Untargeted Bindings

绑定没有实现类的bean，即有bind()没有to()的bean。

This is most useful for concrete classes and types annotated by either @ImplementedBy or @ProvidedBy. An untargetted binding informs the injector about a type, so it may prepare dependencies eagerly. Untargetted bindings have no to clause, like so:

因为没有具体的实现类，所以injector会采用饿汉模式，去找到容器中对应的父类bean或子类bean，赶紧set给这个binding，这对容器中被@ImplementedBy或@ProvidedBy注解过的父类并且还有具体的实现类的情况很有用：

bind(MyConcreteClass.class);
bind(AnotherConcreteClass.class).in(Singleton.class);