Spring处理自动装配歧义

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Spring处理自动装配歧义由David发表在天码营

自动装配中冲突

自动装配（autowiring）要求bean的匹配具备唯一性，否则就会产生歧义，从而抛出异常。

例如，如果我们给QunarBookingService和CtripBookingService都标注上@Component，则应用上下文中会有两个BookingService实例。

QunarBookingService.java

@Component
package com.tianmaying.iocdemo;public class QunarBookingService implements BookingService {public void bookFlight() {System.out.println("book fight by Qunar!");}
}

CtripBookingService.java

@Component
package com.tianmaying.iocdemo;public class CtripBookingService implements BookingService {public void bookFlight() {System.out.println("book fight by Ctrip!");}
}

在SmartBoss的代码中用@Autowired标准的bookingService属性，那么当Spring进行装配时，注入哪一个实现BookingService接口的Bean就会面临多个选择。Spring就会抛出异常NoUniqueBeanDefinitionException。

Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.tianmaying.dao.UserDao com.tianmaying.service.UserService.userDao; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.tianmaying.dao.UserDao] is defined: expected single matching bean but found 2: userDaoImpl,userMemoryRepositoryat org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:573) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE]at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE]at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE]... 43 common frames omitted
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.tianmaying.dao.UserDao] is defined: expected single matching bean but found 2: userDaoImpl,userMemoryRepositoryat org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1126) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE]at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1014) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE]at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:545) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE]... 45 common frames omitted

通过优先级

在定义bean时，可以通过@Primary指定一个优先级高的bean来消除自动装配过程中遇到的歧义问题。

比如，如果SmartBoos希望使用QunarBookingService的实例，那么我们可以给其增加一个@Primary标注。

QunarBookingService.java

@Component
@Primary
package com.tianmaying.iocdemo;public class QunarBookingService implements BookingService {public void bookFlight() {System.out.println("book fight by Qunar!");}
}

@Primary作用在类上适用于自动装配。也可以通过XML配置或者Java配置的方式实现@Primary标注的功能。

如果使用基于XML文件的显式配置，则可以如下定义：

<bean id="qunarBookingService"class="com.tianmaying.iocdemo.QunarBookingService"primary="true" />

如果使用基于XML文件的显式配置，则可以如下定义：

@Bean
@Primary
public QunarBookingService qunarBookingService() {return new QunarBookingService();
}

通过Qualifier

@Qualifier注解可以跟@Autowired或@Inject一起使用，指定被注入的bean的ID。

每个Bean都有一个默认的Qualifier，内容与Bean的ID相同。

比如，我们也可以这样修改SmartBoss的代码：

package com.tianmaying.iocdemo;...@Component
public class SmartBoss {private BookingService bookingService;@Autowired@Qualifier('qunarBookingService')public void setBookingService(BookingService bookingService) {this.bookingService = bookingService;}...
}