Question

I was teaching myself Java threading and I noticed something that confuses me a little. I made a class called engine implementing Runnable. The run method just prints “Hello World”, sleeps for a second, and repeats.

In my main method, I have:

public static void main(String[] args) {
    Thread thread = new Thread(engine);
    thread.start();
    System.out.println("Done.");
}

As I expected, I see “Hello World” and “Done.” printed quickly, meaning the main method has reached the end, but what I didn’t expect was that the thread I started kept running even after the end of main was reached.

Why does the program continue to execute even after main exits? I would have thought that when main exited the process would terminate and all of the threads would be cleaned up forcefully. Does this mean that every thread has to be joined/killed explicitly for a Java program to terminate?

Answer 1

If you want your program to exit when the main method finished, consider making your threads daemons. But take care of the fact, that daemon threads will be aborted, when main finishes.
You can create a daemon thead like so:

Thread t = new Thread(...);
t.setDaemon(true);

All non-daemon threads are user threads. Those threads are stopping the jvm from closing.