CRON expression

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cron expression

不知道中文如何翻译，时间条件表达式？。。。
相对正则表达式，这种表达式在命令行工具以及一些开发框架上，都很常见，用来做为定时任务的时间的时间设定
例如:

  • command crontab
  • spring schedule task

都有使用过这种表达式

cron expression 详解

每个域的意义

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* * * * * * *
| | | | | | | 
| | | | | | +-- Year              (range: 1970-2099)
| | | | | +---- Day of the Week   (range: 1-7 or SUN-SAT)
| | | | +------ Month of the Year (range: 0-11 or JAN-DEC)
| | | +-------- Day of the Month  (range: 1-31)
| | +---------- Hour              (range: 0-23)
| +------------ Minute            (range: 0-59)
+-------------- Second            (range: 0-59)

每个域能赋值的情况

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Field Name      Mandatory   Allowed Values      Allowed Special Characters
Seconds         YES         0-59                , - * /
Minutes         YES         0-59                , - * /
Hours           YES         0-23                , - * /
Day of month    YES         1-31                , - * ? / L W   //allowed '?'
Month           YES         1-12 or JAN-DEC     , - * /
Day of week     YES         1-7 or SUN-SAT      , - * ? / L #   //allowed '?'
Year            NO          empty, 1970-2099    , - * /

注意能使用字符?的域有一个月的某天和一周的某天

实例

example 1

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0 0 13/2 1/1 * * *

example2

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0 0 0/1 1/1 * ? *

好像不是那么简单吧，出现了特殊字符,下面说明这些特殊字符的意义。

特殊字符

  • /
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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”.
 You can also specify ‘/’ after the ‘’ character - in this case ‘’ is equivalent to having ‘0’ before the ‘/’.
 ‘1/3’ in the day-of-month field means “fire every 3 days starting on the first day of the month”.

  • n/m means start at XX n and repeat each m XX

  • * (“all values”)

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    used to select all values within a field. For example, “” in the minute field means *“every minute”.

  • ? (“no specific value”)

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    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.

对比这些说明，现在能否理解上面的两个实例？

疑点解析

  • ? 和* 能互换吗
    还是这个例子 0 0 0/1 1/1 * ? * 和 0 0 0/1 1/1 * * * 是等效的吗? 答案是等效的.

0 15 10 * * ? 2005 和 0 15 10 * * * 2005 是等效的吗？答案是第二个表达式是错的
不能同时指定一个月的某一天和一周的某一天,所以会有?字符的引入. 也就是这两个字符不能同时出现在表达式里面

使用实战

java 使用 CronExpression 来操作cron expressioni ,这个jar包的api见 link