错误

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'processEngine' defined in file [E:\apache-tomcat-6.0.32-case\webapps\case\WEB-INF\classes\config\applicationContext.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.factory.BeanDefinitionStoreException: Factory method [public org.jbpm.api.ProcessEngine org.jbpm.pvm.internal.processengine.SpringHelper.createProcessEngine()] threw exception; nested exception is java.lang.NullPointerException


applicationContext.xml

<?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:aop="http://www.springframework.org/schema/aop"
	xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/beans 
     http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
     http://www.springframework.org/schema/tx
     http://www.springframework.org/schema/tx/spring-tx-3.0.xsd
     http://www.springframework.org/schema/aop 
     http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
     http://www.springframework.org/schema/context
     http://www.springframework.org/schema/context/spring-context-3.0.xsd">

	<!-- 用注解方式注入bean -->
	<context:annotation-config />
	    <!-- jbpm -->
	<bean id="springHelper" class="org.jbpm.pvm.internal.processengine.SpringHelper">
		<property name="jbpmCfg" value="config/spring.jbpm.cfg.xml"></property>
	</bean>

	<bean id="processEngine" factory-bean="springHelper" factory-method="createProcessEngine"></bean>
	
    </beans>