Spring的文档上说：

DI exists in two major variants, Constructor-based dependency
injection and Setter-based dependency injection.

我的理解是，接口注入其实也是通过setter注入来实现的：

interface InjectPerson {
    public void injectHere(Person p);
}

class Company implements InjectPerson {
   Person injectedPerson; 

   public void injectHere(Person p) {
        this.injectedPerson = p;
    }
}