spring in action 學習筆記八：用@Primary 或者@Qualifier消除@Autowired引起的歧義現象

　　首先解釋一下@Primary和@Qualifier這兩個注解的意思：@Primary的意思是在眾多相同的bean中，優先使用用@Primary注解的bean.而@Qualifier這個注解則指定某個bean有沒有資格進行注入。

示例代碼的思路是：1.一個接口Dessert和這個接口的三個實現類，2.再在一個類（AbrahamLincoln）中自動注入Dessert這個接口，3.用自動掃描機制自動創建bean.

如果不用@Primary和@Qualifier注解，勢必出現如下錯誤：NoUniqueBeanDefinitionException.

 

　　示例代碼如下：【用@Primary來解決問題】

Dessert接口的代碼如下：

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的代碼如下：

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的代碼如下：

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的代碼如下：

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//注意這里用了@Primary這個注解。 public class IceCream implements  Dessert{
    @Override
    public void amI() {
        System.out.println("I am ice cream");
    }
}

DessertConfig的代碼如下：

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自動掃描創建bean
 */
@ComponentScan(basePackageClasses = Dessert.class)
public class DessertConfig {

}

測試類的代碼如下：

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這個注解來解決問題：

核心代碼如下：

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") 13 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") 19     public void setDessert(Dessert dessert) {
        this.dessert = dessert;
    }
}