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.Servicepublic 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……………}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
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>
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
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
<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: http: http:
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)); }}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
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<context:annotation-config/><context:component-scan base-package="xx.xx.xx" />
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 { 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}); 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); 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) { } } }
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
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 }
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
- 57
- 58
- 59
- 60
- 61
- 62
- 63
- 64
- 65
- 66
- 67
- 68
- 69
- 70
- 71
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 { 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); } Reference reference = field.getAnnotation(Reference.class); if (reference != null) { 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); } } return bean; } private Object refer(Reference reference, Class<?> referenceClass) { String interfaceName; if (! "".equals(reference.interfaceName())) { interfaceName = reference.interfaceName(); ……………………省略…………………… return referenceConfig.get(); }
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
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. */@Overridepublic 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 { this.webApplicationContext = initWebApplicationContext(); initFrameworkServlet(); } }protected WebApplicationContext initWebApplicationContext() { WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext()); WebApplicationContext wac = null; if (this.webApplicationContext != null) { wac = this.webApplicationContext; if (wac instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac; if (!cwac.isActive()) { if (cwac.getParent() == null) { cwac.setParent(rootContext); } configureAndRefreshWebApplicationContext(cwac); } } } if (wac == null) { wac = findWebApplicationContext(); } if (wac == null) { wac = createWebApplicationContext(rootContext); } if (!this.refreshEventReceived) { onRefresh(wac); } if (this.publishContext) { String attrName = getServletContextAttributeName(); getServletContext().setAttribute(attrName, wac); if (this.logger.isDebugEnabled()) { this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() + "' as ServletContext attribute with name [" + attrName + "]"); } } return wac; }
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
- 57
- 58
- 59
- 60
- 61
- 62
- 63
- 64
- 65
- 66
- 67
- 68
- 69
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
