Injecting instances

In this tutorial we will inject instances into classes. We will use 2 approaches:

• Using an InstanceInjector
• Adding a class that will have its fields injected to the bindings graph

Modules

We will define two independent modules: one will provide the String message and the other will provide the int id.

public class MessageModule extends AbstractModule {
	@Provides
	String message() {
		return "hello";
	}
}

public class IdModule extends AbstractModule {
	@Provides
	int id() {
		return 123;
	}
}

Using an InstanceInjector

Here is the class whose fields must be injected.

public class Injected {
	@Inject
	String value;

	@Inject
	int id;
}

We need to create an Injector and pass MessageModule and IdModule as parameters.

We also need to add a binding for the instance injector. To do this, we will use a ModuleBuilder to manually create a third module whose sole purpose will be to bind the instance injector.

Injector injector = Injector.of(
	new MessageModule(),
	new IdModule(),
	ModuleBuilder.create()
		.bindInstanceInjector(Injected.class)
		.build());

Finally, we need to obtain the InstanceInjector from the Injector and use it to inject the fields into the newly created Injected instance.

InstanceInjector<Injected> instanceInjector = injector.getInstanceInjector(Injected.class);

Injected injected = new Injected();
instanceInjector.injectInto(injected);

Adding class to bindings graph

Here again the class whose fields are to be injected is represented. Note that this time there is an @Inject annotation annotating the type.

@Inject
public class Injected {
	@Inject
	String value;

	@Inject
	int id;
}

Alternatively, you can add a constructor and annotate it with the @Inject annotation, as shown below:

public class Injected {
	private final String value;
	private final int id;

	@Inject
	public Injected(String value, int id) {
		this.value = value;
		this.id = id;
	}
	//...
}

We still need to create an Injector and pass MessageModule and IdModule as parameters.

However, instead of binding the instance injector, this time we will bind the Injected class. To do this, we will also use a ModuleBuilder to manually build a third module, the sole purpose of which will be to bind the required class.

Injector injector = Injector.of(
	new MessageModule(),
	new IdModule(),
	ModuleBuilder.create()
		.bind(Injected.class)
		.build());

Finally, we can request an instance of Injected from the injector. All fields are correctly injected.

Injected instance = injector.getInstance(Injected.class);