java - 为什么我在 Spring 的 Autowiring 中没有得到 No unique bean of type

Question

Person.java

@Controller
public class Person 
{
    @Autowired
    private Ability ability;

     public void printMessage(){

         ability.printMessasge();
     }

     public void setOutputGenerator( Ability ability) {
            this.ability = ability;
        }  
}

能力.java

@Controller
public class Ability 
{
     void printMessasge(){

         System.out.println("I print message");
     }

}

Spring .xml

<bean id="invisible" class="com.mkyong.common.Ability" >
</bean>  

<context:component-scan base-package="com.mkyong" /> 

应用程序.java

public class App {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "spring.xml");

        Person person = (Person) context.getBean("person");
        person.printMessage( );
    }

在上面的例子中我定义了两个bean Ability第一类使用@Controller和一个 xml文件。根据Autowire按类型我应该得到

Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: 
...
No unique bean of type [com.mkyong.common.Ability] is defined:

但我得到了正确的输出。为什么？

如果我创建一个由 Ability 类实现的接口(interface)，那么我将得到 UnsatisfiedDependencyException 异常。像这样:

父类.java

public interface Parent{

     public void printMessasge();

}

能力.java

@Controller
public class Ability implements Parent
{
      void printMessasge(){

         System.out.println("I print message");
     } 

Person.java

@Controller
public class Person 
{
    @Autowired
    Parent parent;
     public void printMessage(){
         parent.printMessasge();
     }

     public void setOutputGenerator( Parent parent) {
            this.parent= parent;
        }  
}

Answer 1

默认情况下 Spring 确实按类型匹配，但在有多个匹配 bean 的情况下，它会退回到按名称匹配。引用 ( 6.9.4 Fine-tuning annotation-based autowiring with qualifiers) 说:

For a fallback match, the bean name is considered a default qualifier value.

使用 @Qualifier注释使按类型 Autowiring 以及按名称进行的 Autowiring 更加明确。