[Monday commuter radio station] cron expression. It's enough to read this article

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Hello, Hello everyone , Here is Yijun @ Monday commuter radio channel .

0. After reading this article, you will learn

• What is? cron expression
• cron The structure and rules of expressions
• cron The expression in Spring as well as Linux The use of

1. What is? cron expression ？

Example ： 10 10 10 ? * MON-WED cron An expression is a string expression used to control time .

cron Expressions are generally composed of 5 A or 6 Space space , form 6 A or 7 Domains .

1.1 cron Structure of expression

cron The structure of the expression is as follows .

Seconds Minutes Hours DayOfMonth Month DayOfWeek Year(Year Omission )

From left to right ,cron The fields of the expression are expressed as second 、 branch 、 when 、 Date in month 、 month 、 Date of the week 、 year .

For the convenience of memory , We can write it down as “ Seconds minutes ten （ Ten links ） Japan （ Different from the following Sunday ）, July Sunday （ Sunday is the day of the week ） year ”—— Ten suns were quickly divided , The moon is a year every Sunday .

The author also suddenly thought of this method , Because I remember it several times before, but it is still vague . If readers have a better way to remember, they might as well leave a message in the comment area .

1.2 cron The meaning of each field of the expression

Field	Range value	Special characters allowed
second （Seconds）	0~59 The integer of	, - * /     Four characters
branch （Minutes）	0~59 The integer of	, - * /     Four characters
Hours （Hours）	0~23 The integer of	, - * /     Four characters
date （DayofMonth）	1~31 The integer of （ Actual days need to be considered ）	,- * ? / L W C     Eight characters
month （Month）	1~12 Or JAN-DEC	, - * /     Four characters
week （DayofWeek）	1~7 Or SUN-SAT	, - * ? / L C #     Eight characters
year （Year）	1970~2099	, - * /     Four characters

The meaning of special characters ：

Special characters	significance
*	Represents any value that matches the field . If in Minutes If this character is used in the field, it means that the event will be triggered every minute .
?	Only in DayOfMonth and DayOfWeek Use in . It's used to specify “ An ambiguous value ”.
-	Specify a range . Such as ：“10-12” stay Hour Domain means “10 spot 、11 spot 、12 spot ” Triggering event .
,	Specify several hash values . Such as ： stay Minutes Domain use 10,20, said 10 Points and 20 Trigger an event respectively at the time of minute .
/	Refers to the step size . Such as ：“0/15” stay Seconds It means the... Of every minute 0 second , The first 15 second , The first 30 Second and the 45 second .“5/15” stay Minutes The field represents the... Per hour 5 minute , The first 20 minute , The first 35 Minute and second 50 minute . Symbol “*” stay “/” front （ Such as ：*/10） Equivalent to 0 stay “/” front （ Such as ：0/10）
L	Can only appear in DayOfMonth and DayOfWeek Domain , But the meaning is different in the two fields , for example DayOfMonth Field represents the last day of the month . If in DayOfWeek Domain represents 7 perhaps SAT. If in DayOfWeek Fields are preceded by numbers , It means one day in the last week of a month , for example 5L It means that the event is triggered on the last Thursday of a month .
W	Means effective working day , Can only appear in DayOfMonth Domain . This character is used to specify the most recent working day of the date .“W” The most recent working day specified by the character cannot span months . character “W” Can only be used with a single value , Can't be a number field , Such as ：1-15W It's wrong. .
LW	L and W It can be used jointly in the date field ,LW Represents the working day of the last week of the month .
#	Used to determine the week ordinal of each month , Can only appear in DayofMonth Domain . For example, in 4#2, The second Wednesday of a month . namely X#Y, Indicates the day of a month Y A few weeks （X-1）.
C	Only in DayOfMonth Domain and DayOfWeek The domain appears . This character depends on a specified “ The calendar ”. That is to say, the value of this expression depends on the related “ The calendar ” Calculated results of , without “ The calendar ” relation , Then it is equivalent to all included “ The calendar ”. Such as ： The date field is “5C” Express Association “ The calendar ” On the first day of middle school , Or after the first day of the month 5 God . Zhou Yu is “1C” Express Association “ The calendar ” On the first day of middle school , Or after the first day of the week 1 God , It's the day after Sunday （ Monday ）

1.3 Examples of common expressions

Now let's look at a few examples , Examine the above corn Understanding of expression rules , The answer is at the end of the article .

(1) 10 10 10 ? * MON-WED

(2) 0 15 15 ? 5L 2022-2024

(3) 0 0/30 9-17 * * ?

2. corn Application of expressions

2.1 Spring Boot The realization of timing function in

@Configuration      
@EnableScheduling   
public class SaticScheduleTask {
    @Scheduled(cron = "0/5 * * * * ?")
    //  Or directly specify the time interval , for example ：5 second 
    // @Scheduled(fixedRate=5000)
    private void configureTasks() {
        . . .
    }
}

1. @Configuration Indicates that this is a configuration class , Both Component The effect of .
2. @EnableScheduling Indicates that the timing task is turned on .
3. @Scheduled(cron = "0/5 * * * * ?") To add a scheduled task —— The event is triggered every five seconds .

2.2 Linux Of crontab command

stay Linux Medium cron The expression is different from the above , More precisely, it is more streamlined .

crontab The order of is cron The expression + command, there cron The expression has only the following five fields ：

Minutes Hours DayOfMonth Month DayOfWeek The operator has ： * / - ,

Let's take a look at some examples ：

1. Every night 21:30 restart smb

30 21 * * * /etc/init.d/smb restart

2. Every Saturday night 11:00 pm restart smb

0 23 * * 6 /etc/init.d/smb restart

3. evening 11 Point to morning 7 Between points , Restart every hour smb

0 23-7/1 * * * /etc/init.d/smb restart

corn Expression example answer

Come and see if your answer is right ~

(1) 10 10 10 ? * MON-WED

answer ： It means every morning from Monday to Wednesday 10 when 10 branch 10 Second trigger event .

(2) 0 15 15 ? 5L 2022-2024

answer ： Express 2022 Year to 2024 On the last Thursday of each month in 15 when 15 Sub trigger event .

(3) 0 0/30 9-17 * * ?

answer ： It means that the event will be triggered every half an hour from 9 to 5 .