Quartz 之 CronExpression

Provides a parser and evaluator for unix-like cron expressions. Cron
expressions provide the ability to specify complex time combinations such as
"At 8:00am every Monday through Friday" or "At 1:30am every
last Friday of the month".

Cron expressions are comprised of 6 required fields and one optional field
separated by white space. The fields respectively are described as follows:

Field Name   Allowed Values   Allowed Special Characters Seconds   0-59   , - * / Minutes   0-59   , - * / Hours   0-23   , - * / Day-of-month   1-31   , - * ? / L W Month   1-12 or JAN-DEC   , - * / Day-of-Week   1-7 or SUN-SAT   , - * ? / L # Year (Optional)   empty, 1970-2199   , - * /

The ‘’ character is used to specify all values. For example, ""
in the minute field means "every minute".

The ‘?’ character is allowed for the day-of-month and day-of-week fields. It
is used to specify ‘no specific value’. This is useful when you need to
specify something in one of the two fields, but not the other.

The ‘-’ character is used to specify ranges For example "10-12" in
the hour field means "the hours 10, 11 and 12".

The ‘,’ character is used to specify additional values. For example
"MON,WED,FRI" in the day-of-week field means "the days Monday,
Wednesday, and Friday".

The ‘/’ character is used to specify increments. For example "0/15"
in the seconds field means "the seconds 0, 15, 30, and 45". And
"5/15" in the seconds field means "the seconds 5, 20, 35, and
50". Specifying ‘*’ before the ‘/’ is equivalent to specifying 0 is
the value to start with. Essentially, for each field in the expression, there
is a set of numbers that can be turned on or off. For seconds and minutes,
the numbers range from 0 to 59. For hours 0 to 23, for days of the month 0 to
31, and for months 1 to 12. The "/" character simply helps you turn
on every "nth" value in the given set. Thus "7/6" in the
month field only turns on month "7", it does NOT mean every 6th
month, please note that subtlety.

The ‘L’ character is allowed for the day-of-month and day-of-week fields.
This character is short-hand for "last", but it has different
meaning in each of the two fields. For example, the value "L" in
the day-of-month field means "the last day of the month" - day 31
for January, day 28 for February on non-leap years. If used in the
day-of-week field by itself, it simply means "7" or
"SAT". But if used in the day-of-week field after another value, it
means "the last xxx day of the month" - for example "6L"
means "the last friday of the month". You can also specify an offset
from the last day of the month, such as “L-3” which would mean the third-to-last
day of the calendar month. When using the ‘L’ option, it is important not to
specify lists, or ranges of values, as you’ll get confusing/unexpected results.

The ‘W’ character is allowed for the day-of-month field. This character
is used to specify the weekday (Monday-Friday) nearest the given day. As an
example, if you were to specify "15W" as the value for the
day-of-month field, the meaning is: "the nearest weekday to the 15th of
the month". So if the 15th is a Saturday, the trigger will fire on
Friday the 14th. If the 15th is a Sunday, the trigger will fire on Monday the
16th. If the 15th is a Tuesday, then it will fire on Tuesday the 15th.
However if you specify "1W" as the value for day-of-month, and the
1st is a Saturday, the trigger will fire on Monday the 3rd, as it will not
‘jump’ over the boundary of a month’s days. The ‘W’ character can only be
specified when the day-of-month is a single day, not a range or list of days.

The ‘L’ and ‘W’ characters can also be combined for the day-of-month
expression to yield ‘LW’, which translates to "last weekday of the
month".

The ‘#’ character is allowed for the day-of-week field. This character is
used to specify "the nth" XXX day of the month. For example, the
value of "6#3" in the day-of-week field means the third Friday of
the month (day 6 = Friday and "#3" = the 3rd one in the month).
Other examples: "2#1" = the first Monday of the month and
"4#5" = the fifth Wednesday of the month. Note that if you specify
"#5" and there is not 5 of the given day-of-week in the month, then
no firing will occur that month. If the ‘#’ character is used, there can
only be one expression in the day-of-week field ("3#1,6#3" is
not valid, since there are two expressions).

The legal characters and the names of months and days of the week are not
case sensitive.

NOTES:

• Support for specifying both a day-of-week and a day-of-month value is
  not complete (you’ll need to use the ‘?’ character in one of these fields).
  
• Overflowing ranges is supported - that is, having a larger number on
  the left hand side than the right. You might do 22-2 to catch 10 o’clock
  at night until 2 o’clock in the morning, or you might have NOV-FEB. It is
  very important to note that overuse of overflowing ranges creates ranges
  that don’t make sense and no effort has been made to determine which
  interpretation CronExpression chooses. An example would be
  “0 0 14-6 ? * FRI-MON”.