How to run a background task in a servlet based web application?

Question

I'm using Java and I want to keep a servlet continuously running in my application, but I'm not getting how to do it. My servlet has a method which gives counts of the user from a database on a daily basis as well as the total count of the users from the whole database. So I want to keep the servlet continuously running for that.

Elle

What do you mean, "continuously running"? – Xylene

what do you mean by continously running? It will run as long as your app server runs – Roscoeroscommon

I don't understand why it has to run continuously... if someone wants the 'user count' then they call your servlet method and you give it to them? – Sanjak

@Sanjak Actually i want the usercount on daily basis,so for that i will have to run the servlet manually everyday so instead of doing that i want to run the servlet contineously.so i wont need to run the servlet everyday. – Elle

@Elle Servlets are not the way to solve this problem - if you want 'stats' collected throughout the day then use some other technology. – Sanjak

@skaffman,@faith Actually to get the count i will have to run the servlet manually everyday as database will be updated everyday as users will be created everyday.so instead of running servlet manually everyday i want to keep the servlet running. – Elle

@pritsag: A servlet is there to serve user requests, not to run batch jobs. – Xylene

@Sanjak can u suggest me the other technologies ? – Elle

@Elle If the database is accessible from the client then a simple Java program using JDBC (running from the client itself) might be appropriate. Other than that you could try a scripting language (perl, python, etc.). If the database is not accessible from the client then you need to create a batch architecture on the server that perhaps stores its results on the database itself - you then then use a Servlet to allow clients access these stats. – Sanjak

Answer 1

Your problem is that you misunderstand the purpose of the servlet. It's intented to act on HTTP requests, nothing more. You want just a background task which runs once on daily basis.

EJB available? Use @Schedule

If your environment happen to support EJB (i.e. a real Java EE server such as WildFly, JBoss, TomEE, Payara, GlassFish, etc), then use @Schedule instead. Here are some examples:

@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.
    }

    @Schedule(hour="*", minute="*", second="*/5", persistent=false)
    public void someFiveSecondelyJob() {
        // Do your job here which should run every 5 seconds.
    }

} 

Yes, that's really all. The container will automatically pickup and manage it.

EJB unavailable? Use ScheduledExecutorService

If your environment doesn't support EJB (i.e. you're not using not a real Java EE server, but a barebones servletcontainer such as Tomcat, Jetty, etc), then use ScheduledExecutorService. This can be initiated by a ServletContextListener. Here's a kickoff example:

@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);
        scheduler.scheduleAtFixedRate(new SomeHourlyJob(), 0, 1, TimeUnit.HOURS);
        scheduler.scheduleAtFixedRate(new SomeQuarterlyJob(), 0, 15, TimeUnit.MINUTES);
        scheduler.scheduleAtFixedRate(new SomeFiveSecondelyJob(), 0, 5, TimeUnit.SECONDS);
    }

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

}

Where the job classes look like this:

public class SomeDailyJob implements Runnable {

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

}

public class SomeHourlyJob implements Runnable {

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

}

public class SomeQuarterlyJob implements Runnable {

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

}

public class SomeFiveSecondelyJob implements Runnable {

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

}

Do not ever think about using java.util.Timer/java.lang.Thread in a Java EE / Servlet based environment

Last but not least, never directly use java.util.Timer and/or java.lang.Thread in Java EE. This is recipe for trouble. An elaborate explanation can be found in this JSF-related answer on the same question: Spawning threads in a JSF managed bean for scheduled tasks using a timer.

Sakai

@BalucS Thank you sir,your solution helped me and I learned about ScheduledExecutorService which was new to me as i m new to java.Thank you once again. – Elle

@Sakai : Where should the class UpdateCounts be put in web.xml? – Roturier

@Roturier web.xml is a Deployment Descriptor. The class UpdateCount is not related with deployment, so it doesn't have to be put in web.xml – Educt

One crucial issue with a ScheduledExecutorService: Be sure to capture all exceptions in your executor. If an exception escapes from your run method, the executor silently stops executing. This is a feature not a bug. Read the doc and study up with some googling. – Trahurn

Question: This task scheduler.scheduleAtFixedRate(new SomeDailyJob(), 0, 1, TimeUnit.DAYS); at what time will start? at 00:00 or every 24h from the moment the webApp will deploy in the server? – Breastfeed

@yaylitzis: just consult Javadoc: docs.oracle.com/javase/7/docs/api/java/util/concurrent/… (check 2nd argument) – Sakai

Just to be sure.. with long initialDelay = 0 and long period=1 this means at 00.00 every day right? – Breastfeed

@BalusC, what about org.springframework.scheduling.concurrent.ScheduledExecutorTask? – Faustena

I've followed through the code in your answer and used it to create a ServletContextListener, using the same principles to wrap the only instance of a ScheduledExecutorService that exists in my EE app. (There are no other places in the code in which I've used Thread, Runnable or ScheduledExecutorService.) However, stopping/undeploying the application still results in an error message of SEVERE: The web application [/MyApp] appears to have started a thread named [pool-4-thread-1] but has failed to stop it. This is very likely to create a memory leak. I don't know why that happens. – Forney

Following on from my prior comment: Creating a ThreadFactory that sets the thread to be a daemon (Thread t = Executors.defaultThreadFactory().newThread(runnable); t.setDaemon(true);) and then using the created ThreadFactory as an argument to Executors.newSingleThreadScheduledExecutor() solves the issue for me. (The only I/O work done by the task in question is to read from a small JSON file. It uses javax.mail to send an email based on configuration in that file.) I'm not expecting the application to be stopped/reloaded for about a month. I don't forsee an issue with using a daemon. – Forney

@Agi: that will happen if scheduler.shutdownNow() is not correctly invoked as per the example. If this is not invoked, then the schedule thread will indeed keep running. – Sakai

Answer 2

I would suggest using a library like quartz in order to run the task at regular intervals. What does the servlet really do ? It sends you a report ?

Cobber

yes ,it gives me the count of the user created per day and also the count of the total users in my database . – Elle

huuu? Can you describe the FULL architecture of your system. I'm lost. – Cobber

@Cobber i m new to java and in learning phase sir and really dont konw much about the servlets. – Elle

The problem is not about servlet. What is the application your are talking about ? (ps : it is a bad idea to delete your comments, especially comments I answered to) – Cobber

@twister when user will hit the application,he will get all details like how much users are created today,how much users are craeted till now etc.and i want to run the servlet run in background contineously so that the user could get the updates.I kno this is not the proper explation.(ps:i know it was a bad idea.sorry for that .) – Elle

@Elle You may attach user date/time of creation to user account in your database and count it on the fly when user hit a button on the web page. This is more powerful and flexible since you can decide which interval you what to count. However, if you really want a cron-like system, e.g. every day, every hour, etc., you may use quartz to run a task on a certain interval. – Hoffmann

@Hoffmann thanks for your kind reply sir.actually i have implemented the method in servlet which is giving me the count perfectly.but the thing now i want is actually instaed of running the servlet on daily basis i want to keep it running continweously so that whenever the request for updates about the count is sent, the response will be sent to user. – Elle

@Elle I think I now understand what you want. You don't understand the purpose of servlet and how it works. that's not your fault. I suggest you read answer by @Sakai carefully. That's what you need. – Hoffmann

Answer 3

You can use cron4j. http://www.sauronsoftware.it/projects/cron4j/manual.php

Pseudohermaphrodite

Answer 4

Implement two classes and call startTask() in main.

public void startTask()
{
    // Create a Runnable
    Runnable task = new Runnable() {
        public void run() {
            while (true) {
                runTask();
            }
        }
    };

    // Run the task in a background thread
    Thread backgroundThread = new Thread(task);
    // Terminate the running thread if the application exits
    backgroundThread.setDaemon(true);
    // Start the thread
    backgroundThread.start();
}

public void runTask()
{
    try {
        // do something...         
        Thread.sleep(1000);

    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

Stakhanovism

This is definitely NOT the way to do it in a web application - look at the answer above by @Sakai instead - he is correct here and I would say you can trust all of his answers. – Barela

Answer 5

In a production system that may have multiple non-jee containers running. Use anot enterprise scheduler like Quartz scheduler which can be configured to use a database for task maamgememt.

Strontia