Google Guice: How to use MapBinder + Generics + AssistedInject (FactoryModuleBuilder)

Let' say we have a hierarchy of objects as shown below:

Each concrete implementation (such as Ferrari or Mercedes)  has a corresponding Factory which uses assisted inject and have the create method.

The CarModule uses the FactoryModuleBuilder to bind the factories would look as shown below:

	install(new FactoryModuleBuilder().build(FerrariFactory.class));
	install(new FactoryModuleBuilder().build(MercedesFactory.class));

view raw CarModule hosted with ❤ by GitHub

If we wish to create objects using these factories we would do something as shown below:

	package com.guice.test;
	import com.google.inject.Inject;
	public class CarManager {
	private final FerrariFactory ferrariFactory;
	private final MercedesFactory mercedesFactory;
	@Inject
	public CarManager(FerrariFactory ferrariFactory,
	MercedesFactory mercedesFactory) {
	this.ferrariFactory = ferrariFactory;
	this.mercedesFactory = mercedesFactory;
	}
	public void createAllCars() {
	System.out.println("start createCars()");
	String part = "assisted part";
	ferrariFactory.create(part);
	mercedesFactory.create(part);
	}
	public void createCarByType(String type) {
	System.out.println("start createCarByType()");
	String part = "assisted part";
	if(type.equals("ferrari"))
	ferrariFactory.create(part);
	else if(type.equals("mercedes"))
	mercedesFactory.create(part);
	else
	System.out.println("Car not supported");
	}
	}

view raw CarManager hosted with ❤ by GitHub

PROBLEM: In case we want to add new factories (and their implementations) such as ToyotaFactory etc., we will have to add them to the module and also change the CarManager class. If our list is huge, our constructor and the methods in the CarManager class will not look nice. Also, there could be many such other classes like CarManager where we use the factories. Not easy to make changes everywhere.

The solution is to use MapBinder along with generics.

Step 1. We create an abstract factory as shown below:

	package com.guice.test;
	import com.google.inject.assistedinject.Assisted;
	public interface CarFactory<T extends Car> {
	T create(@Assisted String part);
	}

view raw CarFactory hosted with ❤ by GitHub

Step 2. Make the FerrariFactory and the MercedesFactory extend the CarFactory as shown below(shown only for FerrariFactory):

	package com.guice.test.car.beans;
	import com.google.inject.assistedinject.Assisted;
	public interface FerrariFactory extends CarFactory<Ferrari> {
	Ferrari create(@Assisted("part") String part);
	}

view raw FerrariFactory.java hosted with ❤ by GitHub

Step 3. We bind them in the module using mapbinder + generics as shown below:

	package com.guice.test;
	import com.google.inject.AbstractModule;
	import com.google.inject.TypeLiteral;
	import com.google.inject.assistedinject.FactoryModuleBuilder;
	import com.google.inject.multibindings.MapBinder;
	public class CarModule extends AbstractModule {
	@Override
	protected void configure() {
	install(new FactoryModuleBuilder().build(FerrariFactory.class));
	install(new FactoryModuleBuilder().build(MercedesFactory.class));
	MapBinder<String, CarFactory<?>> mapbinder = MapBinder.newMapBinder(binder(), new TypeLiteral<String>(){}, new TypeLiteral<CarFactory<?>>(){});
	mapbinder.addBinding("ferrari").to(FerrariFactory.class);
	mapbinder.addBinding("mercedes").to(MercedesFactory.class);
	}
	}

view raw CarModule hosted with ❤ by GitHub

Use them in the CarManager class as shown below:

	package com.guice.test;
	import java.util.Map;
	import com.google.inject.Inject;
	import com.google.inject.multibindings.MapBinder;
	public class CarManagerWithMapBinder {
	private final Map<String, CarFactory<?>> factories;
	@Inject
	public CarManagerWithMapBinder(Map<String, CarFactory<?>> factories) {
	this.factories = factories;
	}
	public void createAllCars() {
	System.out.println("start createCars()");
	for(Map.Entry<String, CarFactory<?>> entry :factories.entrySet()) {
	String part ="assisted part";
	entry.getValue().create(part);
	}
	}
	public void createCarByType(String type) {
	System.out.println("start createCarByType()");
	for(Map.Entry<String, CarFactory<?>> entry :factories.entrySet()) {
	if(entry.getKey().equals(type)) {
	String part ="assisted part";
	entry.getValue().create(part);
	}
	}
	}
	}

view raw CarManager hosted with ❤ by GitHub

As we can see, the createAllCars() method and the createCarByType() method are not dependent on the factories anymore !!!  Next time we add a new factory, all we need to do is add it to the CarModule and we are done !!

Note: The above solution works with Guice 3 and JDK 7 OR Guice 4 and JDK 8 (not with JDK8 and Guice 4 due to https://github.com/google/guice/issues/904).