dubbo之@Reference注解有什么作用

發(fā)布時(shí)間：2023-03-21 14:18:07 來(lái)源：億速云閱讀：76 作者：iii 欄目：開(kāi)發(fā)技術(shù)

這篇文章主要介紹“dubbo之@Reference注解有什么作用”的相關(guān)知識(shí)，小編通過(guò)實(shí)際案例向大家展示操作過(guò)程，操作方法簡(jiǎn)單快捷，實(shí)用性強(qiáng)，希望這篇“dubbo之@Reference注解有什么作用”文章能幫助大家解決問(wèn)題。

目的

看看dubbo是怎么給加了@Reference注解的屬性注入invoker實(shí)例，為什么有時(shí)候加了@Reference注解的屬性會(huì)是null。

ReferenceAnnotationBeanPostProcessor

看到這個(gè)名字，就很容易知道，是專(zhuān)門(mén)針對(duì)@Reference注解的后置處理。

ReferenceAnnotationBeanPostProcessor的代碼比較多，下面列一下比較重要的內(nèi)容。

public class ReferenceAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
        implements MergedBeanDefinitionPostProcessor, PriorityOrdered, ApplicationContextAware, BeanClassLoaderAware,
        DisposableBean {
    // 緩存加了@Referece注解的元數(shù)據(jù)信息，key是bean的名稱(chēng)或者類(lèi)名，value是加了@Reference的屬性和方法
    private final ConcurrentMap<String, ReferenceInjectionMetadata> injectionMetadataCache =
            new ConcurrentHashMap<String, ReferenceInjectionMetadata>(256);
	// 緩存new過(guò)的ReferenceBean，相同的key，只會(huì)產(chǎn)生一個(gè)ReferenceBean
    private final ConcurrentMap<String, ReferenceBean<?>> referenceBeansCache =
            new ConcurrentHashMap<String, ReferenceBean<?>>();

	// spring設(shè)置屬性到bean之前調(diào)用該方法
    @Override
    public PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
		// 根據(jù)bean的類(lèi)型，獲取需要注入的元數(shù)據(jù)信息
        InjectionMetadata metadata = findReferenceMetadata(beanName, bean.getClass(), pvs);
        try {
            // 注入對(duì)象
            metadata.inject(bean, beanName, pvs);
        } catch (BeanCreationException ex) {
            throw ex;
        } catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Injection of @Reference dependencies failed", ex);
        }
        return pvs;
    }

  private InjectionMetadata findReferenceMetadata(String beanName, Class<?> clazz, PropertyValues pvs) {
        // 如果是自定義的消費(fèi)者，沒(méi)有beanName，退化成使用類(lèi)名作為緩存key
        String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
        // 雙重檢查，判斷是否需要刷新注入信息
        ReferenceInjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
        // 判斷是否需要刷新
        if (InjectionMetadata.needsRefresh(metadata, clazz)) {
        	// 第一次判斷為需要刷新，則鎖住injectionMetadataCache對(duì)象
            synchronized (this.injectionMetadataCache) {
            	// 再次判斷是否需要刷新
                metadata = this.injectionMetadataCache.get(cacheKey);
                if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                	// 需要刷新，而且原來(lái)緩存的信息不為空，清除緩存信息
                    if (metadata != null) {
                        metadata.clear(pvs);
                    }
                    try {
                    	// 生成新的元數(shù)據(jù)信息
                        metadata = buildReferenceMetadata(clazz);
                        // 放入緩存
                        this.injectionMetadataCache.put(cacheKey, metadata);
                    } catch (NoClassDefFoundError err) {
                        throw new IllegalStateException("Failed to introspect bean class [" + clazz.getName() +
                                "] for reference metadata: could not find class that it depends on", err);
                    }
                }
            }
        }
        return metadata;
    }

    private ReferenceInjectionMetadata buildReferenceMetadata(final Class<?> beanClass) {
    	// 查找加了@Reference注解的屬性
        Collection<ReferenceFieldElement> fieldElements = findFieldReferenceMetadata(beanClass);
        // 查找加了@Reference注解的屬性
        // !!!!@Reference還能加到方法上!!!還真沒(méi)試過(guò)
        // 不過(guò)這個(gè)不關(guān)心，只關(guān)注屬性的
        Collection<ReferenceMethodElement> methodElements = findMethodReferenceMetadata(beanClass);
        return new ReferenceInjectionMetadata(beanClass, fieldElements, methodElements);
    }

    private List<ReferenceFieldElement> findFieldReferenceMetadata(final Class<?> beanClass) {
		
		// 保存加了@Reference注解的屬性列表
        final List<ReferenceFieldElement> elements = new LinkedList<ReferenceFieldElement>();

        ReflectionUtils.doWithFields(beanClass, new ReflectionUtils.FieldCallback() {
            @Override
            public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
				// 獲取屬性上的@Reference注解
                Reference reference = getAnnotation(field, Reference.class);
				// 如果存在@Reference注解
                if (reference != null) {
					// 不支持靜態(tài)屬性的注入
                    if (Modifier.isStatic(field.getModifiers())) {
                        if (logger.isWarnEnabled()) {
                            logger.warn("@Reference annotation is not supported on static fields: " + field);
                        }
                        return;
                    }
					// 添加到隊(duì)列里
                    elements.add(new ReferenceFieldElement(field, reference));
                }

            }
        });

        return elements;

    }

ReferenceInjectionMetadata

dubbo在ReferenceAnnotationBeanPostProcessor里定義了一個(gè)私有的子類(lèi)

ReferenceInjectionMetadata繼承spring定義的InjectionMetadata類(lèi)。

之所以需要自定義ReferenceInjectionMetadata類(lèi)，是因?yàn)閐ubbo的@Reference注解可以使用在屬性和方法上，需要區(qū)分開(kāi)。但是spring定義的InjectionMetadata類(lèi)，只支持一個(gè)injectedElements集合，代碼如下：

public class InjectionMetadata {

	private static final Log logger = LogFactory.getLog(InjectionMetadata.class);

	private final Class<?> targetClass;

	private final Collection<InjectedElement> injectedElements;

	@Nullable
	private volatile Set<InjectedElement> checkedElements;


	public InjectionMetadata(Class<?> targetClass, Collection<InjectedElement> elements) {
		// 構(gòu)造函數(shù)接收兩個(gè)參數(shù)，類(lèi)型，注入的元素
		this.targetClass = targetClass;
		this.injectedElements = elements;
	}
    ......
    ......
    ......
	public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
		Collection<InjectedElement> checkedElements = this.checkedElements;
		// 優(yōu)先使用checkedElements來(lái)注入，如果checkedElements為空，則直接使用injectedElements（沒(méi)有調(diào)用checkConfigMembers方法，checkedElements會(huì)空）
		Collection<InjectedElement> elementsToIterate =
				(checkedElements != null ? checkedElements : this.injectedElements);
		if (!elementsToIterate.isEmpty()) {
		    // 循環(huán)遍歷所有待注入的元素
			for (InjectedElement element : elementsToIterate) {
				if (logger.isTraceEnabled()) {
					logger.trace("Processing injected element of bean '" + beanName + "': " + element);
				}
				// 調(diào)用注入方法
				element.inject(target, beanName, pvs);
			}
		}
	}

基于這個(gè)原因，dubbo定義了ReferenceInjectionMetadata類(lèi)，代碼如下：

 private static class ReferenceInjectionMetadata extends InjectionMetadata {

        private final Collection<ReferenceFieldElement> fieldElements;

        private final Collection<ReferenceMethodElement> methodElements;


        public ReferenceInjectionMetadata(Class<?> targetClass, Collection<ReferenceFieldElement> fieldElements,
                                          Collection<ReferenceMethodElement> methodElements) {
            // 構(gòu)造函數(shù)接收3個(gè)參數(shù)，類(lèi)型，待注入的屬性元素，待注入的方法元素
            // 把fieldElements和methodElements的內(nèi)容合并，作為InjectionMetadata的injectedElements
            super(targetClass, combine(fieldElements, methodElements));
            this.fieldElements = fieldElements;
            this.methodElements = methodElements;
        }

        private static <T> Collection<T> combine(Collection<? extends T>... elements) {
            List<T> allElements = new ArrayList<T>();
            for (Collection<? extends T> e : elements) {
                allElements.addAll(e);
            }
            return allElements;
        }

        public Collection<ReferenceFieldElement> getFieldElements() {
            return fieldElements;
        }

        public Collection<ReferenceMethodElement> getMethodElements() {
            return methodElements;
        }
    }

代碼很簡(jiǎn)單，入?yún)⒆優(yōu)?個(gè)，屬性和方法列表區(qū)分開(kāi)，然后把兩者合并起來(lái)，調(diào)用父類(lèi)的構(gòu)造函數(shù)，用fieldElements保存屬性列表，用methodElements保存方法列表。

ReferenceFieldElement

屬性注入實(shí)際發(fā)生在ReferenceFieldElement類(lèi)，代碼如下：

 private class ReferenceFieldElement extends InjectionMetadata.InjectedElement {

        private final Field field;

        private final Reference reference;

        private volatile ReferenceBean<?> referenceBean;

        // 構(gòu)造函數(shù)會(huì)傳入要設(shè)置的Field對(duì)象，Reference注解對(duì)象
        protected ReferenceFieldElement(Field field, Reference reference) {
            super(field, null);
            this.field = field;
            this.reference = reference;
        }

        @Override
        protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {

            Class<?> referenceClass = field.getType();
			// 構(gòu)建ReferenceBean對(duì)象
            referenceBean = buildReferenceBean(reference, referenceClass);
			// 將屬性設(shè)置為可訪(fǎng)問(wèn)的
            ReflectionUtils.makeAccessible(field);
			// 給Field對(duì)象設(shè)置屬性
            field.set(bean, referenceBean.getObject());

        }

    }

ReferenceMethodElement

方法注入實(shí)際發(fā)生在ReferenceMethodElement類(lèi)，代碼如下：

private class ReferenceMethodElement extends InjectionMetadata.InjectedElement {

        private final Method method;

        private final Reference reference;

        private volatile ReferenceBean<?> referenceBean;

        protected ReferenceMethodElement(Method method, PropertyDescriptor pd, Reference reference) {
            super(method, pd);
            this.method = method;
            this.reference = reference;
        }

        @Override
        protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
			// 獲取類(lèi)型
            Class<?> referenceClass = pd.getPropertyType();
			// 構(gòu)建ReferenceBean對(duì)象
            referenceBean = buildReferenceBean(reference, referenceClass);
			// 將方法設(shè)置為可訪(fǎng)問(wèn)
            ReflectionUtils.makeAccessible(method);
			// 把referenceBean生成的對(duì)象作為入?yún)?，調(diào)用bean對(duì)象的method方法
			// 看到這里，就能明白，@Reference也可以加在那些setXXX方法上
			// 例如有個(gè)userService屬性，有個(gè)setUserService方法，可以在setUserService方法上加@Reference注解
            method.invoke(bean, referenceBean.getObject());

        }

    }

為什么加了@Reference注解的屬性是null

從上面的代碼分析，可以知道屬性的注入，是靠ReferenceAnnotationBeanPostProcessor后置處理來(lái)觸發(fā)，往filed設(shè)置值。

如果這一過(guò)程中，發(fā)生異常，導(dǎo)致沒(méi)有成功為field設(shè)置值，則加了@Referencce的屬性就會(huì)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: 10.0.45.150
java.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,multiSearch,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 omitted
2020-07-30 17:00:00.014 INFO 13092 --- [ main] c.a.d.c.s.b.f.a.ReferenceBeanBuilder : [] <dubbo:reference singleton="true" interface="com.xdchen.searchplatform.searcher.protocol.SearcherService" uniqueServiceName="com.xdchen.searchplatform.searcher.protocol.SearcherService:1.0.0111" generic="false" version="1.0.0111" timeout="5000" id="com.xdchen.searchplatform.searcher.protocol.SearcherService" /> has been built.

看這一段錯(cuò)誤日志，當(dāng)SearcherService沒(méi)有任何provider啟動(dòng)的時(shí)候調(diào)用ReferenceBean.getObject方法，就會(huì)拋IllegalStateException異常，設(shè)置屬性失敗。

網(wǎng)上很多說(shuō)，遇到加@Reference注解的屬性為null的，應(yīng)該就是這個(gè)情況。

什么情況會(huì)拋No provider的IllegalStateException異常

ReferenceAnnotationBeanPostProcessor.buildReferenceBean

   private ReferenceBean<?> buildReferenceBean(Reference reference, Class<?> referenceClass) throws Exception {

        String referenceBeanCacheKey = generateReferenceBeanCacheKey(reference, referenceClass);

        ReferenceBean<?> referenceBean = referenceBeansCache.get(referenceBeanCacheKey);

        if (referenceBean == null) {

            ReferenceBeanBuilder beanBuilder = ReferenceBeanBuilder
                    .create(reference, classLoader, applicationContext)
                    .interfaceClass(referenceClass);
			// 這里報(bào)錯(cuò)
            referenceBean = beanBuilder.build();

            referenceBeansCache.putIfAbsent(referenceBeanCacheKey, referenceBean);

        }

        return referenceBean;

    }

buildReferenceBean方法調(diào)用ReferenceBeanBuilder.build報(bào)錯(cuò)

ReferenceBeanBuilder.build

ReferenceBeanBuilder.build方法是它的父類(lèi)AbstractAnnotationConfigBeanBuilder的

 public final B build() throws Exception {

        checkDependencies();
		
        B bean = doBuild();

        configureBean(bean);
        
        if (logger.isInfoEnabled()) {
        	// 這里報(bào)錯(cuò)
            logger.info(bean + " has been built.");
        }

        return bean;

    }

ReferenceBeanBuilder.doBuild

ReferenceBeanBuilder重寫(xiě)了doBuild方法，返回ReferenceBean對(duì)象

    @Override
    protected ReferenceBean doBuild() {
        return new ReferenceBean<Object>();
    }

所以，問(wèn)題是出在了ReferenceBean.toString方法上

AbstractConfig.toString

ReferenceBean并沒(méi)有重寫(xiě)toString方法，但他的根父類(lèi)是AbstractConfig，看錯(cuò)誤日志，可以看到這個(gè)：

at com.alibaba.dubbo.config.AbstractConfig.toString(AbstractConfig.java:474)

AbstractConfig.toString代碼如下：

 @Override
    public String toString() {
        try {
            StringBuilder buf = new StringBuilder();
            buf.append("<dubbo:");
            buf.append(getTagName(getClass()));
            Method[] methods = getClass().getMethods();
            // 拿到當(dāng)前類(lèi)的所有方法
            for (Method method : methods) {
                try {
                    String name = method.getName();
                    // 過(guò)濾剩下get和is開(kāi)頭的方法，但不包括getClass、get和is
                    if ((name.startsWith("get") || name.startsWith("is"))
                            && !"getClass".equals(name) && !"get".equals(name) && !"is".equals(name)
                            && Modifier.isPublic(method.getModifiers())
                            && method.getParameterTypes().length == 0
                            && isPrimitive(method.getReturnType())) {
                        int i = name.startsWith("get") ? 3 : 2;
                        String key = name.substring(i, i + 1).toLowerCase() + name.substring(i + 1);
                        // 反射獲取方法返回值，拼接字符串
                        // 就是這里報(bào)空指針
                        Object value = method.invoke(this, new Object[0]);
                        if (value != null) {
                            buf.append(" ");
                            buf.append(key);
                            buf.append("=\"");
                            buf.append(value);
                            buf.append("\"");
                        }
                    }
                } catch (Exception e) {
                    logger.warn(e.getMessage(), e);
                }
            }
            buf.append(" />");
            return buf.toString();
        } catch (Throwable t) {
            logger.warn(t.getMessage(), t);
            return super.toString();
        }
    }

ReferenceBean.getObjet

ReferenceBean類(lèi)實(shí)現(xiàn)類(lèi)FactoryBean接口，實(shí)現(xiàn)了getObject方法，getObject方法滿(mǎn)足get開(kāi)頭的條件，會(huì)被AbstractConfig.toString方法調(diào)用到

public class ReferenceBean<T> extends ReferenceConfig<T> implements FactoryBean, ApplicationContextAware, InitializingBean, DisposableBean {
    @Override
    public Object getObject() throws Exception {
        return get();
    }
    
    public synchronized T get() {
        if (destroyed) {
            throw new IllegalStateException("Already destroyed!");
        }
        if (ref == null) {
            init();
        }
        return ref;
    }

    private void init() {
        if (initialized) {
            return;
        }
        initialized = true;
        ......
        ......
        ref = createProxy(map);
        ConsumerModel consumerModel = new ConsumerModel(getUniqueServiceName(), this, ref, interfaceClass.getMethods());
        ApplicationModel.initConsumerModel(getUniqueServiceName(), consumerModel);
    }

    private T createProxy(Map<String, String> map) {
        ......
		......
        Boolean c = check;
        if (c == null && consumer != null) {
            c = consumer.isCheck();
        }
        if (c == null) {
            c = true; // default true
        }
        if (c && !invoker.isAvailable()) {
            throw new IllegalStateException("Failed to check the status of the service " + interfaceName + ". No provider available for the service " + (group == null ? "" : group + "/") + interfaceName + (version == null ? "" : ":" + version) + " from the url " + invoker.getUrl() + " to the consumer " + NetUtils.getLocalHost() + " use dubbo version " + Version.getVersion());
        }
        if (logger.isInfoEnabled()) {
            logger.info("Refer dubbo service " + interfaceClass.getName() + " from url " + invoker.getUrl());
        }
        // create service proxy
        return (T) proxyFactory.getProxy(invoker);
    }

省略了大部分代碼，只保留了比較重要的，調(diào)用getObject方法，會(huì)判斷是否初始化過(guò)，如果初始化過(guò)，直接返回ref；如果沒(méi)有初始化，則會(huì)進(jìn)行初始化，然后調(diào)用createProxy方法來(lái)創(chuàng)建代理，如果我們沒(méi)有配置consumer的check或者check=true，則會(huì)檢查invoker對(duì)象的可用性“invoker.isAvailable()”，如果不可用，就會(huì)拋IllegalStateException異常。

避免@Reference注解的屬性為null

配置消費(fèi)者的檢查為false，即@Reference(check=false)

ReferenceBean.getObject調(diào)用時(shí)機(jī)的猜測(cè)

看的ReferenceFieldElement.inject方法，很容易以為IllegalStateException是在 “field.set(bean, referenceBean.getObject());”這一行報(bào)錯(cuò)的，但實(shí)際上是在 “referenceBean = buildReferenceBean(reference, referenceClass);”

 @Override
        protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {

            Class<?> referenceClass = field.getType();
			// 構(gòu)建ReferenceBean對(duì)象
            referenceBean = buildReferenceBean(reference, referenceClass);
			// 將屬性設(shè)置為可訪(fǎng)問(wèn)的
            ReflectionUtils.makeAccessible(field);
			// 給Field對(duì)象設(shè)置屬性
            field.set(bean, referenceBean.getObject());

        }

為什么要在AbstractConfig.toString就調(diào)用了getObject方法，觸發(fā)報(bào)錯(cuò)呢？

如果AbstractConfig.toString過(guò)濾掉getObject方法，會(huì)發(fā)生什么事情呢？

InjectionMetadata.inject方法是遍歷checkedElements列表，挨個(gè)調(diào)用element.inject方法。

如果AbstractConfig.toString過(guò)濾掉getObject方法，則首次調(diào)用ReferenceBean.getObject方法是在“field.set(bean, referenceBean.getObject());”。異常沒(méi)有被catch住，checkedElements列表的遍歷會(huì)被打斷。

會(huì)出現(xiàn)這樣的情況，有一個(gè)bean需要注入5個(gè)代理對(duì)象，但是調(diào)用第一個(gè)ReferenceBean.getObject的時(shí)候拋異常，則注入行為被中斷，另外4個(gè)屬性也沒(méi)有被注入。

關(guān)于“dubbo之@Reference注解有什么作用”的內(nèi)容就介紹到這里了，感謝大家的閱讀。如果想了解更多行業(yè)相關(guān)的知識(shí)，可以關(guān)注億速云行業(yè)資訊頻道，小編每天都會(huì)為大家更新不同的知識(shí)點(diǎn)。

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