Java/J2ee - Servlet Listener & Quartz Scheduler

Here the concepts of Servlet Listeners & schedulers are shown with example.

Brief Description:

1. Servlet Listeners are loaded at the time of application deployment in web server.
2. contextInitialized() & contextDestroyed() are the overloaded functions which are called at the time of application load and unload respectively.
3. Schedulers are the threads which could be scheduled to run at a particular time.
4. Job class is actually having the duty of the job in the
   execute() method
   .
5. Defined Job could be assigned to a scheduler.
6. CronExpression is an interesting one which is used to define the schedule interval for a particular scheduler.

I used this scheduler for scheduling some alerts in my First real time project "Telematics - ACRM" in my First company Defiance.

Code:

Note: We need to add quarts jars in the class path.

Servlet Listener Class

import java.text.ParseException;

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

import org.apache.log4j.Logger;

import org.quartz.CronExpression;

import org.quartz.CronTrigger;

import org.quartz.JobDetail;

import org.quartz.Scheduler;

import org.quartz.SchedulerException;

import org.quartz.SchedulerFactory;

import org.quartz.impl.StdSchedulerFactory;

public class AlertServletListener implements ServletContextListener {
private SchedulerFactory sf = null;
private Scheduler sched = null;
private static transient Logger logger = Logger.getLogger(AlertServletListener.class);

@Override
public void contextDestroyed(final ServletContextEvent sce) {
try { sched.shutdown(true); logger.info("AlertServer Shut Down");
} catch (final SchedulerException e) { logger.info("Err @ AlertServer Shut Down " + e.getMessage());
}
}

@Override
public void contextInitialized(final ServletContextEvent sce) {
logger.info("AlertServer Starting");
try { sf = new StdSchedulerFactory(); sched = sf.getScheduler(); final JobDetail job3 = new JobDetail("myJob3", "myJobGroup3", MessageProcessJob.class); final CronTrigger ct3 = new CronTrigger("myTrigger3", "myTriggerGroup3"); final CronExpression cexp3 = new CronExpression("1/35 * * * * ?"); ct3.setCronExpression(cexp3); sched.scheduleJob(job3, ct3); sched.start(); logger.info("AlertServer Started Up"); Thread.sleep(10000L);
} catch (final ParseException e) { e.printStackTrace();
} catch (final SchedulerException e) { e.printStackTrace();
} catch (final InterruptedException e) { e.printStackTrace();
}
}
}

Job Class

import org.apache.log4j.Logger;

import org.quartz.Job;

import org.quartz.JobExecutionContext;

import org.quartz.JobExecutionException;

public class DistanceAlertsJob implements Job {

/**

* logger logger

*/

private static transient Logger logger = Logger.getLogger(DistanceAlertsJob.class);

@Override

public void execute(final JobExecutionContext arg0) throws JobExecutionException {

logger.info("Distance Alert Job");

Sysytem.out.println("Hello");

}

}

web.xml (Add the listener entry in web.xml)
..... <listener>
<listener-class>
com.boss.pageflows.schedular.AlertServletListener
</listener-class>
</listener>
.....