dubbo之@Reference注解有什么作用

这篇文章主要介绍“dubbo之@Reference注解有什么作用”的相关知识，小编通过实际案例向大家展示操作过程，操作方法简单快捷，实用性强，希望这篇“dubbo之@Reference注解有什么作用”文章能帮助大家解决问题。看看dubbo是怎么给加了@Reference注解的属性注入invoker实例，为什么有时候加了@Reference注解的属性会是null。看到这个名字，就很容易知道，是专门针对@Reference注解的后置处理。ReferenceAnnotationBeanPostProcessor的代码比较多，下面列一下比较重要的内容。dubbo在ReferenceAnnotationBeanPostProcessor里定义了一个私有的子类ReferenceInjectionMetadata继承spring定义的InjectionMetadata类。之所以需要自定义ReferenceInjectionMetadata类，是因为dubbo的@Reference注解可以使用在属性和方法上，需要区分开。但是spring定义的InjectionMetadata类，只支持一个injectedElements集合，代码如下：基于这个原因，dubbo定义了ReferenceInjectionMetadata类，代码如下：代码很简单，入参变为3个，属性和方法列表区分开，然后把两者合并起来，调用父类的构造函数，用fieldElements保存属性列表，用methodElements保存方法列表。属性注入实际发生在ReferenceFieldElement类，代码如下：方法注入实际发生在ReferenceMethodElement类，代码如下：从上面的代码分析，可以知道属性的注入，是靠ReferenceAnnotationBeanPostProcessor后置处理来触发，往filed设置值。如果这一过程中，发生异常，导致没有成功为field设置值，则加了@Referencce的属性就会一直是null。2020-07-30 17:00:00.013 WARN 13092 — [ main] com.alibaba.dubbo.config.AbstractConfig : [] [DUBBO] null, dubbo version: 2.6.2, current host: 10.0.45.150java.lang.reflect.InvocationTargetException: null
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.alibaba.dubbo.config.AbstractConfig.toString(AbstractConfig.java:474)
at java.lang.String.valueOf(String.java:2994)
at java.lang.StringBuilder.append(StringBuilder.java:131)
at com.alibaba.dubbo.config.spring.beans.factory.annotation.AbstractAnnotationConfigBeanBuilder.build(AbstractAnnotationConfigBeanBuilder.java:79)
at com.alibaba.dubbo.config.spring.beans.factory.annotation.ReferenceAnnotationBeanPostProcessor.buildReferenceBean(ReferenceAnnotationBeanPostProcessor.java:385)
at com.alibaba.dubbo.config.spring.beans.factory.annotation.ReferenceAnnotationBeanPostProcessor.access$100(ReferenceAnnotationBeanPostProcessor.java:65)
at com.alibaba.dubbo.config.spring.beans.factory.annotation.ReferenceAnnotationBeanPostProcessor$ReferenceFieldElement.inject(ReferenceAnnotationBeanPostProcessor.java:363)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)
at com.alibaba.dubbo.config.spring.beans.factory.annotation.ReferenceAnnotationBeanPostProcessor.postProcessPropertyValues(ReferenceAnnotationBeanPostProcessor.java:92)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1400)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:277)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1247)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1167)
at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:857)
at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:760)
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:218)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1325)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1171)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:849)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:877)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775)
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:316)
at com.xdchen.bp.award.api.server.Application.main(Application.java:20)
Caused by: java.lang.IllegalStateException: Failed to check the status of the service com.xdchen.searchplatform.searcher.protocol.SearcherService. No provider available for the service com.xdchen.searchplatform.searcher.protocol.SearcherService:1.0.0111 from the url zookeeper://zk1.esf.fdd:2181/com.alibaba.dubbo.registry.RegistryService?application=award.ddxf.bp.fdd&default.reference.filter=traceIdConsumer,default,consumerCatFilter&default.timeout=6000&dubbo=2.6.2&interface=com.xdchen.searchplatform.searcher.protocol.SearcherService&methods=search,searchByBytes,m免费云主机域名ultiSearch,scrollIndex,searchByHttp,searchByIds,multiSearchByBytes&organization=fangdd&owner=chenxudong&pid=13092&register.ip=10.0.45.150&revision=3.8.0&side=consumer&timeout=5000&timestamp=1596099599500&version=1.0.0111 to the consumer 10.0.45.150 use dubbo version 2.6.2
at com.alibaba.dubbo.config.ReferenceConfig.createProxy(ReferenceConfig.java:422)
at com.alibaba.dubbo.config.ReferenceConfig.init(ReferenceConfig.java:333)
at com.alibaba.dubbo.config.ReferenceConfig.get(ReferenceConfig.java:163)
at com.alibaba.dubbo.config.spring.ReferenceBean.getObject(ReferenceBean.java:66)
… 42 common frames omitted2020-07-30 17:00:00.014 INFO 13092 — [ main] c.a.d.c.s.b.f.a.ReferenceBeanBuilder : [] has been built.看这一段错误日志，当SearcherService没有任何provider启动的时候调用ReferenceBean.getObject方法，就会抛IllegalStateException异常，设置属性失败。网上很多说，遇到加@Reference注解的属性为null的，应该就是这个情况。buildReferenceBean方法调用ReferenceBeanBuilder.build报错ReferenceBeanBuilder.build方法是它的父类AbstractAnnotationConfigBeanBuilder的ReferenceBeanBuilder重写了doBuild方法，返回ReferenceBean对象所以，问题是出在了ReferenceBean.toString方法上ReferenceBean并没有重写toString方法，但他的根父类是AbstractConfig，看错误日志，可以看到这个：at com.alibaba.dubbo.config.AbstractConfig.toString(AbstractConfig.java:474)AbstractConfig.toString代码如下：ReferenceBean类实现类FactoryBean接口，实现了getObject方法，getObject方法满足get开头的条件，会被AbstractConfig.toString方法调用到省略了大部分代码，只保留了比较重要的，调用getObject方法，会判断是否初始化过，如果初始化过，直接返回ref；如果没有初始化，则会进行初始化，然后调用createProxy方法来创建代理，如果我们没有配置consumer的check或者check=true，则会检查invoker对象的可用性“invoker.isAvailable()”，如果不可用，就会抛IllegalStateException异常。配置消费者的检查为false，即@Reference(check=false)看的ReferenceFieldElement.inject方法，很容易以为IllegalStateException是在 “field.set(bean, referenceBean.getObject());”这一行报错的，但实际上是在 “referenceBean = buildReferenceBean(reference, referenceClass);”为什么要在AbstractConfig.toString就调用了getObject方法，触发报错呢？如果AbstractConfig.toString过滤掉getObject方法，会发生什么事情呢？InjectionMetadata.inject方法是遍历checkedElements列表，挨个调用element.inject方法。如果AbstractConfig.toString过滤掉getObject方法，则首次调用ReferenceBean.getObject方法是在“field.set(bean, referenceBean.getObject());”。异常没有被catch住，checkedElements列表的遍历会被打断。会出现这样的情况，有一个bean需要注入5个代理对象，但是调用第一个ReferenceBean.getObject的时候抛异常，则注入行为被中断，另外4个属性也没有被注入。关于“dubbo之@Reference注解有什么作用”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识，可以关注百云主机行业资讯频道，小编每天都会为大家更新不同的知识点。

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