How to inject Injector?

You should not be using the Injector directly. Rather pass in the Provider<FooClass> instead. Also, you should be injecting the provider in the places where you use FooClass.

private final Provider<FooClass> provider;

@Inject
public ClassWhereFooIsUsed(Provider<FooClass> provider) {
    this.provider = provider;
}

.... somewhere else
FooClass f = provider.get(); // This is lazy