Using Cron in place of Quartz Enterprise Job Scheduler

If you need to schedule tasks in your Java web application, then there are few options.

On previous projects I have used Spring Framework's scheduling integration, which provides convenience classes for Quartz. This worked well, but a good alternative on Unix-like systems is to use cron.

Here are the steps to get something simple working:

1. Open a terminal and log in to your server

2. Open your crontab for editing:

 
    crontab -e

3. Add a line like this in your crontab:

 
    * * * * * wget -O /tmp/your-domain_scheduler_out http://your-domain.com/scheduler

Make sure you enter the correct domain name. Using localhost will work in some cases, depending on your server's configuration.

4. Now you need to create your scheduler servlet:

// Accessed using path: /scheduler
public class SchedulerServlet extends HttpServlet implements Servlet
{
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException
  {
    // Invoke your scheduler service - something you will need to implement
    SchedulerService.getInstance().doTasks();

    response.setContentType("text/plain");

    PrintWriter out = response.getWriter();

    // Write the current date/time to the response
    out.println((new Date()).toString() + " :: Scheduler invoked");
    out.close();
  }
}

5. Update your web.xml:

  
  <servlet>
    <servlet-name>schedulerServlet</servlet-name>
    <servlet-class>com.your-domain.some.path.SchedulerServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>schedulerServlet</servlet-name>
    <url-pattern>/scheduler</url-pattern>
  </servlet-mapping>

6. Deploy and test:
Compile your serlvet, reload your application and make sure your scheduler servlet works. If the cron job is working correctly you should see the scheduler_out file in your /tmp directory (which will be updated every minute).

7. Improvements for production deployment:
You should consider doing the following before deploying in a production environment:

• Secure the scheduler servlet by restricting access to the local IP address
• Instead of invoking wget directly in your crontab, you could invoke a shell script 
• Handle exceptions and maintain a log file
• Decide how often the scheduler should run and change your cron entry accordingly.