我有以下内容;
private String cronExpression = "";
private final String jobID = "MyJObID";
...
Scheduler scheduler = ServiceLocator.getInstance().getScheduler();
CronTrigger trigger = new CronTrigger(jobID , Scheduler.DEFAULT_GROUP, cronExpression);
JobDetail jobDetail = new JobDetail(jobID , Scheduler.DEFAULT_GROUP, MyJob.class);
scheduler.scheduleJob(jobDetail, trigger);
我的问题是什么时候为空的 cron 表达式触发这项工作?