Copied and modified from {@link com.google.inject.assistedinject.FactoryModuleBuilder}. Usage is mostly the same, with the exception of forwarded bindings (see at the bottom of this documentation). Provides a factory that combines the caller's arguments with injector-supplied values to construct objects.
Defining a factory
Create an interface whose methods return the constructed type, or any of its supertypes. The method's parameters are the arguments required to build the constructed type.
public interface PaymentFactory { Payment create(Date startDate, Money amount); } You can name your factory methods whatever you like, such as
create,
createPayment or
newPayment.
Creating a type that accepts factory parameters
{@code constructedType} is a concrete class with an{@literal @}{@link Inject}-annotated constructor. In addition to injector- supplied parameters, the constructor should have parameters that match each of the factory method's parameters. Each factory-supplied parameter requires an {@literal @}{@link Assisted} annotation. This serves to document that theparameter is not bound by your application's modules.
public class RealPayment implements Payment { {@literal @}Inject public RealPayment( CreditService creditService, AuthService authService, {@literal @}Assisted Date startDate, {@literal @}Assisted Money amount) { ... } } Multiple factory methods for the same type
If the factory contains many methods that return the same type, you can create multiple constructors in your concrete class, each constructor marked with with {@literal @}{@link AssistedInject}, in order to match the different parameters types of the factory methods.
public interface PaymentFactory { Payment create(Date startDate, Money amount); Payment createWithoutDate(Money amount); } public class RealPayment implements Payment { {@literal @}AssistedInject public RealPayment( CreditService creditService, AuthService authService, {@literal @}Assisted Date startDate, {@literal @}Assisted Money amount) { ... } {@literal @}AssistedInject public RealPayment( CreditService creditService, AuthService authService, {@literal @}Assisted Money amount) { ... } } Configuring simple factories
In your {@link Module module}, install a {@code GinFactoryModuleBuilder}that creates the factory:
install(new GinFactoryModuleBuilder() .implement(Payment.class, RealPayment.class) .build(PaymentFactory.class);
As a side-effect of this binding, Gin will inject the factory to initialize it for use. The factory cannot be used until the injector has been initialized.
Using the factory
Inject your factory into your application classes. When you use the factory, your arguments will be combined with values from the injector to construct an instance.
public class PaymentAction { {@literal @}Inject private PaymentFactory paymentFactory; public void doPayment(Money amount) { Payment payment = paymentFactory.create(new Date(), amount); payment.apply(); } } Making parameter types distinct
The types of the factory method's parameters must be distinct. To use multiple parameters of the same type, use a named {@literal @}{@link Assisted} annotation to disambiguate the parameters. Thenames must be applied to the factory method's parameters:
public interface PaymentFactory { Payment create( {@literal @}Assisted("startDate") Date startDate, {@literal @}Assisted("dueDate") Date dueDate, Money amount); } ...and to the concrete type's constructor parameters:
public class RealPayment implements Payment { {@literal @}Inject public RealPayment( CreditService creditService, AuthService authService, {@literal @}Assisted("startDate") Date startDate, {@literal @}Assisted("dueDate") Date dueDate, {@literal @}Assisted Money amount) { ... } } Values are created by Gin
Returned factories use child injectors to create values. The values are eligible for method interception. In addition, {@literal @}{@link Inject}members will be injected before they are returned.
More configuration options
In addition to simply specifying an implementation class for any returned type, factories' return values can be automatic or can be configured to use annotations:
If you just want to return the types specified in the factory, do not configure any implementations:
public interface FruitFactory { Apple getApple(Color color); } ... protected void configure() { install(new GinFactoryModuleBuilder().build(FruitFactory.class)); } Note that any type returned by the factory in this manner needs to be an implementation class.
To return two different implementations for the same interface from your factory, use binding annotations on your return types:
interface CarFactory { {@literal @}Named("fast") Car getFastCar(Color color); {@literal @}Named("clean") Car getCleanCar(Color color); } ... protected void configure() { install(new GinFactoryModuleBuilder() .implement(Car.class, Names.named("fast"), Porsche.class) .implement(Car.class, Names.named("clean"), Prius.class) .build(CarFactory.class)); } In difference to regular Guice Assisted Inject, in Gin, return types in your factory are
not further resolved using your regular injector configuration. This means that in the following example you'll still get a {@code Chicken} and not a {@code Rooster}:
interface Animal {} public class Chicken implements Animal {} public class Rooster extends Chicken {} interface AnimalFactory { Animal getAnimal(); } ... protected void configure() { bind(Chicken.class).to(Rooster.class); install(new GinFactoryModuleBuilder() .implement(Animal.class, Chicken.class) .build(AnimalFactory.class)); }