Master Java Task Scheduling: From Crontab to Quartz, Spring and Distributed Solutions

Background

Scheduled tasks are ubiquitous in real‑world applications, such as starting flash sales at midnight on Double 11, automatically pulling payroll data on the first of each month, or sending heartbeat requests over TCP connections.

Typical characteristics of scheduled tasks include triggering at a specific moment, periodic execution at a fixed frequency, and starting at a designated time with recurring intervals.

1. Crontab Scheduler

1.1 Introduction

Crontab is a Linux utility that periodically executes shell scripts or commands. It is widely used by operations engineers and shares a similar expression syntax with Quartz and Spring Schedule.

Command format:

.---------------- minute (0 - 59)
|  .------------- hour (0 - 23)
|  |  .---------- day of month (1 - 31)
|  |  |  .------- month (1 - 12)
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)
|  |  |  |  |
*  *  *  *  *  command

Parameters:

minute: 0‑59

hour: 0‑23

day of month: 1‑31

month: 1‑12

day of week: 0‑7 (0 and 7 both represent Sunday)

command: the command to run

Example expressions:

# Every 5 minutes
*/5 * * * * Command
# At the 5th minute of every hour
5 * * * * Command
# Every day at 18:30
30 18 * * * Command
# On the 8th of each month at 07:30
30 7 8 * * Command
# On June 8th each year at 05:30
30 5 8 6 * Command
# Every Sunday at 06:30
30 6 * * 0 Command

1.2 Practical Example on CentOS

Install crond: yum -y install cronie yum-cron Create a shell script that logs the current time:

# Create test.sh script
vim /root/shell/test.sh
# Script content
#!/bin/bash
echo `date '+%Y-%m-%d %H:%M:%S'` >> /root/shell/file.log

Test the script: sh /root/shell/test.sh Check the log: cat /root/shell/file.log If the log shows timestamps, the script runs correctly. Add the task to crontab:

crontab -e
# Run test.sh every minute
*/1 * * * * sh /root/shell/test.sh

Verify the task list: crontab -l Reload and restart the crond service:

systemctl reload crond
systemctl restart crond

Monitor the log in real time:

tail -f /root/shell/file.log