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;
    }
}