com.google.inject.assistedinject Documentation Differences

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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 @} 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 @}Assisted annotation. This serves to document that the parameter 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 @}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 module, install a {@code FactoryModuleBuilder} that creates the factory:

install(new FactoryModuleBuilder()
     .implement(Payment.class, RealPayment.class)
     .build(PaymentFactory.class));

As a side-effect of this binding, Guice will inject the factory to initialize it for use. The factory cannot be used until the injector has been initialized.

Configuring complex factories

Factories can create an arbitrary number of objects, one per each method. Each factory method can be configured using .implement.

public interface OrderFactory {
    Payment create(Date startDate, Money amount);
    Shipment create(Customer customer, Item item);
    Receipt create(Payment payment, Shipment shipment);
 }
 
 [...]
 
 install(new FactoryModuleBuilder()
     .implement(Payment.class, RealPayment.class)
     // excluding .implement for Shipment means the implementation class
     // will be 'Shipment' itself, which is legal if it's not an interface.
     .implement(Receipt.class, RealReceipt.class)
     .build(OrderFactory.class));

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 @}Assisted annotation to disambiguate the parameters. The names 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 Guice

Returned factories use child injectors to create values. The values are eligible for method interception. In addition, {@literal @}{@literal 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 FactoryModuleBuilder().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 FactoryModuleBuilder()
       .implement(Car.class, Names.named("fast"), Porsche.class)
       .implement(Car.class, Names.named("clean"), Prius.class)
       .build(CarFactory.class));
 }

Implementation limitations

As a limitation of the implementation, it is prohibited to declare a factory method that accepts a {@code Provider} as one of its arguments. @since 3.0 @author schmitt@google.com (Peter Schmitt)