java - Why non-daemon thread is terminating if in jUnit test?

Question

The following daemon-bean is running:

public class DaemonBean extends Thread {

    private final static Logger log = LoggerFactory.getLogger(DaemonBean.class);

    {
        setDaemon(true);
        start();
    }

    @Override
    public void run() {

        for(int i=0; i<10 && !isInterrupted(); ++i) {
            log.info("Hearbeat {}", i);
            try {
                sleep(1000);
            } catch (InterruptedException e) {
                return;
            }
        }

    }
}

It is daemon, so would terminate if singleton.

So, the following non-daemon bean is waiting for him:

public class Waitor1 extends Thread {

    private final static Logger log = LoggerFactory.getLogger(Waitor1.class);

    private Thread joinable;

    {
        setDaemon(false);
        setUncaughtExceptionHandler(new UncaughtExceptionHandler() {

            @Override
            public void uncaughtException(Thread t, Throwable e) {
                log.error("Error in thread", e);
            }
        });
    }

    public Thread getJoinable() {
        return joinable;
    }

    public void setJoinable(Thread value) {
        this.joinable = value;
        if( this.joinable != null ) {
            start();
        }
    }

    @Override
    public void run() {

        log.info("Waiting started");

        try {
            joinable.join();
        } catch (InterruptedException e) {
            log.info("Thread interrupted");
            return;
        }

        log.info("Waiting ended");

    }

}

The Spring configuration for beans is:

<bean id="daemon" class="beans.DaemonBean"/>

    <bean id="waitor" class="beans.Waitor1">
        <property name="joinable" ref="daemon"/>
    </bean>

The question is: why is it working if runned from main and not working if ran from jUnit test?

Running code is

 public static void main(String[] args) {
        new ClassPathXmlApplicationContext("/beans/Waiting1.xml");
    }

or

@Test
    public void testWaiting1() {
        new ClassPathXmlApplicationContext("/beans/Waiting1.xml");
    }

In case of main I see all hearbeats. In case of jUnit I see only heartbeat 0, then message "Waiting started" and the program is terminated as if nobody waiting for non-daemon threads here.

What can be the reason of it?

Answer 1

When you run your code from main it creates both beans, thus two threads - daemon and non-daemon. As long as non-daemon thread is running, your application won't exit. So it works.

It's different when run from JUnit. As soon as JUnit test method completes (and it completes immediately after the Spring context is up), JUnit assumes your tests are done. Thus it kills all your threads and basically the whole JVM.

Remember your Waitor1 bean spawns a background thread which JUnit doesn't care about. As soon as you leave @Test method JUnit will just stop everything.