Spawning threads in a JSF managed bean for scheduled tasks using a timer

Question

I would like to know if it\'s ok to use Timer inside application scoped beans.

Example, lets say that I want to create a timer task that sends out a bu

Answer 1

Introduction

As to spawning a thread from inside a JSF managed bean, it would only make sense if you want to be able to reference it in your views by #{managedBeanName} or in other managed beans by @ManagedProperty("#{managedBeanName}"). You should only make sure that you implement @PreDestroy to ensure that all those threads are shut down whenever the webapp is about to shutdown, like as you would do in contextDestroyed() method of ServletContextListener (yes, you did?). See also Is it safe to start a new thread in a JSF managed bean?

Never use java.util.Timer in Java EE

As to using java.util.Timer in a JSF managed bean, you should absolutely not use the old fashioned Timer, but the modern ScheduledExecutorService. The Timer has the following major problems which makes it unsuitable for use in a long running Java EE web application (quoted from Java Concurrency in Practice):

• Timer is sensitive to changes in the system clock, ScheduledExecutorService isn't.
• Timer has only one execution thread, so long-running task can delay other tasks. ScheduledExecutorService can be configured with any number of threads.
• Any runtime exceptions thrown in a TimerTask kill that one thread, thus making Timer dead, i.e. scheduled tasks will not run anymore. ScheduledThreadExecutor not only catches runtime exceptions, but it lets you handle them if you want. Task which threw exception will be canceled, but other tasks will continue to run.

Apart from the book quotes, I can think of more disadvantages:

• If you forget to explicitly cancel() the Timer, then it keeps running after undeployment. So after a redeploy a new thread is created, doing the same job again. Etcetera. It has become a "fire and forget" by now and you can't programmatically cancel it anymore. You'd basically need to shutdown and restart the whole server to clear out previous threads.

• If the Timer thread is not marked as daemon thread, then it will block the webapp's undeployment and server's shutdown. You'd basically need to hard kill the server. The major disadvantage is that the webapp won't be able to perform graceful cleanup via e.g. contextDestroyed() and @PreDestroy methods.

EJB available? Use @Schedule

If you target Java EE 6 or newer (e.g. JBoss AS, GlassFish, TomEE, etc and thus not a barebones JSP/Servlet container such as Tomcat), then use a @Singleton EJB with a @Schedule method instead. This way the container will worry itself about pooling and destroying threads via ScheduledExecutorService. All you need is then the following EJB:

@Singleton
public class BackgroundJobManager {

    @Schedule(hour="0", minute="0", second="0", persistent=false)
    public void someDailyJob() {
        // Do your job here which should run every start of day.
    }

    @Schedule(hour="*/1", minute="0", second="0", persistent=false)
    public void someHourlyJob() {
        // Do your job here which should run every hour of day.
    }

    @Schedule(hour="*", minute="*/15", second="0", persistent=false)
    public void someQuarterlyJob() {
        // Do your job here which should run every 15 minute of hour.
    }

} 

This is if necessary available in managed beans by @EJB:

@EJB
private BackgroundJobManager backgroundJobManager;

EJB unavailable? Use ScheduledExecutorService

Without EJB, you'd need to manually work with ScheduledExecutorService. The application scoped managed bean implementation would look something like this:

@ManagedBean(eager=true)
@ApplicationScoped
public class BackgroundJobManager {

    private ScheduledExecutorService scheduler; 

    @PostConstruct
    public void init() {
        scheduler = Executors.newSingleThreadScheduledExecutor();
        scheduler.scheduleAtFixedRate(new SomeDailyJob(), 0, 1, TimeUnit.DAYS);
    }

    @PreDestroy
    public void destroy() {
        scheduler.shutdownNow();
    }

}

where the SomeDailyJob look like this:

public class SomeDailyJob implements Runnable {

    @Override
    public void run() {
        // Do your job here.
    }

}

If you don't need to reference it in the view or other managed beans at all, then better just use ServletContextListener to keep it decoupled from JSF.

@WebListener
public class BackgroundJobManager implements ServletContextListener {

    private ScheduledExecutorService scheduler;

    @Override
    public void contextInitialized(ServletContextEvent event) {
        scheduler = Executors.newSingleThreadScheduledExecutor();
        scheduler.scheduleAtFixedRate(new SomeDailyJob(), 0, 1, TimeUnit.DAYS);
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        scheduler.shutdownNow();
    }

}