dubbo在controller中reference注解为空的问题深度解析

素颜马尾好姑娘i 2021-06-11 15:11 524阅读 0赞

## dubbo注解的使用 ##

使用非常简单，下面贴出关键部分

//provider的配置文件
    <?xml version="1.0" encoding="UTF-8"?>  
    <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:dubbo="http://code.alibabatech.com/schema/dubbo" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://code.alibabatech.com/schema/dubbo http://code.alibabatech.com/schema/dubbo/dubbo.xsd">  
          
        <dubbo:application name="provider-app" />  
          
        <dubbo:registry protocol="zookeeper" address="127.0.0.1:2181" />
          
        <dubbo:protocol name="dubbo" port="20880" />
    
        <dubbo:annotation package="com.i.springboot.controller" />
    </beans>
    //服务接口类
    @com.alibaba.dubbo.config.annotation.Service
    public class PostService {
        ………………省略……………………
    }
    
    //consumer端配置
    <?xml version="1.0" encoding="UTF-8"?>  
    <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:dubbo="http://code.alibabatech.com/schema/dubbo" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://code.alibabatech.com/schema/dubbo http://code.alibabatech.com/schema/dubbo/dubbo.xsd">  
          
        <dubbo:application name="consumer-app"/>  
        <dubbo:registry address="zookeeper://127.0.0.1:2181" />
        <dubbo:annotation package="com.i.springboot.controller"/>
    </beans>  
    
    //服务引用
    @Controller
    @RequestMapping("/post")
    @SessionAttributes("currentUser")
    public class PostController {
        //注解使用dubbo服务端服务
        @Reference
        PostService postService;
        ……………省略……………
    }
     
      
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以上如果只是用spring的容器，而不使用springmvc进行结合使用时是不会出现引用为空的问题的；但是如果不了解spring和springmvc加载配置文件和初始化bean的流程，则极有可能出现postService为Null的情况，错误配置如下

我src/main/resources下面有spring-consumer.xml、spring-mvc.xm两个配置文件，spring-mvc.xml和web.xml配置部分如下

<?xml version="1.0" encoding="UTF-8"?>    
    <beans xmlns="http://www.springframework.org/schema/beans" xmlns:context="http://www.springframework.org/schema/context" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:p="http://www.springframework.org/schema/p" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd"> 
    
        
        <context:component-scan base-package="com.i.springboot.controller" />
        ……………………省略…………………………
    </beans>
    
    <!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd" >
    <web-app>
      <display-name>Archetype Created Web Application</display-name>
      <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>
            classpath*:/spring-*.xml
        </param-value>
        </context-param>
        <listener>
            <listener-class>org.springframework.web.context.ContextLoaderListener
            </listener-class>
        </listener>
    
        <servlet>
            <servlet-name>spring</servlet-name>
            <servlet-class>org.springframework.web.servlet.DispatcherServlet
            </servlet-class>
            <init-param>
                <param-name>contextConfigLocation</param-name>
                <param-value>classpath:spring-mvc.xml</param-value>
            </init-param>
            <load-on-startup>2</load-on-startup>
        </servlet>
        <servlet-mapping>
            <servlet-name>spring</servlet-name>
            <url-pattern>*.do</url-pattern>
        </servlet-mapping>
    </web-app>
     
      
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初步一看完全没有问题，spring启动的时候会扫描controller包，然后初始化我的postService服务类，一切想的都是那么美好，程序一运行，一个大大的空指针异常抛出，然后网上、qq上一堆乱问，终不得解，生无可恋，下面一步步解决问题   
开始之前提出如下疑问

1.  spring是什么时候跟dubbo勾搭在一起的
2.  为什么PostService上面加上dubbo的Service注解，服务类就会被加载
3.  为什么dubbo官方文件所说的通过Reference引用服务却为NULL

## dubbo和spring结合 ##

到底dubbo是怎么和spring组合的呢，先看如下

<beans xmlns="http://www.springframework.org/schema/beans"  
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
        xmlns:dubbo="http://code.alibabatech.com/schema/dubbo"  
        xsi:schemaLocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans.xsd 
    
        http://code.alibabatech.com/schema/dubbo 
        http://code.alibabatech.com/schema/dubbo/dubbo.xsd">
     
      
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注意到xmlns:dubbo=”http://code.alibabatech.com/schema/dubbo”了吗，仔细想想spring怎么解析xml配置文件的，NamespaceHandler、NamespaceHandlerSupport、BeanDefinitionParser，spring提供的一种SPI规范，dubbo定义了自己的schema、namespacehandler、beandefinitionparser

/** * DubboNamespaceHandler * * @author william.liangf * @export */
    public class DubboNamespaceHandler extends NamespaceHandlerSupport { 
    
        static {
            Version.checkDuplicate(DubboNamespaceHandler.class);
        }
    
        public void init() {
            registerBeanDefinitionParser("application", new DubboBeanDefinitionParser(ApplicationConfig.class, true));
            registerBeanDefinitionParser("module", new DubboBeanDefinitionParser(ModuleConfig.class, true));
            registerBeanDefinitionParser("registry", new DubboBeanDefinitionParser(RegistryConfig.class, true));
            registerBeanDefinitionParser("monitor", new DubboBeanDefinitionParser(MonitorConfig.class, true));
            registerBeanDefinitionParser("provider", new DubboBeanDefinitionParser(ProviderConfig.class, true));
            registerBeanDefinitionParser("consumer", new DubboBeanDefinitionParser(ConsumerConfig.class, true));
            registerBeanDefinitionParser("protocol", new DubboBeanDefinitionParser(ProtocolConfig.class, true));
            registerBeanDefinitionParser("service", new DubboBeanDefinitionParser(ServiceBean.class, true));
            registerBeanDefinitionParser("reference", new DubboBeanDefinitionParser(ReferenceBean.class, false));
            registerBeanDefinitionParser("annotation", new DubboBeanDefinitionParser(AnnotationBean.class, true));
        }
    
    }
     
      
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![结合][20161104214922715]  
DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions，xml配置文件就这样开始解析，具体不做描述，主要看dubbo是如何搭上spring的车的，解析过程咱们重点看AnnotationsBean的解析，也就是dubbo的DubboBeanDefinitionParser类

## dubbo的service、reference是如何初始化的 ##

DubboBeanDefinitionParser的parse方法被调用后，dubbo定义的几个大的标签application、registry、provider、consumer、annotation等都会被初始化，并包装成RootBeanDefinition在spring的bean容器中

@SuppressWarnings("unchecked")
        private static BeanDefinition parse(Element element, ParserContext parserContext, Class<?> beanClass, boolean required) {
            RootBeanDefinition beanDefinition = new RootBeanDefinition();
            beanDefinition.setBeanClass(beanClass);
            beanDefinition.setLazyInit(false);
           ……………………………省略具体解析代码…………………………………
            return beanDefinition;
        }
     
      
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重点看annotation实例化过程，也是dubbo注解的关键，进入此类，看到一长串的类

public class AnnotationBean extends AbstractConfig implements DisposableBean, BeanFactoryPostProcessor, BeanPostProcessor, ApplicationContextAware { 
     
      
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不得不提下spring的processer和aware两个SPI点，设计的非常巧妙，完全符合设计六大原则之一的[开闭原则][Link 1]

//任选一种都支持process和aware两种方式
    <context:annotation-config/>
    <context:component-scan base-package="xx.xx.xx" />
     
      
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dubbo充分利用了spring提供的机制进行service的初始化和reference的实例化的

### service的实例化过程 ###

第一步

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)
                throws BeansException {
            if (annotationPackage == null || annotationPackage.length() == 0) {
                return;
            }
            if (beanFactory instanceof BeanDefinitionRegistry) {
                try {
                    // init scanner
                    Class<?> scannerClass = ReflectUtils.forName("org.springframework.context.annotation.ClassPathBeanDefinitionScanner");
                    Object scanner = scannerClass.getConstructor(new Class<?>[] {BeanDefinitionRegistry.class, boolean.class}).newInstance(new Object[] {(BeanDefinitionRegistry) beanFactory, true});
                    // add filter
                    Class<?> filterClass = ReflectUtils.forName("org.springframework.core.type.filter.AnnotationTypeFilter");
                    Object filter = filterClass.getConstructor(Class.class).newInstance(Service.class);
                    Method addIncludeFilter = scannerClass.getMethod("addIncludeFilter", ReflectUtils.forName("org.springframework.core.type.filter.TypeFilter"));
                    addIncludeFilter.invoke(scanner, filter);
                    // scan packages
                    String[] packages = Constants.COMMA_SPLIT_PATTERN.split(annotationPackage);
                    Method scan = scannerClass.getMethod("scan", new Class<?>[]{
       String[].class});
                    scan.invoke(scanner, new Object[] {packages});
                } catch (Throwable e) {
                    // spring 2.0
                }
            }
        }
     
      
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实例化一个ClassPathBeanDefinitionScanner类，通过反射实现有参构造的初始化，将注册bean的(BeanDefinitionRegistry) beanFactory作为参数传递给实例，最后反射调用scanner的scan方法，将service注解的实例增加至spring容器中   
第二步

public Object postProcessAfterInitialization(Object bean, String beanName)
                throws BeansException {
            //判断是不是dubbo需要处理的bean类，如果是则继续进行处理，不是则不做任何处理
            if (! isMatchPackage(bean)) {
                return bean;
            }
            Service service = bean.getClass().getAnnotation(Service.class);
            if (service != null) {
                ServiceBean<Object> serviceConfig = new ServiceBean<Object>(service);
                if (void.class.equals(service.interfaceClass())
                        && "".equals(service.interfaceName())) {
                    if (bean.getClass().getInterfaces().length > 0) {
                        serviceConfig.setInterface(bean.getClass().getInterfaces()[0]);
                    } else {
                        throw new IllegalStateException("Failed to export remote service class " + bean.getClass().getName() + ", cause: The @Service undefined interfaceClass or interfaceName, and the service class unimplemented any interfaces.");
                    }
                }
                if (applicationContext != null) {
                    serviceConfig.setApplicationContext(applicationContext);
                    if (service.registry() != null && service.registry().length > 0) {
                        List<RegistryConfig> registryConfigs = new ArrayList<RegistryConfig>();
                        for (String registryId : service.registry()) {
                            if (registryId != null && registryId.length() > 0) {
                                registryConfigs.add((RegistryConfig)applicationContext.getBean(registryId, RegistryConfig.class));
                            }
                        }
                        serviceConfig.setRegistries(registryConfigs);
                    }
                    if (service.provider() != null && service.provider().length() > 0) {
                        serviceConfig.setProvider((ProviderConfig)applicationContext.getBean(service.provider(),ProviderConfig.class));
                    }
                    if (service.monitor() != null && service.monitor().length() > 0) {
                        serviceConfig.setMonitor((MonitorConfig)applicationContext.getBean(service.monitor(), MonitorConfig.class));
                    }
                    if (service.application() != null && service.application().length() > 0) {
                        serviceConfig.setApplication((ApplicationConfig)applicationContext.getBean(service.application(), ApplicationConfig.class));
                    }
                    if (service.module() != null && service.module().length() > 0) {
                        serviceConfig.setModule((ModuleConfig)applicationContext.getBean(service.module(), ModuleConfig.class));
                    }
                    if (service.provider() != null && service.provider().length() > 0) {
                        serviceConfig.setProvider((ProviderConfig)applicationContext.getBean(service.provider(), ProviderConfig.class));
                    } else {
    
                    }
                    if (service.protocol() != null && service.protocol().length > 0) {
                        List<ProtocolConfig> protocolConfigs = new ArrayList<ProtocolConfig>();
                        for (String protocolId : service.registry()) {
                            if (protocolId != null && protocolId.length() > 0) {
                                protocolConfigs.add((ProtocolConfig)applicationContext.getBean(protocolId, ProtocolConfig.class));
                            }
                        }
                        serviceConfig.setProtocols(protocolConfigs);
                    }
                    try {
                        serviceConfig.afterPropertiesSet();
                    } catch (RuntimeException e) {
                        throw (RuntimeException) e;
                    } catch (Exception e) {
                        throw new IllegalStateException(e.getMessage(), e);
                    }
                }
                //
                serviceConfig.setRef(bean);
                serviceConfigs.add(serviceConfig);
                //通过注册中心暴露dubbo的服务
                serviceConfig.export();
            }
            //返还spring容器，有点类似于装饰
            return bean;
        }
     
      
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至此，dubbo通过service注解实现spring容器管理完毕

### reference实例过程 ###

reference和service最大的不同是，reference注解生成的实例就没有交给spring容器去管理，而只是作为spring管理bean的一个属性赋值操作，通过反射来实现，代码如下

public Object postProcessBeforeInitialization(Object bean, String beanName)
                throws BeansException {
            //和service一样
            if (! isMatchPackage(bean)) {
                return bean;
            }
            Method[] methods = bean.getClass().getMethods();
            for (Method method : methods) {
                String name = method.getName();
                if (name.length() > 3 && name.startsWith("set")
                        && method.getParameterTypes().length == 1
                        && Modifier.isPublic(method.getModifiers())
                        && ! Modifier.isStatic(method.getModifiers())) {
                    try {
                        Reference reference = method.getAnnotation(Reference.class);
                        if (reference != null) {
                            Object value = refer(reference, method.getParameterTypes()[0]);
                            if (value != null) {
                                method.invoke(bean, new Object[] {  });
                            }
                        }
                    } catch (Throwable e) {
                        logger.error("Failed to init remote service reference at method " + name + " in class " + bean.getClass().getName() + ", cause: " + e.getMessage(), e);
                    }
                }
            }
            Field[] fields = bean.getClass().getDeclaredFields();
            for (Field field : fields) {
                try {
                    if (! field.isAccessible()) {
                        field.setAccessible(true);
                    }
                    //本项目使用的是PostController，field就是controller中的postService，refer方法则是通过连接注册中心，检测服务是否存在，当然如果配置中check为false就不会现在进行检测
                    Reference reference = field.getAnnotation(Reference.class);
                    if (reference != null) {
                        //refer方法有兴趣可以自己看，牵扯到zk和netty
                        Object value = refer(reference, field.getType());
                        if (value != null) {
                            field.set(bean, value);
                        }
                    }
                } catch (Throwable e) {
                    logger.error("Failed to init remote service reference at filed " + field.getName() + " in class " + bean.getClass().getName() + ", cause: " + e.getMessage(), e);
                }
            }
            //将controller中有reference标示的字段赋值后返回，并没有将字段类实例注入spring容器，确实也没有必要
            return bean;
        }
    
        private Object refer(Reference reference, Class<?> referenceClass) { //method.getParameterTypes()[0]
            String interfaceName;
            if (! "".equals(reference.interfaceName())) {
                interfaceName = reference.interfaceName();
            ……………………省略……………………
            return referenceConfig.get();
        }
     
      
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至此reference标注的实例也初始化完成，service和referece返回的都是dubbo的代理类com.alibaba.dubbo.common.bytecode.Proxy，用到了jdk的Proxy、InvocationHandler来生成代理类（没有javassist的情况），再次声明，reference标注的对象不会被spring容器管理，是无法通过factory.getBean获取的

## Null出现的原因 ##

首先得知道spring容器初始化过程

public class ContextLoaderListener extends ContextLoader implements ServletContextListener /** * Initialize the root web application context. */ @Override public void contextInitialized(ServletContextEvent event) { 
        initWebApplicationContext(event.getServletContext());
    }
     
      
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我配置了监听，会读取spring-\*.xml配置文件，会初始化一个xmlwebapplicationcontext也就是应用的rootContext顶级容器，这个容器在serverletcontext上下文中，等监听初始化完毕后，我们配置的dispatcherservlet开始初始化

/** * Overridden method of {@link HttpServletBean}, invoked after any bean properties * have been set. Creates this servlet's WebApplicationContext. */
        @Override
        protected final void initServletBean() throws ServletException {
            try {
                //初始化mvc的容器
                this.webApplicationContext = initWebApplicationContext();
                initFrameworkServlet();
            }
        }
    
    protected WebApplicationContext initWebApplicationContext() {
            //获取监听初始化的顶级容器
            WebApplicationContext rootContext =
                    WebApplicationContextUtils.getWebApplicationContext(getServletContext());
            WebApplicationContext wac = null;
    
            if (this.webApplicationContext != null) {
                // A context instance was injected at construction time -> use it
                wac = this.webApplicationContext;
                if (wac instanceof ConfigurableWebApplicationContext) {
                    ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                    if (!cwac.isActive()) {
                        // The context has not yet been refreshed -> provide services such as
                        // setting the parent context, setting the application context id, etc
                        if (cwac.getParent() == null) {
                            // The context instance was injected without an explicit parent -> set
                            // the root application context (if any; may be null) as the parent
                            cwac.setParent(rootContext);
                        }
                        configureAndRefreshWebApplicationContext(cwac);
                    }
                }
            }
            if (wac == null) {
                // No context instance was injected at construction time -> see if one
                // has been registered in the servlet context. If one exists, it is assumed
                // that the parent context (if any) has already been set and that the
                // user has performed any initialization such as setting the context id
                wac = findWebApplicationContext();
            }
            if (wac == null) {
                // No context instance is defined for this servlet -> create a local one
                //初始化XmlWebApplicationContext开始，它会读取配置文件spring-mvc.xml
                wac = createWebApplicationContext(rootContext);
            }
    
            if (!this.refreshEventReceived) {
                // Either the context is not a ConfigurableApplicationContext with refresh
                // support or the context injected at construction time had already been
                // refreshed -> trigger initial onRefresh manually here.
                onRefresh(wac);
            }
    
            if (this.publishContext) {
                // Publish the context as a servlet context attribute.
                String attrName = getServletContextAttributeName();
                //将mvc容器放入servletcontext上下文中
                getServletContext().setAttribute(attrName, wac);
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                            "' as ServletContext attribute with name [" + attrName + "]");
                }
            }
    
            return wac;
        }
     
      
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监听初始化的容器读取了所有的配置文件，并初始化了controller类，同时将dubbo注解reference的实例set给了controller，而后servlet初始化过程中又再一次读取了spring-mvc.xml配置文件，同时也对controller进行了初始化，但是与顶级容器初始化不同的是，它没有加载dubbo实现的DubboNamespaceHandler，也就是说reference实例化的过程都没有进行，因此在mvc容器中的controller是没有注入reference标注的实例的，因此出现NULL的情况

## 总结 ##

遇到这样的问题是一种幸运，也是一种不幸，幸运的是通过debug看源码的过程增加了对spring的了解以及框架的优秀设计，不幸的是对于spring一些细节的地方还不够了解；遇到问题不放弃、不抛弃，加油

[20161104214922715]: /images/20210527/baae45b2028c44c387dce0667d48ebe9.png
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