spring 3 autowiring and junit testing

My code:

@Component
public class A {
    @Autowired
    private B b;

    public void method() {}
}

public interface X {...}

@Component
public class B implements X {
    ...
}

I want to test in isolation class A. Do I have to mock class B? If yes, how? Because it is autowired and there is no setter where i could send the mocked object.