Might a finally block not get executed when a thread is interrupted/killed?

Question

In the Java tutorial it says about try { ... } finally { ... }:

Note: If the JVM exits while the try or catch code is being executed, then the finally block may not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.

Is it true that a thread can be interrupted or killed (I thought that was impossible?) such that the finally block will not be executed while the JVM running this thread is not exited/killed? (I am puzzled because the quote above is pretty explicit about this, not much room for misunderstanding.)

Edit: Broke the question down to its core intend.

Answer 1

Rafael, I believe this is one of the edge cases you are after. If a thread is blocked on something native (eg reading from STDIN or a Socket), and the JVM is in a state of shutdown, and the thread is interrupted, then finally may not be invoked.

The following example indicates this without invoking deprecated methods:

1. Sleep - finally is invoked.
2. SystemIn - finally is not invoked.

The example is very contrived, and is purely provided for demonstrative purposes :)

public class Interrupted {

  static final List<Thread> THREADS = Arrays.asList(
      new Thread(new Sleep()),
      new Thread(new SystemIn())
  );
  static final CountDownLatch LATCH = new CountDownLatch(THREADS.size());

  public static void main(String[] args) throws Exception {
    Runtime.getRuntime().addShutdownHook(new Thread(new ShutdownHook()));
    for (Thread thread : THREADS) {
      thread.start();
    }
    System.out.println("[main] Waiting for threads to start...");
    LATCH.await();
    System.out.println("[main] All started, time to exit");
    System.exit(0);
  }

  static abstract class BlockingTask implements Runnable {
    @Override
    public void run() {
      final String name = getClass().getSimpleName();
      try {
        LATCH.countDown();
        System.out.printf("[%s] is about to block...%n",name);
        blockingTask();
      } catch (Throwable e) {
        System.out.printf("[%s] ", name);
        e.printStackTrace(System.out);
      } finally {
        System.out.printf("[%s] finally%n", name);
      }
    }
    abstract void blockingTask() throws Throwable;
  }

  static class Sleep extends BlockingTask {
    @Override
    void blockingTask() throws Throwable {
      Thread.sleep(60 * 60 * 1000); // 1 hour
    }
  }

  static class SystemIn extends BlockingTask {
    @Override
    void blockingTask() throws Throwable {
      System.in.read();
    }
  }

  static class ShutdownHook implements Runnable {
    @Override
    public void run() {
      System.out.println("[shutdown-hook] About to interrupt blocking tasks...");
      for (Thread thread : THREADS) {
        thread.interrupt();
      }
      System.out.println("[shutdown-hook] Interrupted");
      try {
        for (int i=0; i<10; i++) {
          Thread.sleep(50L);
          System.out.println("[shutdown-hook] Still exiting...");
        }
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

}

Answer 2

Well, I stand corrected. It is possible by using deprecated methods:

@Test
public void testThread() throws Exception {
    Thread thread = new Thread(new MyRunnable());
    thread.start();
    Thread.sleep(100);
    thread.suspend();
    Thread.sleep(2000);
}

class MyRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Start");
        try {
            Thread.sleep(1500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("Done");
        }
    }
}

Due to the pausing which will (most likely) occure while the thread is asleep, the finally block will never be executed.