[java] Spring: Why do we autowire the interface and not the implemented class?

Example

interface IA
{
  public void someFunction();
}

@Resource(name="b")
class B implements IA
{
  public void someFunction()
  {
    //busy code block
  }
  public void someBfunc()
  {
     //doing b things
  }
}

@Resource(name="c")
class C implements IA
{
  public void someFunction()
  {
    //busy code block
  }
  public void someCfunc()
  {
     //doing C things
  }
}

class MyRunner
{

  @Autowire
  @Qualifier("b") 
  IA worker;

  worker.someFunction();
}

Can someone explain this to me.

• How does spring know which polymorphic type to use.
• Do I need @Qualifier or @Resource?
• Why do we autowire the interface and not the implemented class?

Also it may cause some warnigs in logs like a Cglib2AopProxy Unable to proxy method. And many other reasons for this are described here Why always have single implementaion interfaces in service and dao layers?