在方法中调用getter使用Activiti Engine返回java.lang.NullPointerException

Question

我在Spring + JSF项目中使用Activiti引擎，我正在尝试通过我的process.BPMN20.xml调用方法并将参数传递给我的工作流：

 <?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" 
        targetNamespace="http://www.bpmnwithactiviti.org"
    xmlns:activiti="http://activiti.org/bpmn">
    <process id="my-process" name="my-process">
        <startEvent id="startEvent" name="Start" activiti:formKey="taskForm_newOrder.jsf" />
    <sequenceFlow id="toValidateTask" sourceRef="startEvent" targetRef="validateTask"/>

    <serviceTask id="validateTask" activiti:expression="#{operationController.sayHello()}" >
    <extensionElements>
        <activiti:field name="input" expression="${adminModele.operationLabel}" />
    </extensionElements>
    </serviceTask>

    <sequenceFlow id="toCompletedOrderTask" sourceRef="validateTask" targetRef="completeOrderTask"/>
        <userTask id="completeOrderTask" name="Complete order" activiti:assignee="${adminModele.operationLabel}" activiti:formKey="taskForm_completeOrder.jsf">
      <documentation>Completing order for book with isbn : ${adminModele.operationLabel}  </documentation>
    </userTask>
    <sequenceFlow id="toApproveTask" sourceRef="completeOrderTask" targetRef="approveTask"/>
    <serviceTask id="approveTask" activiti:expression="#{bookOrder.validates()}" />
    <sequenceFlow id="toEndEvent" sourceRef="approveTask" targetRef="endEvent"/>
        <endEvent id="endEvent" name="End"/>
    </process>
</definitions>

在这里我将值设置为bookOrder

 bookOrder = new BookOrder();
    bookOrder.setIsbn("123");
    sayHello(); // this call doesn't return an error        

sayHello（）方法：

public void sayHello(){
    System.out.println("SayHello ..."+bookOrder.getIsbn());
    }

当我调用sayHello（）时，它显示我想要的意味着bookOrder.getIsbn()不为空，但是当Activiti引擎在完成整个过程时调用它时，我得到：

 javax.faces.el.EvaluationException: org.activiti.engine.ActivitiException: Error while evaluating expression: #{operationController.sayHello()}
            at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
            at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
    Caused by: java.lang.NullPointerException
            at ma.net.s2m.selectsystem.web.controller.admin.OperationController.sayHello(OperationController.java:887)
            at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
            at java.lang.reflect.Method.invoke(Method.java:597)
    at org.activiti.engine.impl.javax.el.BeanELResolver.invoke(BeanELResolver.java:483)

任何想法是什么问题以及如何解决？