Automatically starting up Java EE 5 Timers

When you're developing a Java EE application and need a simple scheduler you may decide to use Java EE Timer Service API.

Of course Java EE Timer Service is not as powerful as Quartz, but it's provided out of the box :)

Last week I needed to implement a simple scheduler in my Java EE 5 app. I encountered a small problem - I wanted to start it automatically just after the application start up.

Here is how I did it.

1st approach: create timer inside @PostConstruct method

I tried this at first:

@Local
public interface PeriodicWorker {
 void callback(Timer timer);
}

@Stateless
public class PeriodicWorkerImpl implements PeriodicWorker {
 private static final Log log = LogFactory.getLog(PeriodicWorkerImpl.class);
 @Resource
 private TimerService timerService;
 @PostConstruct
 void init() {
  log.debug("Timer Service injected " + (timerService != null));
  timerService.createTimer(new Date(), 10000, PeriodicWorkerImpl.class);
 }
 @Timeout
 @Override
 public void callback(Timer timer) {
  log.debug("Got timeout from timer " + timer.getInfo());  
 }
}

Of course it didn't work. Java EE container does not instantiate all SLSB when the app is started those components are started when they are needed (lazy loading).

So my problem was, where to inject it...

2nd approach: injecting worker SLSB it into another EJB component

Injecting into another EJB? Might be tempting, but again, that EJB component had to be first called in order to be instantiated and all its @Resource fields populated.

But then it struck me, the reference to my SLSB can be injected into web container-managed ServletContextListener object.

3rd approach: injecting worker SLSB into ServletContextListener

ServletContextListener defines lifecycle methods which are called after the servlet context is initialised/destroyed.

I was almost done, but this time I got an exception saying that it is illegal to create a timer within SLSB's @PostConstruct method.

This time it was simple to fix.

4th and final solution

I added createTimer method to my PeriodicWorker and moved creation of timer there:

@Stateless
public class PeriodicWorkerImpl implements PeriodicWorker {
 private static final Log log = LogFactory.getLog(PeriodicWorkerImpl.class);
 @Resource
 private TimerService timerService;
 @PostConstruct
 void init() {
  log.debug("Timer Service injected " + (timerService != null));
 }
 @Override
 public void createTimer() {
  timerService.createTimer(new Date(), 10000, PeriodicWorkerImpl.class);
 }
 @Timeout
 @Override
 public void callback(Timer timer) {
  log.debug("Got timeout from timer " + timer.getInfo());  
 }
}

And then I simply call createTimer() method from my ServletContextListener.

public class TimerInitialiserContextListener implements ServletContextListener { 
 private static final Log log = LogFactory.getLog(TimerInitialiserContextListener.class);
 @EJB
 private PeriodicWorker worker;
 public void contextDestroyed(ServletContextEvent ctx) {
 }
 public void contextInitialized(ServletContextEvent ctx) {
  worker.createTimer();
 }
}

I deployed the app and it worked like a charm.

It turned out not to be that simple (took me 15 minutes including deploying on application server!), but it works out of the box. I pity there is no EJB-specific component which is started when the EJB module is started. Wouldn't it be nice?

Note: Yes it would, and it's possible in Java EE 6. See this post's comments below :)

cheers,
Łukasz