Question

I started learning java and I am now at the concurrency chapter. After reading some stuff about concurrency I tried an example of my own.

public class Task implements Runnable{

public void run() {
    while(!Thread.interrupted()) {
        try {
            System.out.println("task");
            TimeUnit.SECONDS.sleep(2);
        }catch (InterruptedException e) {
            System.out.println("interrupted");
        }
    }
}

}

public static void main(String[] args) throws Exception {
    ExecutorService exec = Executors.newCachedThreadPool();
    exec.execute(new Task());
    TimeUnit.SECONDS.sleep(10);
    exec.shutdownNow();
}

The problem is that i was expecting to see the following output:

task
task
task
task
task
interrupted

but after I get this, the program continues printing until I close it.
So, my question is what am I doing wrong? why does the program continues printing?

Answer 1

The section about interrupts in the Java tutorials about concurrency explain the problem quite well:

The Interrupt Status Flag

The interrupt mechanism is implemented using
an internal flag known as the interrupt status. Invoking
Thread.interrupt sets this flag. When a thread checks for an interrupt
by invoking the static method Thread.interrupted, interrupt status is
cleared. The non-static isInterrupted method, which is used by one
thread to query the interrupt status of another, does not change the
interrupt status flag.

By convention, any method that exits by throwing an
InterruptedException clears interrupt status when it does so. However,
it’s always possible that interrupt status will immediately be set
again, by another thread invoking interrupt.

So when you catch the InterruptedException within the loop, the interrupted status is already reset and thus, the next call of Thread.interrupted() will return false, which in turn keeps the while loop running. To stop the loop, you have the following options:

• Use break to exit the loop
• Use return to exit the whole method
• Move the try-catch-block to outside the while loop (as suggested by Nathan Hughes)
• Call interrupt() on the current thread to set the interrupted flag again
• Use a separate boolean to control the loop and set that flag accordingly in the catch-block
• Make the task a recurring task by using a ScheduledExecutorService and dropping the loop from the run-method of your Runnable