SpringBoot @Scheduled Cron expression usage

Detailed explanation of Cron expression

Cron expressions are strings used to define the execution time of a timed task and are widely used in Spring's timed task frameworks such as @Scheduled and Quartz.

Its core is to implement complex scheduling rules through the combination of ‌time fields‌ and ‌wildcard characters‌.

1. Expression format

The Cron expression consists of ‌6 or 7 fields‌, which represent different time units (the ‌6-bit‌ format is usually used in Spring).

The format is as follows:

2. Special character parsing

3. Common examples

‌4. Key rules

‌1. The mutual exclusion between day and week

• If you specify both day and weekly, you need to ignore one of the fields.
• ✅ Correct: 0 0 0 ? * MON (execute every Monday, ignore the day)
• ❌ Error: 0 0 0 * * MON (Day and weekly take effect at the same time, may conflict)

2‌. Abbreviation of month and week‌

• Month: JAN, FEB, MAR… DEC
• Week: SUN, MON, TUE… SAT
  ‌

3. A combination of L and W

• LW represents the last working day of the month.
• L-3 means the third day to the end. ‌

4. Year field (optional)‌

• Spring's @Scheduled ‌ does not support the year field‌, and requires a 6-bit expression.

5. Dynamic and complex scenarios‌

1‌. Dynamic Cron expression‌

• It can be read from the configuration file in Spring by @Scheduled(cron = "${}").
• Combined with the database dynamic update task:

@Scheduled(cron = "#{@()}")
public void dynamicTask() {
    // Business logic}

2‌. Avoid the peak of the mission‌

• Add random delay (avoid multiple tasks being triggered simultaneously):

@Scheduled(cron = "0 #{T().current().nextInt(55)} * * * ?")
public void randomMinuteTask() {
    // Random minutes per hour}

3‌. Leap year treatment

• Cron cannot directly deal with leap years, so it needs to be judged in combination with the code logic.

6. Debugging and Verification

1‌. Online Tools‌

• Crontab Guru: Quickly validate expressions.
• CronMaker: Generate expressions and view the next execution time.

2. Log debugging

• Add a log in the task method to see if the trigger time is as expected:

@Scheduled(cron = "0 0/5 * * * ?")
public void logTask() {
    ("Task execution time: {}", ());
}

7. FAQ

‌1. Why is the task not executed? ‌

• Check whether to add@EnableScheduling。
• Check if the Cron expression is correct (such as Spring does not support the year field).
• Check the time zone settings (the server time zone is used by default, which can be modified through the zone property).

‌2. How to implement execution every N days? ‌

• Plan 1:use0 0 0 */N * ?(like0 0 0 */5 * ?Perform every 5 days).
• Plan 2: Record the last execution time through code.

‌3. Idepotency in distributed environments

Using Redis distributed locks:

@Scheduled(cron = "0 0 * * * ?")
public void distributedTask() {
    if (("taskLock", 10)) {
        try {
            // Business logic        } finally {
            ("taskLock");
        }
    }
}

Summarize

Cron expressions implement flexible timing rules through concise syntax, but attention should be paid to the applicable scenarios of ‌field mutex‌ and ‌special characters.

In complex services, dynamic configuration, distributed locking and log monitoring can be combined to ensure stable task execution.

The above is personal experience. I hope you can give you a reference and I hope you can support me more.