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這篇文章主要介紹“dubbo之@Reference注解有什么作用”的相關知識，小編通過實際案例向大家展示操作過程，操作方法簡單快捷，實用性強，希望這篇“dubbo之@Reference注解有什么作用”文章能幫助大家解決問題。
看看dubbo是怎么給加了@Reference注解的屬性注入invoker實例，為什么有時候加了@Reference注解的屬性會是null。
看到這個名字，就很容易知道，是專門針對@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容。
public class ReferenceAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, ApplicationContextAware, BeanClassLoaderAware,
DisposableBean {
// 緩存加了@Referece注解的元數據信息，key是bean的名稱或者類名，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過的ReferenceBean，相同的key，只會產生一個ReferenceBean
private final ConcurrentMap<String, ReferenceBean<?>> referenceBeansCache =
new ConcurrentHashMap<String, ReferenceBean<?>>();
// spring設置屬性到bean之前調用該方法
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
// 根據bean的類型，獲取需要注入的元數據信息
InjectionMetadata metadata = findReferenceMetadata(beanName, bean.getClass(), pvs);
try {
// 注入對象
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者，沒有beanName，退化成使用類名作為緩存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 {
// 生成新的元數據信息
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!!!還真沒試過
// 不過這個不關心，只關注屬性的
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;
}
// 添加到隊列里
elements.add(new ReferenceFieldElement(field, reference));
}
}
});
return elements;
}dubbo在ReferenceAnnotationBeanPostProcessor里定義了一個私有的子類
ReferenceInjectionMetadata繼承spring定義的InjectionMetadata類。
之所以需要自定義ReferenceInjectionMetadata類，是因為dubbo的@Reference注解可以使用在屬性和方法上，需要區分開。但是spring定義的InjectionMetadata類，只支持一個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) {
// 構造函數接收兩個參數，類型，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;
// 優先使用checkedElements來注入，如果checkedElements為空，則直接使用injectedElements（沒有調用checkConfigMembers方法，checkedElements會空）
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
// 循環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);
}
// 調用注入方法
element.inject(target, beanName, pvs);
}
}
}基于這個原因，dubbo定義了ReferenceInjectionMetadata類，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) {
// 構造函數接收3個參數，類型，待注入的屬性元素，待注入的方法元素
// 把fieldElements和methodElements的內容合并，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;
}
}代碼很簡單，入參變為3個，屬性和方法列表區分開，然后把兩者合并起來，調用父類的構造函數，用fieldElements保存屬性列表，用methodElements保存方法列表。
屬性注入實際發生在ReferenceFieldElement類，代碼如下：
private class ReferenceFieldElement extends InjectionMetadata.InjectedElement {
private final Field field;
private final Reference reference;
private volatile ReferenceBean<?> referenceBean;
// 構造函數會傳入要設置的Field對象，Reference注解對象
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();
// 構建ReferenceBean對象
referenceBean = buildReferenceBean(reference, referenceClass);
// 將屬性設置為可訪問的
ReflectionUtils.makeAccessible(field);
// 給Field對象設置屬性
field.set(bean, referenceBean.getObject());
}
}方法注入實際發生在ReferenceMethodElement類，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 {
// 獲取類型
Class<?> referenceClass = pd.getPropertyType();
// 構建ReferenceBean對象
referenceBean = buildReferenceBean(reference, referenceClass);
// 將方法設置為可訪問
ReflectionUtils.makeAccessible(method);
// 把referenceBean生成的對象作為入參，調用bean對象的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userService屬性，有個setUserService方法，可以在setUserService方法上加@Reference注解
method.invoke(bean, referenceBean.getObject());
}
}從上面的代碼分析，可以知道屬性的注入，是靠ReferenceAnnotationBeanPostProcessor后置處理來觸發，往filed設置值。
如果這一過程中，發生異常，導致沒有成功為field設置值，則加了@Referencce的屬性就會一直是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.
看這一段錯誤日志，當SearcherService沒有任何provider啟動的時候調用ReferenceBean.getObject方法，就會拋IllegalStateException異常，設置屬性失敗。
網上很多說，遇到加@Reference注解的屬性為null的，應該就是這個情況。
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);
// 這里報錯
referenceBean = beanBuilder.build();
referenceBeansCache.putIfAbsent(referenceBeanCacheKey, referenceBean);
}
return referenceBean;
}buildReferenceBean方法調用ReferenceBeanBuilder.build報錯
ReferenceBeanBuilder.build方法是它的父類AbstractAnnotationConfigBeanBuilder的
public final B build() throws Exception {
checkDependencies();
B bean = doBuild();
configureBean(bean);
if (logger.isInfoEnabled()) {
// 這里報錯
logger.info(bean + " has been built.");
}
return bean;
}ReferenceBeanBuilder重寫了doBuild方法，返回ReferenceBean對象
@Override
protected ReferenceBean doBuild() {
return new ReferenceBean<Object>();
}所以，問題是出在了ReferenceBean.toString方法上
ReferenceBean并沒有重寫toString方法，但他的根父類是AbstractConfig，看錯誤日志，可以看到這個：
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();
// 拿到當前類的所有方法
for (Method method : methods) {
try {
String name = method.getName();
// 過濾剩下get和is開頭的方法，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類實現類FactoryBean接口，實現了getObject方法，getObject方法滿足get開頭的條件，會被AbstractConfig.toString方法調用到
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，調用getObject方法，會判斷是否初始化過，如果初始化過，直接返回ref；如果沒有初始化，則會進行初始化，然后調用createProxy方法來創建代理，如果我們沒有配置consumer的check或者check=true，則會檢查invoker對象的可用性“invoker.isAvailable()”，如果不可用，就會拋IllegalStateException異常。
配置消費者的檢查為false，即@Reference(check=false)
看的ReferenceFieldElement.inject方法，很容易以為IllegalStateException是在 “field.set(bean, referenceBean.getObject());”這一行報錯的，但實際上是在 “referenceBean = buildReferenceBean(reference, referenceClass);”
@Override
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
Class<?> referenceClass = field.getType();
// 構建ReferenceBean對象
referenceBean = buildReferenceBean(reference, referenceClass);
// 將屬性設置為可訪問的
ReflectionUtils.makeAccessible(field);
// 給Field對象設置屬性
field.set(bean, referenceBean.getObject());
}為什么要在AbstractConfig.toString就調用了getObject方法，觸發報錯呢？
如果AbstractConfig.toString過濾掉getObject方法，會發生什么事情呢？
InjectionMetadata.inject方法是遍歷checkedElements列表，挨個調用element.inject方法。
如果AbstractConfig.toString過濾掉getObject方法，則首次調用ReferenceBean.getObject方法是在“field.set(bean, referenceBean.getObject());”。異常沒有被catch住，checkedElements列表的遍歷會被打斷。
會出現這樣的情況，有一個bean需要注入5個代理對象，但是調用第一個ReferenceBean.getObject的時候拋異常，則注入行為被中斷，另外4個屬性也沒有被注入。
關于“dubbo之@Reference注解有什么作用”的內容就介紹到這里了，感謝大家的閱讀。如果想了解更多行業相關的知識，可以關注億速云行業資訊頻道，小編每天都會為大家更新不同的知識點。
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