dubbo学习篇1 注解之 @Reference 原理解析

一. 使用注解

在dubbo springboot 使用时，在需要调用的服务接口上使用 @Reference 即可直接调用远程服务

@Reference(version = "1.0.0",
            application = "${dubbo.application.id}")
    private HelloService helloService;

比如上述样式 调试发现调用时 helloSevice为一个生产的代理对象如下图:

二.分析原理

@Reference 的行为跟 @Autowired 类似 均实现了自动注入的过程 。

首先参考下https://blog.csdn.net/qq_27529917/article/details/78454912 学习 @Autowired 工作原理 ，可跳过。

@Reference 的注解处理依赖于 ReferenceAnnotationBeanPostProcessor 类 该类的定义如下:

public class ReferenceAnnotationBeanPostProcessor extends AnnotationInjectedBeanPostProcessor implements
        ApplicationContextAware, ApplicationListener {
 
    /**
     * The bean name of {@link ReferenceAnnotationBeanPostProcessor}
     */
    public static final String BEAN_NAME = "referenceAnnotationBeanPostProcessor";
 
    /**
     * Cache size
     */
    private static final int CACHE_SIZE = Integer.getInteger(BEAN_NAME + ".cache.size", 32);
 
    private final ConcurrentMap<String, ReferenceBean<?>> referenceBeanCache =
            new ConcurrentHashMap<>(CACHE_SIZE);
 
    private final ConcurrentHashMap<String, ReferenceBeanInvocationHandler> localReferenceBeanInvocationHandlerCache =
            new ConcurrentHashMap<>(CACHE_SIZE);
 
    private final ConcurrentMap<InjectionMetadata.InjectedElement, ReferenceBean<?>> injectedFieldReferenceBeanCache =
            new ConcurrentHashMap<>(CACHE_SIZE);
 
    private final ConcurrentMap<InjectionMetadata.InjectedElement, ReferenceBean<?>> injectedMethodReferenceBeanCache =
            new ConcurrentHashMap<>(CACHE_SIZE);
 
    private ApplicationContext applicationContext;

其中 referenceBeanCache缓存了接口对应bean的定义 localReferenceBeanInvocationHandlerCache缓存了接口对应InvocationHandler的定义。

参考 autowired的解析过程 分析首先找到 (2.63定义在当前类 原理一致)

 class AnnotationInjectedBeanPostProcessor{
 
@Override
    public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
        if (beanType != null) {
            InjectionMetadata metadata = findInjectionMetadata(beanName, beanType, null);
            metadata.checkConfigMembers(beanDefinition);
        }
    }
 
 
 
}

执行的时间节点是bean刚刚初始化完 但在属性填充之前， spring会为每个beanDefinition依次调用实现了 MergedBeanDefinitionPostProcessor接口的PostProcessor的改方法（注册过程单独放在spring boot 自动配置章节）

该函数依次处理spring传来的beanClass 并解析class field method尝试找到

private List<ReferenceAnnotationBeanPostProcessor.ReferenceFieldElement> findFieldReferenceMetadata(Class<?> beanClass) {
        final List<ReferenceAnnotationBeanPostProcessor.ReferenceFieldElement> elements = new LinkedList();
        ReflectionUtils.doWithFields(beanClass, new FieldCallback() {
            public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
                Reference reference = (Reference)AnnotationUtils.getAnnotation(field, Reference.class);
                if (reference != null) {
                    if (Modifier.isStatic(field.getModifiers())) {
                        if (ReferenceAnnotationBeanPostProcessor.this.logger.isWarnEnabled()) {
                            ReferenceAnnotationBeanPostProcessor.this.logger.warn("@Reference annotation is not supported on static fields: " + field);
                        }
 
                        return;
                    }
 
                    elements.add(ReferenceAnnotationBeanPostProcessor.this.new ReferenceFieldElement(field, reference));
                }
 
            }
        });
        return elements;
    }

有这个定义的filed或者method 如果存在的话 就建立类定义中对应field method的缓存。同时加入到beanclass/beanname对应该meta信息的缓存 以便同样的请求过来不在解析。

之后 处理调用该函数

public void checkConfigMembers(RootBeanDefinition beanDefinition) {
        Set<InjectionMetadata.InjectedElement> checkedElements = new LinkedHashSet(this.injectedElements.size());
        Iterator var3 = this.injectedElements.iterator();
 
        while(var3.hasNext()) {
            InjectionMetadata.InjectedElement element = (InjectionMetadata.InjectedElement)var3.next();
            Member member = element.getMember();
            if (!beanDefinition.isExternallyManagedConfigMember(member)) {
                beanDefinition.registerExternallyManagedConfigMember(member);
                checkedElements.add(element);
                if (logger.isDebugEnabled()) {
                    logger.debug("Registered injected element on class [" + this.targetClass.getName() + "]: " + element);
                }
            }
        }
 
        this.checkedElements = checkedElements;
    }

这个函数会将上个函数标注出来需要被注入的域标注为"外部管理"，只有被标注为外部管理的成员 在后续才会在当前bean实例化调用postProcessPropertyValues来处理外部注入

最后一步 postProcessPropertyValues 注入属性

public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
        InjectionMetadata metadata = this.findReferenceMetadata(beanName, bean.getClass(), pvs);
 
        try {
            metadata.inject(bean, beanName, pvs);
            return pvs;
        } catch (BeanCreationException var7) {
            throw var7;
        } catch (Throwable var8) {
            throw new BeanCreationException(beanName, "Injection of @Reference dependencies failed", var8);
        }
    }
 
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
        Collection<InjectionMetadata.InjectedElement> checkedElements = this.checkedElements;
        Collection<InjectionMetadata.InjectedElement> elementsToIterate = checkedElements != null ? checkedElements : this.injectedElements;
        if (!((Collection)elementsToIterate).isEmpty()) {
            boolean debug = logger.isDebugEnabled();
 
            InjectionMetadata.InjectedElement element;
            for(Iterator var7 = ((Collection)elementsToIterate).iterator(); var7.hasNext(); element.inject(target, beanName, pvs)) {
                element = (InjectionMetadata.InjectedElement)var7.next();
                if (debug) {
                    logger.debug("Processing injected element of bean '" + beanName + "': " + element);
                }
            }
        }
 
    }

具体过程为判断当前bean 是否需要当前的processor注入 如果需要的话 即执行注入

protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs) throws Throwable {
            if (this.isField) {
                Field field = (Field)this.member;
                ReflectionUtils.makeAccessible(field);
                field.set(target, this.getResourceToInject(target, requestingBeanName));
            } else {
                if (this.checkPropertySkipping(pvs)) {
                    return;
                }
 
                try {
                    Method method = (Method)this.member;
                    ReflectionUtils.makeAccessible(method);
                    method.invoke(target, this.getResourceToInject(target, requestingBeanName));
                } catch (InvocationTargetException var5) {
                    throw var5.getTargetException();
                }
            }
 
        }

其中 本实例中实际处理inject动作的为

private class ReferenceFieldElement extends InjectedElement 
 
 
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
            Class<?> referenceClass = this.field.getType();
            this.referenceBean = ReferenceAnnotationBeanPostProcessor.this.buildReferenceBean(this.reference, referenceClass);
            ReflectionUtils.makeAccessible(this.field);
            this.field.set(bean, this.referenceBean.getObject());
        }

其中具体的创建代码为

private ReferenceBean<?> buildReferenceBean(Reference reference, Class<?> referenceClass) throws Exception {
        String referenceBeanCacheKey = this.generateReferenceBeanCacheKey(reference, referenceClass);
        ReferenceBean<?> referenceBean = (ReferenceBean)this.referenceBeansCache.get(referenceBeanCacheKey);
        if (referenceBean == null) {
            ReferenceBeanBuilder beanBuilder = (ReferenceBeanBuilder)ReferenceBeanBuilder.create(reference, this.classLoader, this.applicationContext).interfaceClass(referenceClass);
            referenceBean = (ReferenceBean)beanBuilder.build();
            this.referenceBeansCache.putIfAbsent(referenceBeanCacheKey, referenceBean);
        }
 
        return referenceBean;
    }

该过程生成了一个referencebean的实例 并根据注解参数配置了

public class ReferenceBean<T> extends ReferenceConfig<T> 

ReferenceConfig的参数。

此时执行最后一步

this.field.set(bean, this.referenceBean.getObject());

此时尝试去当前referenceBean去除绑定的 dubbo调用invoke对象 如果不存在则创建一个 

具体创建过程比较长 

 private void init() {
   // 构建参数map
   this.ref = this.createProxy(map);
 
 }

最终创建动态代理 并将ref对象赋值给@Reference注解的成员 。

完。