Question

I am seeing interesting behavior. I am running this code

public class ThreadsTest {

    public static void main(String[] args) {
        Runnable mr = new MyRunnable();
        Thread t1 = new Thread(mr);
        Thread t2 = new Thread(mr);
        t1.setName("first");
        t2.setName("second");
        t1.start();
        t2.start();
        t1.run();
    } 
} 

class MyRunnable implements Runnable {
    public void run() {
    for (int i=0; i < 2; i++) {
        System.out.println("Running: " + Thread.currentThread().getName());
    }
    }
}

The output I get is:

Running: first
Running: first
Running: second
Running: second

I am expecting to see something similar to this:

Running: first
Running: first
Running: second
Running: second
Running: main
Running: main

Does anyone knows why I don’t see Running: main somewhere in my output. Thank you.

Answer 1

The explanation is a bit subtle.

The default behaviour of the Thread.run() method is described as running the supplied Runnable if it exists, and doing nothing otherwise.

The subtlety is that when a Thread exits, the exit() method “aggressively” nulls out reference fields to prevent storage leaks. (Here’s the source … line 720 onwards.) It appears that by the time that you called t1.run() in main, the thread had exited so the call was a no-op.

Naturally, the output depends on whether or not the main thread gets to run again before the child thread exits … and that will depend on your platform; e.g. what the JVM and the OS-level thread scheduler do and how many cores are available.