject EmployeeDatabase mEmployeeDatabase; ...}
DI frameworks
● Google Guice● Spring DI● Java EE6 CDI● Dagger● etc.
DI frameworks
● Google Guice● Spring DI● Java EE6 CDI● Dagger● etc.
Dagger 2. History
Developed by SquareAdopted by Google (Dagger 2)
Dagger 2. Main features
● Android friendly● JSR-330● Compile-time DI validation● Full stack code generation● User mimic code● Easy to debug & understand
Dagger 2. APIclass Thermosiphon implements Pump { private final Heater heater;
@Inject Thermosiphon(Heater heater) { this.heater = heater; }
...}
Dagger 2. APIclass CoffeeMaker { @Inject Heater heater; @Inject Pump pump;
...}
Dagger 2. API@Moduleclass DripCoffeeModule { @Provides Heater provideHeater() { return new ElectricHeater(); }
@Provides Pump providePump(Thermosiphon pump) { return pump; }}
Dagger 2. API@Component(modules = DripCoffeeModule.class)interface CoffeeShop { CoffeeMaker maker();}
Dagger 2. APICoffeeShop coffeeShop = DaggerCoffeeShop.builder() .dripCoffeeModule(new DripCoffeeModule()) .build();
...
CoffeeShop coffeeShop = DaggerCoffeeShop.create();
Dagger 2. APIpublic class CoffeeApp { public static void main(String[] args) { CoffeeShop coffeeShop = DaggerCoffeeShop.create(); coffeeShop.maker().brew(); }}
Dagger 2. Scope@Provides @Singleton Heater provideHeater() { return new ElectricHeater();}
Dagger 2. Lazyclass GridingCoffeeMaker { @Inject Lazy<Grinder> lazyGrinder;
public void brew() { while (needsGrinding()) { // Grinder created once on first call to .get() and cached. lazyGrinder.get().grind(); } }}
Dagger 2. Provider Injectionclass 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(); ... } }}
Dagger 2. Qualifiers@Qualifier@Documented@Retention(RUNTIME)public @interface Named { String value() default "";}
Dagger 2. Qualifiersclass ExpensiveCoffeeMaker { @Inject @Named("water") Heater waterHeater; @Inject @Named("hot plate") Heater hotPlateHeater; ...}
Dagger 2. Qualifiers@Provides @Named("hot plate") Heater provideHotPlateHeater() { return new ElectricHeater(70);}
@Provides @Named("water") Heater provideWaterHeater() { return new ElectricHeater(93);}
Dagger 2. Compile-time validation@Moduleclass 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.
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); }
Dagger 2. Debugging
Here should be example of Guice stacktrace:VERY-VERY-BAD-STACKTRACE
And here is Dagger stacktrace:Mmmm… What a lovely stacktrace!
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● Diferent obj-graphs for diferent flavors
Dagger 2. Where should I inject?public class MyApp extends Application {@Overridepublic void onCreate() {registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {... @Override public void onActivityCreated(Activity activity, Bundle savedInstanceState) { inject(activity); }...});}
Dagger 2
Example
Dagger 2
Questions?
