Spring Boot - Best way to start a background thread on deployment

Question

I have a Spring Boot application deployed in Tomcat 8. When the application starts I want to start a worker Thread in the background that Spring Autowires with some dependencies. Currently I have this :

@SpringBootApplication
@EnableAutoConfiguration
@ComponentScan
public class MyServer extends SpringBootServletInitializer {   

    public static void main(String[] args) {
        log.info("Starting application");
        ApplicationContext ctx = SpringApplication.run(MyServer.class, args);
        Thread subscriber = new Thread(ctx.getBean(EventSubscriber.class));
        log.info("Starting Subscriber Thread");
        subscriber.start();
    }

In my Docker test environment this works just fine - but when I deploy this to my Linux (Debian Jessie, Java 8) host in Tomcat 8 I never see the "Starting Subscriber Thread" message (and the thread is not started).

You are aware of the fact that starting a thread in a EE environment is basically a bad thing to do? Just create a service in spring annotate it with `@Scheduled` and let it run on certain intervals. Or even better why do you need this thread? Generally there are better (and more spring integrated) solutions. — M. Deinum, Sep 28 '16 at 06:58

Magnus · Accepted Answer · 2018-05-09T23:42:48.187

28

The main method is not called when deploying the application to a non-embedded application server. The simplest way to start a thread is to do it from the beans constructor. Also a good idea to clean up the thread when the context is closed, for example:

@Component
class EventSubscriber implements DisposableBean, Runnable {

    private Thread thread;
    private volatile boolean someCondition;

    EventSubscriber(){
        this.thread = new Thread(this);
        this.thread.start();
    }

    @Override
    public void run(){
        while(someCondition){
            doStuff();
        }
    }

    @Override
    public void destroy(){
        someCondition = false;
    }

}

edited May 09 '18 at 23:42

answered Sep 28 '16 at 05:12

Magnus

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It is not possible to add @Bean on the class level, see https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/context/annotation/Bean.html – Daniel Dietrich Oct 31 '17 at 16:16
Good point, I have changed it to `@Component` instead, although you can register the bean however you want really. – Magnus Nov 01 '17 at 00:08
Doing significant work in the constructor, such as starting a thread makes it very hard to test the object. The reply from Snickers3192 should be the accepted answer – xpmatteo Mar 25 '18 at 16:18
1

@xpmatteo Fair point. If you want to be able to construct the object without starting the thread, you could use a `@PostConstruct` annotation. – Magnus Mar 25 '18 at 22:31

Derrops · Answer 2 · 2019-02-08T05:12:09.280

5

You could have a bean which impelements ApplicationListener<ContextRefreshedEvent> It's onApplicationEvent will be called just start your thread there if it hasn't been started already. I think you want the ApplicationReadyEvent by the way.

Edit How to add a hook to the application context initialization event?

@Component
public class FooBar implements ApplicationListener<ContextRefreshedEvent> {

    Thread t = new Thread();

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        if (!t.isAlive()) {
            t.start();
        }
    }
}

edited Feb 08 '19 at 05:12

answered Sep 28 '16 at 02:26

Derrops

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I know SO frowns upon examples in links rather than answers, but [here's spring's guide to using ApplicationListnener](https://spring.io/blog/2015/02/11/better-application-events-in-spring-framework-4-2) – Gab Jul 24 '18 at 16:50
There you go @KCK – Derrops Feb 08 '19 at 05:12

score 0 · Answer 3 · answered Mar 09 '21 at 14:53

0

Starting a Thread manually is probably not the best way to do it. Instead, use Springs Task execution and Scheduling functionionality.

answered Mar 09 '21 at 14:53

Kees van Dieren

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Spring Boot - Best way to start a background thread on deployment

3 Answers3

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