Fast graceful shutdown of ThreadPerTaskExecutor (when expected WAITING Threads)

[Rob Bygrave]

A "solution / fix" to this issue that modifies the internals of
*java.util.concurrent.ThreadPerTaskExector* is to:


*1) Add a new interface InterruptableTask*

public interface InterruptableTask {

    /**
     * Return true if this is a task that is deemed to be in an idle state
that can be interrupted.
     */
    boolean canInterrupt();
}


*2) Modify java.util.concurrent.ThreadPerTaskExector by:*

*2i)* Changing the threads field from storing just Thread to additionally
storing the associated task (Runnable or Callable)

// instead of ...
private final Set<Thread> threads = ConcurrentHashMap.newKeySet();

// use a Map with values being the tasks (Runnable or Callable)
private final Map<Thread, Object> threadTasks = new ConcurrentHashMap<>();



*2ii)* Add a new internal method tryStopInterruptableTask():

private void tryStopInterruptableTask() {
  threadTasks.entrySet().stream()
       .filter(entry -> entry.getKey().isAlive()) // Thread isAlive
       .filter(entry -> entry.getKey().getState() == Thread.State.WAITING)
// Thread WAITING state
       .forEach(entry -> {
         if (entry.getValue() instanceof InterruptableTask task) {
           if (task.canInterrupt()) { // application deems this task to NOT
be in-flight (e.g. not IO waiting on database response)
             entry.getKey().interrupt();
           }
         }
       });
}

*2 iii)* Call this method as part of shutdown() (or perhaps the internal
tryShutdownAndTerminate())



With these changes, then the Nima specific ConnectionHandler just needs to
implement InterruptableTask and then the usual
shutdown of the ExecutorService operates nicely.

executor.shutdown();
if (!executor.awaitTermination(gracefulShutdownMillis,
TimeUnit.MILLISECONDS)) {
    List<Runnable> running = executor.shutdownNow();
    if (!running.isEmpty()) {
        LOGGER.log(INFO, running.size() + " channel tasks did not terminate
gracefully");
    }
}



Cheers, Rob.
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