一、

假设有如下三个类实现同一个接口，则自动装配时会产生歧义

 @Component

 public class Cake implements Dessert { ... }

 @Component

 public class Cookies implements Dessert { ... }

 @Component

 public class IceCream implements Dessert { ... }

 @Autowired

 public void setDessert(Dessert dessert) {

 this.dessert = dessert;

 }

二、@Primary的3种用法

Let’s say that ice cream is your favorite dessert. You can express that favorite choice
in Spring using the @Primary annotation. @Primary can be used either alongside
@Component for beans that are component-scanned or alongside @Bean for beans
declared in Java configuration. For example, here’s how you might declare the
@Component -annotated IceCream bean as the primary choice:

1.在自动扫描

@Component

@Primary

public class IceCream implements Dessert { ... }

2.在java配置文件中

@Bean

@Primary

public Dessert iceCream() {

    return new IceCream();

}

3.xml配置文件

 <bean id="iceCream"

 class="com.desserteater.IceCream"

 primary="true" />

如果有两个合适的bean都标记为@Primary，则Spring还是无法确定要装配哪个bean,这时要用@Qulifier来解决歧义