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
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">
//服务接口类
@com.alibaba.dubbo.config.annotation.Service
public class PostService {
………………省略……………………
}
//consumer端配置
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">
//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……………
}
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
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"> ……………………省略…………………………
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd" >
classpath*:/spring-*.xml
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
spring是什么时候跟dubbo勾搭在一起的为什么PostService上面加上dubbo的Service注解，服务类就会被加载为什么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
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"> 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)); } } 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; } 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 { 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** //任选一种都支持process和aware两种方式 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 } } } 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 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 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 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; } 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(); } 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()); } 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; } 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 精彩链接
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