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。
目的
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。
ReferenceAnnotationBeanPostProcessor
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。
public class ReferenceAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, ApplicationContextAware, BeanClassLoaderAware,
DisposableBean {
// 缓存加了@Referece注解的元数据信息，key是bean的名称或者类名，value是加了@Reference的属性和方法
private final ConcurrentMap<String, ReferenceInjectionMetadata> injectionMetadataCache =
new ConcurrentHashMap<String, ReferenceInjectionMetadata>(256);
// 缓存new过的ReferenceBean，相同的key，只会产生一个ReferenceBean
private final ConcurrentMap<String, ReferenceBean<?>> referenceBeansCache =
new ConcurrentHashMap<String, ReferenceBean<?>>();
// spring设置属性到bean之前调用该方法
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
// 根据bean的类型，获取需要注入的元数据信息
InjectionMetadata metadata = findReferenceMetadata(beanName, bean.getClass(), pvs);
try {
// 注入对象
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) {
// 如果是自定义的消费者，没有beanName，退化成使用类名作为缓存key
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// 双重检查，判断是否需要刷新注入信息
ReferenceInjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
// 判断是否需要刷新
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
// 第一次判断为需要刷新，则锁住injectionMetadataCache对象
synchronized (this.injectionMetadataCache) {
// 再次判断是否需要刷新
metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
// 需要刷新，而且原来缓存的信息不为空，清除缓存信息
if (metadata != null) {
metadata.clear(pvs);
}
try {
// 生成新的元数据信息
metadata = buildReferenceMetadata(clazz);
// 放入缓存
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注解的属性
Collection<ReferenceFieldElement> fieldElements = findFieldReferenceMetadata(beanClass);
// 查找加了@Reference注解的属性
// !!!!@Reference还能加到方法上!!!还真没试过
// 不过这个不关心，只关注属性的
Collection<ReferenceMethodElement> methodElements = findMethodReferenceMetadata(beanClass);
return new ReferenceInjectionMetadata(beanClass, fieldElements, methodElements);
}
private List<ReferenceFieldElement> findFieldReferenceMetadata(final Class<?> beanClass) {
// 保存加了@Reference注解的属性列表
final List<ReferenceFieldElement> elements = new LinkedList<ReferenceFieldElement>();
ReflectionUtils.doWithFields(beanClass, new ReflectionUtils.FieldCallback() {
@Override
public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
// 获取属性上的@Reference注解
Reference reference = getAnnotation(field, Reference.class);
// 如果存在@Reference注解
if (reference != null) {
// 不支持静态属性的注入
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("@Reference annotation is not supported on static fields: " + field);
}
return;
}
// 添加到队列里
elements.add(new ReferenceFieldElement(field, reference));
}
}
});
return elements;
}ReferenceInjectionMetadata
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：
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) {
// 构造函数接收两个参数，类型，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;
// 优先使用checkedElements来注入，如果checkedElements为空，则直接使用injectedElements（没有调用checkConfigMembers方法，checkedElements会空）
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
// 循环遍历所有待注入的元素
for (InjectedElement element : elementsToIterate) {
if (logger.isTraceEnabled()) {
logger.trace("Processing injected element of bean '" + beanName + "': " + element);
}
// 调用注入方法
element.inject(target, beanName, pvs);
}
}
}基于这个原因，dubbo定义了ReferenceInjectionMetadata类，代码如下：
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) {
// 构造函数接收3个参数，类型，待注入的属性元素，待注入的方法元素
// 把fieldElements和methodElements的内容合并，作为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。
ReferenceFieldElement
属性注入实际发生在ReferenceFieldElement类，代码如下：
private class ReferenceFieldElement extends InjectionMetadata.InjectedElement {
private final Field field;
private final Reference reference;
private volatile ReferenceBean<?> referenceBean;
// 构造函数会传入要设置的Field对象，Reference注解对象
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();
// 构建ReferenceBean对象
referenceBean = buildReferenceBean(reference, referenceClass);
// 将属性设置为可访问的
ReflectionUtils.makeAccessible(field);
// 给Field对象设置属性
field.set(bean, referenceBean.getObject());
}
}ReferenceMethodElement
方法注入实际发生在ReferenceMethodElement类，代码如下：
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 {
// 获取类型
Class<?> referenceClass = pd.getPropertyType();
// 构建ReferenceBean对象
referenceBean = buildReferenceBean(reference, referenceClass);
// 将方法设置为可访问
ReflectionUtils.makeAccessible(method);
// 把referenceBean生成的对象作为入参，调用bean对象的method方法
// 看到这里，就能明白，@Reference也可以加在那些setXXX方法上
// 例如有个userService属性，有个setUserService方法，可以在setUserService方法上加@Reference注解
method.invoke(bean, referenceBean.getObject());
}
}为什么加了@Reference注解的属性是null
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: 10.0.45.150
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®ister.ip=10.0.45.150&revision=3.8.0&side=consumer&timeout=5000×tamp=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 : [] <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。
什么情况会抛No provider的IllegalStateException异常
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);
// 这里报错
referenceBean = beanBuilder.build();
referenceBeansCache.putIfAbsent(referenceBeanCacheKey, referenceBean);
}
return referenceBean;
}buildReferenceBean方法调用ReferenceBeanBuilder.build报错
ReferenceBeanBuilder.build
ReferenceBeanBuilder.build方法是它的父类AbstractAnnotationConfigBeanBuilder的
public final B build() throws Exception {
checkDependencies();
B bean = doBuild();
configureBean(bean);
if (logger.isInfoEnabled()) {
// 这里报错
logger.info(bean + " has been built.");
}
return bean;
}ReferenceBeanBuilder.doBuild
ReferenceBeanBuilder重写了doBuild方法，返回ReferenceBean对象
@Override
protected ReferenceBean doBuild() {
return new ReferenceBean<Object>();
}所以，问题是出在了ReferenceBean.toString方法上
AbstractConfig.toString
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：
@Override
public String toString() {
try {
StringBuilder buf = new StringBuilder();
buf.append("<dubbo:");
buf.append(getTagName(getClass()));
Method[] methods = getClass().getMethods();
// 拿到当前类的所有方法
for (Method method : methods) {
try {
String name = method.getName();
// 过滤剩下get和is开头的方法，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);
// 反射获取方法返回值，拼接字符串
// 就是这里报空指针
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
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。
避免@Reference注解的属性为null
配置消费者的检查为false，即@Reference(check=false)
Referenc