Spring应用抛出NoUniqueBeanDefinitionException异常的解决方案
作者:源码面前了无秘密
前言
我们在开发Spring应用时可能会不小心注入两个相同类型的Bean,比如实现了两个相同Service接口的类,示例伪代码如下:
interface SampleService { String getName(); } class ServiceA implements SampleService{ String getName(){ return "john"; } } class ServiceB implements SampleService{ String getName(){ return "wonder"; } }
这时候我们用SampleService接口注入
@Autowired SampleService sampleService;
启动应用后,Spring就会优雅地提示如下错误:
Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.john.primary.SampleService' available: expected single matching bean but found 2: ServiceA,ServiceB
但是我们不想报错且想获取其中某一个Bean,这时候我们该怎么办呢?
解决方案
既然包含了两个相同类型的Bean,通常来说我们只要把其中一个Bean不注入就好,那如果我们想保留这两个相同类型的Bean,但是又想让SampleService正常注入呢?
如果我们是用早期Spring的Xml配置Bean时,可以使用如下两种方式解决:
1.那么我们可以在其中一个Bean配置里加上autowire-candidate="false"
<bean id="serviceA" class="com.john.primary.ServiceA" /> <bean id="serviceB" class="com.john.primary.ServiceB" autowire-candidate="false" />
2.或者在其中一个Bean配置里加上primary="true":
<bean id="serviceA" class="com.john.primary.ServiceA" primary="true"/> <bean id="serviceB" class="com.john.primary.ServiceB" />
3.采用javax.annotation.Priority注解
这种方式需要我们在BeanFactory里加上dependencyComparator,示例代码如下:
DefaultListableBeanFactory beanFactory = (DefaultListableBeanFactory)context.getBeanFactory(); //@Priority注解比较 beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE); SampleService sampleService= context.getBean(SampleService.class);
4.实现注解Order或者实现org.springframework.core.Ordered接口
public class ServiceA implements SampleService,Ordered { @Override public int getOrder() { return 0; } @Override public String toString() { return "ServiceA{}"; } }
这种方式需要我们重写AnnotationAwareOrderComparator的getPriority方法,示例代码如下:
public class PriorityOrderComparator extends AnnotationAwareOrderComparator { /** * Shared default instance of {@code PriorityOrderComparator}. */ public static final PriorityOrderComparator INSTANCE = new PriorityOrderComparator(); @Override public Integer getPriority(Object obj) { //先获取Priority Integer order = super.getPriority(obj); if(order == null) //获取Order注解或者Ordered接口返回值 return super.findOrder(obj); return order; } }
我们还可以使用目前流行的注解方式来实现,Spring文档中也提到过:
Because autowiring by type may lead to multiple candidates, it is often necessary to have more control over the selection process. One way to accomplish this is with Spring's @Primary annotation. @Primary indicates that a particular bean should be given preference when multiple beans are candidates to be autowired to a single-valued dependency. If exactly one primary bean exists among the candidates, it becomes the autowired value.
那么可以使用如下方式:
1.@Primary注解:
该注解可以标注在类上或者方法上,示例如下:
@Primary @Component class ServiceA implements SampleService{ String getName(){ return "john"; } }
注解在有@Bean注解的方法上:
@Bean @Primary SampleService sampleService(){ return new ServiceA(); }
2.还是采用Xml配置中的第三或者第四种解决方案,只是采用第四种方案的话还是需要重新扩展AnnotationAwareOrderComparator
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