1. Overview

With a cron scheduler, we can automate repetitive tasks we’d otherwise need to handle manually. Additionally, the cron expression allows us to schedule jobs executing at the desired date and time.

For scheduling jobs in Java, we usually use the Quartz library. It’s an open-source solution for job scheduling written entirely in Java. Furthermore, if we’re working with the Spring framework, we can use the @Scheduled annotation to easily schedule tasks.

Although cron expression represents a powerful way to schedule tasks, its syntax can sometimes be confusing and overwhelming.

In this tutorial, we’ll examine the differences between the ? and the * symbols in cron expressions.

2. Fields in Cron Expression

Before we dive in, let’s explore the fields that can appear in the cron expressions.

In Quartz, a cron expression represents a string that involves up to seven fields separated by whitespace, each representing a specific unit of date and time:

Field Required 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
Month Yes 0-11 (or JAN-DEC) , – * /
Day of Week Yes 1-7 (or SUN-SAT) , – * / ? L C #
Year No 1970-2099 (or empty) , – * /

As we can see in the table above, all fields are mandatory except the field that specifies a year. If we don’t provide a value, the job will be executed every year.

Additionally, the syntax for the Unix cron expressions is a bit different:

Field Required Allowed Values Allowed Special Characters
Minutes Yes 0-59 , – * /
Hours Yes 0-23 , – * /
Day of Month Yes 1-31 , – * /
Month Yes 1-12 (or JAN-DEC) , – * /
Day of Week Yes 0-6 (or SUN-SAT) , – * /

The Unix cron expression consists of five fields followed by the command we’d like to execute. Unlike Quartz, there aren’t specific fields where we’d specify seconds and years. It focuses on scheduling tasks for the current year.

It’s worth noting that the cron expression in Unix doesn’t allow the ? symbol to appear in the expression.

In the next sections, we’ll primarily focus on the cron expressions with the Quartz library.

3. The ? in Cron Expression

Next, let’s examine the question mark symbol (?) in the cron expression. Simply put, it represents no specific value.

We can use it only within the fields that specify the day of the month and the day of the week.

However, it’s important to note the day of the month and the day of the week fields are mutually exclusive. In other words, we can’t specify values for both fields in the same expression.

For instance, the following expression results in an error:

0 30 10 1 OCT 2 2023

Additionally, to easily understand the expression, let’s see it in the table:

Seconds Minutes Hours Day of Month Month Day of Week Year
0 30 10 1 OCT 2 2023

We set values for both the day of the month and the day of the week parameters, which isn’t supported with Quartz.

The cron expression would be invalid even if we use the day of the month that falls on the correct weekday:

0 30 10 30 OCT 2 2023

Here, the 30th of October in 2023 falls on a Monday, but the expression is still not valid.

Furthermore, since we’re required to set values for both fields, we need to put the ? symbol on one of them to indicate the value is unset. The field where we set ? will be ignored:

0 0 0 30 OCT ?

From the example, the job runs at midnight on the 30th of October, every year.

Additionally, the ? can appear only once in a cron expression. Setting both values with ? would result in an error as well:

0 30 * ? OCT ?

4. The * in Cron Expression

On the other hand, the asterisk (*) in the cron expression means all the values. To put it differently, we’d use it to set all the values defined for a specific field.

Furthermore, unlike the ?, we can use * within any field in the cron expression.

As an example, let’s create a cron expression where we’ll set all the values from the field representing the hours:

0 30 * 1 OCT ?

Next, let’s see in the tabular format:

Seconds Minutes Hours Day of Month Month Day of Week Year
0 30 * 1 OCT ? empty

The job executes on the first of October, at every hour, 30 minutes and 0 seconds.

Additionally, we can use the * for multiple fields as well:

* * * * OCT ?

This job runs every second, every day in October.

4.1. Day of Month and Day of Week in Linux Cron

When it comes to the day of the month and the weekday fields in Linux cron, they behave differently than the ones from Quartz.

Firstly, they’re not mutually exclusive. We can set both values in the same cron expression.

Secondly, if both fields contain values other than the asterisks, they form a union:

30 10 1 10 5

The job from the example above executes at 10:30 on the first of October and every Friday.

Lastly, if one of the values starts with the asterisk, they form an intersection:

30 10 */1 * 1

Here, the job runs at 10:30 on every day of the month only if it falls on Monday.

5. Comparison Between * and ?

To conclude, let’s list the main differences between the * and the ? special characters in the cron expression:

The * Symbol The ? Symbol
Stands for all allowed values of a specific field Means no specific value
Can be used in any field Can be used only in fields representing the day of the month and the day of the week
Used to specify all the values from the field Used to set empty value
Can appear multiple times in the same expression Only one can exist per expression

6. Conclusion

In this article, we learned the differences between the asterisk and the question mark special characters in cron expressions.

To sum up, we’d use the * in the field of a cron expression to include all allowed values for that specific field. On the contrary, the ? represents no specific value and can be used only within the day of month and day of week fields.

Since Quartz doesn’t support implementation for both of those fields, we need to use ? in one of them to leave the field empty.

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