spring in action 学习笔记八：用@Primary 或者@Qualifier消除@Autowired引起的歧义现象

　　首先解释一下@Primary和@Qualifier这两个注解的意思：@Primary的意思是在众多相同的bean中，优先使用用@Primary注解的bean.而@Qualifier这个注解则指定某个bean有没有资格进行注入。

示例代码的思路是：1.一个接口Dessert和这个接口的三个实现类，2.再在一个类（AbrahamLincoln）中自动注入Dessert这个接口，3.用自动扫描机制自动创建bean.

如果不用@Primary和@Qualifier注解，势必出现如下错误：NoUniqueBeanDefinitionException.

　　示例代码如下：【用@Primary来解决问题】

Dessert接口的代码如下：

 package com.advancedWiring.ambiguityInAutowiring;

 import org.springframework.stereotype.Component;

 /**
  * Created by ${秦林森} on 2017/6/9.
  */
 @Component
 public interface Dessert {
     void amI();
 }

Cake的代码如下：

 package com.advancedWiring.ambiguityInAutowiring;

 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.context.annotation.Primary;
 import org.springframework.stereotype.Component;

 /**
  * Created by ${秦林森} on 2017/6/9.
  */
 @Component
 public class Cake implements Dessert {
     @Override
     public void amI() {
         System.out.println("I am cake");
     }
 }

Cookie的代码如下：

package com.advancedWiring.ambiguityInAutowiring;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

/**
 * Created by ${秦林森} on 2017/6/9.
 */
@Component
public class Cookie implements Dessert {
    @Override
    public void amI() {
        System.out.println("I am cookie");
    }
}

IceCream的代码如下：

package com.advancedWiring.ambiguityInAutowiring;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

/**
 * Created by ${秦林森} on 2017/6/9.
 */
@Component
@Primary//注意这里用了@Primary这个注解。
public class IceCream implements  Dessert{
    @Override
    public void amI() {
        System.out.println("I am ice cream");
    }
}

DessertConfig的代码如下：

 package com.advancedWiring.ambiguityInAutowiring;

 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;

 /**
  * Created by ${秦林森} on 2017/6/9.
  */
 @Configuration
 /**
  * 用@ComponentScan自动扫描创建bean
  */
 @ComponentScan(basePackageClasses = Dessert.class)
 public class DessertConfig {

 }

测试类的代码如下：

 package com.advancedWiring.ambiguityInAutowiring;

 import org.springframework.context.annotation.AnnotationConfigApplicationContext;

 /**
  * Created by ${秦林森} on 2017/6/9.
  */
 public class AmbiguityTest {
     public static void main(String[] args) {
         AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(DessertConfig.class);
         Dessert dessert = ac.getBean(Dessert.class);
         dessert.amI();
     }
 }

二：用@Qualifier这个注解来解决问题：

核心代码如下：

 package com.advancedWiring.ambiguityInAutowiring;

 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.context.annotation.Primary;
 import org.springframework.stereotype.Component;

 /**
  * Created by ${秦林森} on 2017/6/9.
  */
 @Component
 @Qualifier("crispy")
 public class Cookie implements Dessert {
     @Override
     public void amI() {
         System.out.println("I am cookie");
     }
 }

 package com.advancedWiring.ambiguityInAutowiring;

 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.stereotype.Component;

 /**
  * Created by ${秦林森} on 2017/6/9.
  */
 @Component
 public class AbrahamLincoln {
     private Dessert dessert;

     public Dessert getDessert() {
         return dessert;
     }
     @Autowired
     @Qualifier("crispy")
     public void setDessert(Dessert dessert) {
         this.dessert = dessert;
     }
 }