问题
Source Wikipedia:
Question mark (?) is used instead of '*' for leaving either day-of-month or day-of-week blank.
The above statement is not making much sense to me.
So if I write some cron as 0 0 0 ? * * then does it mean first of every month or it means it will execute daily?
It is a bit confusing as Java crons start with seconds while other crons start with minute.
回答1:
According to the Quartz CronTrigger tutorial (Quartz is used by www.cronmaker.com, referenced above), the ? wildcard is only used in the day of month and day of week fields:
? ("no specific value") - useful when you need to specify something in one of the two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a particular day of the month (say, the 10th), but don't care what day of the week that happens to be, I would put "10" in the day-of-month field, and "?" in the day-of-week field.
I guess * isn't appropriate here because * means execute on every day of the week, which contradicts the rule saying that it should only run on the 10th of the month.
回答2:
I know it's an old question but for posterity, it's explained here : http://www.nncron.ru/help/EN/working/cron-format.htm#STARTTIME
Both in classic and extended modes, nnCron users can use a "nonstandard" character "?" in the first four fields of cron format. It stands for time of nnCron startup, i. e. when a field is processed, startup time will be substituted for the question mark: minutes for Minute field, hour for Hour field, day of the month for Day of month field and month for Month field.
For example, if you write the following:
Time: ? ? * * * * the task will be executed on nnCron startup and then will be executed daily at the same time (unless the user doesn't restart nnCron, of course): question marks will be "replaced" by nnCron startup time. For example, if nnCron was started at 8:25, question marks will be substituted this way:
Time: 25 8 * * * *
来源：https://stackoverflow.com/questions/11499740/significance-of-question-mark-in-java-cron