dubbo（消费方）启动时报如下错误：
18/02/11 14:50:01 ERROR org.springframework.web.servlet.DispatcherServlet:502: Context initialization failed java.lang.IllegalStateException: ref not allow null! at com.alibaba.dubbo.config.ServiceConfig.checkRef(ServiceConfig.java:248) at com.alibaba.dubbo.config.ServiceConfig.doExport(ServiceConfig.java:203) at com.alibaba.dubbo.config.ServiceConfig.export(ServiceConfig.java:143) at com.alibaba.dubbo.config.spring.ServiceBean.onApplicationEvent(ServiceBean.java:109) at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:167) at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139) at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:383) at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:337) at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:882) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:545) at org.springframework.web.servlet.FrameworkServlet.configureAndRefreshWebApplicationContext(FrameworkServlet.java:668) at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:634) at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:682) at org.springframework.web.servlet.FrameworkServlet.initWebApplicationContext(FrameworkServlet.java:553) at org.springframework.web.servlet.FrameworkServlet.initServletBean(FrameworkServlet.java:494) at org.springframework.web.servlet.HttpServletBean.init(HttpServletBean.java:136) at javax.servlet.GenericServlet.init(GenericServlet.java:158) at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1183) at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1099) at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:779) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:133) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:748) 18/02/11 14:50:01 ERROR o
