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处理自动装配歧义 由发表在
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
@Componentpackage com.tianmaying.iocdemo;public class QunarBookingService implements BookingService { public void bookFlight() { System.out.println("book fight by Qunar!"); }}
CtripBookingService.java
@Componentpackage 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,userMemoryRepository at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:573) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE] at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE] at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE] ... 43 common frames omittedCaused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.tianmaying.dao.UserDao] is defined: expected single matching bean but found 2: userDaoImpl,userMemoryRepository at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1126) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE] at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1014) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE] at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:545) ~[spring-beans-4.2.4.RELEASE.jar:4.2.4.RELEASE] ... 45 common frames omitted
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@Primarypackage 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:
如果使用基于XML文件的显式配置,则可以如下定义:
@Bean@Primarypublic QunarBookingService qunarBookingService() { return new QunarBookingService();}
@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;...@Componentpublic class SmartBoss { private BookingService bookingService; @Autowired @Qualifier('qunarBookingService') public void setBookingService(BookingService bookingService) { this.bookingService = bookingService; } ...}
这样就告诉Spring给SmartBoss
注入Qualifer为qunarBookingService
的Bean。
我们也可以自定义Bean的Qualifier。在使用自定义的@Qualifier值时,最好选择一个含义准确意义明确的词。
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