Activiti 6 TaskListener expression springbean fixedvalue null

    <activiti:taskListener event="create" expression="${myService.method(task)}">
      <activiti:field name="taskName">
        <activiti:string><![CDATA[text]]></activiti:string>
      </activiti:field>

I want to use spring bean get the fixedvalue,but the fixedvalue taskName is NULL。

 @Service("myService")
 public class MyService{
private FixedValue taskName;
    public FixedValue getTaskName() {
        return taskName;
    }

    public void setTaskName(FixedValue taskName) {
        this.taskName = taskName;
    }

public void method(DelegateTask task){
  //taskName get NullpointException
  taskName.getExpressionText();
  return;
}
}

Maybe the Activiti just run the springuel expression，not set the fixedvalue into the springbean field。