osworkflow提示空指针异常是什么原因

long flowId=workflow.initialize("DefaultWorkFlow", 100, null);执行main方法时候到上面那句就包空指针异常，为什么呢？public static void main(String args[]) throws InvalidActionException, InvalidRoleException, InvalidInputException, InvalidEntryStateException, WorkflowException, MalformedURLException{
Workflow workflow=new BasicWorkflow("mywf");
Configuration config = new DefaultConfiguration();
workflow.setConfiguration(config);
long flowId=workflow.initialize("DefaultWorkFlow", 100, null); System.out.println(flowId);
int[] steps = workflow.getAvailableActions(flowId, null);
System.out.println(steps[0]);

workflow.doAction(flowId, steps[0], null);

System.out.println(workflow.getAvailableActions(flowId, null).length);
}
java.lang.NullPointerException
at com.opensymphony.workflow.AbstractWorkflow.initialize(AbstractWorkflow.java:595)
at com.yourcompany.struts.Test.main(Test.java:21)
Exception in thread "main"