jee6 学习笔记 6.2 - @Schedule

This tests the ejb3.1 annotation @Schedule.
The idea is to print the server time regularly, in a scheduled task method.

package com.jxee.ejb.test.singleton;

import java.util.Date;

import javax.annotation.PostConstruct;
import javax.ejb.Lock;
import javax.ejb.LockType;
import javax.ejb.Schedule;
import javax.ejb.Singleton;
import javax.ejb.Startup;

import org.apache.log4j.Logger;

/**
 *  Test jee 6 annotation @Schedule - automatic timers.
 *  <b>it's said that this annotation works with any EJBs except for
 *  Stateful session beans.</b>
 *  <br/>
 *  <br/>
 *  From the JEE6 API doc:
 *  <br/><br/>
 *  @Schedule schedules a timer for automatic creation with a timeout schedule 
 *  based on a cron-like time expression. The annotated method is 
 *  used as the timeout callback method. 
 *  </li>
 *  <li>
 *  A timeout callback method can have public, private, protected, 
 *  or package level access. The method must not be declared as final
 *  or static, and must not throw application exceptions.<br/>
 *  </li>
 *  <li>
 *  Schedule-based timer times are evaluated in the context of the 
 *  default time zone associated with the container in which the 
 *  application is executing. A schedule-based timer may optionally 
 *  override this default and associate itself with a specific time zone.
 *  If the schedule-based timer is associated with a specific time zone, 
 *  all its times are evaluated in the context of that time zone, 
 *  regardless of the default time zone in which the container is executing.
 *  </li>
 */
//@Startup   // eager initialization during the application startup sequence
@Singleton // a singleton scheduler, rather than a pool of schedulers...
public class ScheduleEJB {

  private static final Logger log = Logger.getLogger(ScheduleEJB.class);
  private String timeStr;
  private Integer counter = 0;
  
  @PostConstruct
  public void init() {
    log.debug(">>> inited: " + this.printInstance());
  }
  
  /**
   * scheduled to execute on Thursday at every 20th second for every 3 minutes
   */
  @Schedule(dayOfWeek="Thu", hour="*", minute="*/3", second="20", persistent=true)
  public void printTime() {
    doScheduledTask();
  }
  
  @Lock(LockType.WRITE)
  private void doScheduledTask() {
    ++counter;
    timeStr = "scheduled ["+ counter + "]" + ", timeStr(sec)=" + (new Date().getTime())/1000;
    log.debug(this.timeStr);
  }

  @Lock(LockType.READ)
  public String getTimeStr() {
    return timeStr;
  }
  
  public String printInstance() {
    return this.toString();
  }
}

here's the screen shot of the server log:



小结：

1. @Schedule can apply to stateless session beans and message driven beans. but not stateful session beans.

2. @Schedule creates so called "automatic" timers to schedule tasks.

3. if you wanna make sure that the ejb is fully initialized before servicing, you can use the annotation @Startup

4. this example uses a singleton ejb to schedule the task. otherwise you end up with the scheduled method called by multiple times at the timer expires.

5. A timeout callback method can have public, private, protected, or package level access. The method must not be declared as final or static, and must not throw application exceptions.

6. Schedule-based timer times are evaluated in the context of the default time zone associated with the container in which the application is executing. A schedule-based timer may optionally override this default and associate itself with a specific time zone.

7. attribute "persistent" defaults to "true", meaning the timer/the scheduled task/method execution would survive server restart.

8. one thing to mention is that one might need to remove the "tmp/work/data" folders from JBoss6 server configuration, after a redeployment. otherwise, it won't work as expected - the singleton ejb looks like have multiple instances.